Deep Diving into API Security

APIs allow systems to communicate seamlessly. However, this central role also makes them attractive targets for cyber attacks. This article examines the critical threats, highlights common API security vulnerabilities, and highlights best practices that can be used to strengthen your API security strategy. We also take a look at key compliance requirements.

Table of contents

Summary

Application Programming Interfaces (APIs) are omnipresent in a digitally connected IT system landscape. They enable seamless communication and data exchange between different systems and are the foundation of complex digital ecosystems.

However, increasing reliance on APIs also increases the number of attack vectors. APIs are often accessible over public networks and are well documented. This makes them attractive targets for cybercriminals, as unsecured APIs often expose sensitive data and provide deep insight into system architecture. Threats such as injection attacks, cross-site scripting (XSS), distributed denial-of-service (DDoS) attacks and man-in-the-middle (MitM) attacks pose real threats to the integrity, confidentiality and availability of data and services.

A failure in API security can have serious consequences: Data leaks, manipulation of system states, and disruption of critical services can lead to significant financial loss, reputational damage, and legal ramifications. Given these risks, implementing robust security measures for APIs is critical.

What are the most common threats to APIs?

As a developer and/or provider of an API, you should understand how API security vulnerabilities can be exploited. Understanding these threats is essential to appreciate the need for robust security measures and to protect against them effectively.

Injection Attacks

Attackers attempt to inject malicious code into an API request. This code can be contained in various parts of an API request, such as the headers, cookies or the message body. Common examples include SQL injection and command injection. SQL injection, for example, attempts to insert malicious SQL commands into database queries if the data entered is not sufficiently sanitised. Similarly, command injection attacks aim to execute unwanted commands in the operating system.

Overview of injection attacks:

Definition: Attackers send malicious data in an API request to exploit vulnerabilities.
Affected areas of the API request: headers, cookies, message body.
Cause: Inadequate validation of incoming data. Direct use of user input in database queries or system commands.
Possible consequences: Data leakage. Misuse of data. System compromise. Complete application shutdown. Unauthorised access to databases. Manipulation of databases.
Possible targets: Data theft. Data manipulation. Unauthorised access to systems. Bypass security mechanisms.

Cross-Site Scripting (XSS)

At its core, XSS is about attackers injecting malicious scripts into web applications. In the context of API security, the scripts are designed to manipulate API requests in another user's browser. Unlike injection attacks, which primarily exploit server-side vulnerabilities, XSS typically targets client-side scripts.

Overview of XSS:

Definition: Injecting malicious scripts into web applications to run them in the context of another user's browser.
Affected areas of the API request: headers, cookies, message body.
Cause: Insufficient validation and sanitisation of incoming data.
Possible consequences: Data leakage. Data theft. Unwanted actions in the context of other users.
Possible targets: Access to sensitive data. Performing unauthorised actions on behalf of another user. Compromise data. Hijacking sessions. Deface websites. Negatively affect user experience. Spread malware.

Distributed Denial-of-Service (DDoS) Attacks

The goal of these attacks is to overload an API or service with a flood of requests, making it inaccessible to legitimate users. Unlike traditional denial-of-service (DoS) attacks, DDoS requests come from many different, often compromised systems (a botnet).

API endpoints are increasingly being targeted, as a successful DDoS attack can significantly disrupt operations and result in prolonged downtime. The sheer volume of malicious traffic can exhaust system resources, impact server performance and ultimately prevent service availability.

Overview of DDoS:

Definition: Overloading an API or service with excessive traffic.
Affected areas of the API request: The attack may target the entire API or specific, particularly resource-intensive endpoints.
Cause: Inadequate resource control and rate limiting measures. Missing API gateways.
Possible consequences: Increased resource consumption. Increased costs. Significant downtime of services and applications. Services not accessible or only partially accessible to legitimate users. Damage to reputation.
Possible targets: To limit or prevent the availability of a service. This can be done for a variety of reasons, such as to disrupt operations, cause financial damage, spread political or ideological messages, or simply cause chaos. Attackers may also try to distract from the actual attack in order to carry out other, less obvious attacks at the same time.

Man-in-the-Middle (MitM) Attacks

In this type of attack, an attacker intercepts communication between two systems. In the context of APIs, for example, this could be the connection between a client application and the API server, but also the communication between the API server and its downstream services. The goal of such an attack is to read or even manipulate the exchanged data unnoticed.

Overview of MitM:

Definition: The attacker intercepts the communication between two systems.
Affected parts of the API request: Entire communication chain.
Cause: Insufficient encryption. Vulnerabilities in network infrastructure. Sending sensitive information in the URL.
Possible consequences: Unauthorised reading of sensitive data such as login credentials, API keys, financial and health data. Manipulation of data. Gain unauthorised access rights. Compromise downstream systems. Control all communications.
Possible targets: Data theft. Spying on communications. Manipulate processes. Bypass security controls. Financial gain. Planting false information.

API Abuse

Unlike direct vulnerability attacks, exploit attempts often aim to use legitimate functions of an API in unexpected or malicious ways. For example, attackers may attempt to retrieve sensitive data in bulk, even if they are authorised to retrieve individual records, or overload systems with an excessive number of requests, even if these requests are valid in themselves.

Overview of API abuse:

Definition: Exploiting the legitimate functionality of an API in a way that is not intended and results in negative consequences.
Areas of API request affected: API endpoints and functions, especially those that provide access to data or can trigger compute-intensive operations.
Cause: Inadequate rate limiting. Lack of monitoring for unusual usage patterns. Careless handling of access controls. Unclear definition of authorised API usage.
Possible consequences: Service interruptions due to congestion. Depletion of system resources. Unexpectedly high costs. Slowdown or degradation of API performance for legitimate users. Data misuse, disclosure or unauthorised access to sensitive information.
Possible targets: Extracting large amounts of data. Achieving financial gain. Limit or prevent the availability of a service.

Common vulnerabilities of APIs

The following section highlights the most common vulnerabilities that can occur in APIs. These vulnerabilities can have a variety of causes, ranging from design flaws and incorrect configurations to inadequate security measures.

Broken Object Level Authorization (BOLA)

Incorrect authorisation at object level allows a legitimate user to access other people's objects.

Attack Description: In an attack exploiting BOLA, an attacker attempts to access objects for which they have no authorisation. This is typically done by manipulating the object ID (e.g. a user ID or order number) in the API request.
Cause: The main cause of BOLA is the lack or poor implementation of permission checks at the level of individual data objects. It is often only checked whether a user is authenticated in general, but not whether they have the rights to edit or view the specific data requested. Poor access controls and a failure to check permissions granularly for each data access also contribute.
Possible consequences: The consequences of a successful BOLA attack range from unauthorised access to sensitive personal data to the manipulation or even deletion of important information. This can lead to data breaches, financial loss and significant reputational damage for the affected organisation.
Example attack scenario: A user logs into an online shopping platform and loads their order history. The API request includes an order ID. Due to a BOLA vulnerability, the attacker could now attempt to modify the order ID in the request (e.g. by incrementing or systematically trying other IDs) to view other users' order histories without the API performing sufficient authorisation checks for that particular order ID.

Broken User Authentication

Another critical aspect of API security is poor user authentication. This is an easy target for attackers because it is accessible to everyone. Although some authentication issues require advanced technical knowledge to exploit, tools to do so are usually available.

Attack Description: In an attack that exploits weak user authentication, attackers attempt to spoof or bypass the identity of a legitimate user. This can be done by guessing weak passwords, exploiting token management vulnerabilities, or bypassing multi-factor authentication mechanisms.
Cause: The root causes of this vulnerability are varied. These include the use of weak password policies, errors in the management of authentication tokens (e.g. insecure generation, inadequate validation or failure to invalidate after logout) or the absence or poor implementation of multi-factor authentication procedures. Relying on simple API keys as the sole authentication method can also be a cause.
Possible consequences: Attackers can gain unauthorised access to sensitive data or functions, perform fraudulent transactions, or take control of user accounts. This can lead to data breaches, financial losses and a significant loss of user confidence.
Example attack scenario: An API uses simple, predictable patterns to generate session IDs upon login. An attacker could analyse these patterns and attempt to guess valid session IDs of other users in order to access their accounts without their actual credentials.

Broken Function Level Authorization

Function-level privilege escalation is a lack of control over access to specific functions or operations within the API.

Attack Description: In a function-level privilege escalation attack, an attacker attempts to call API endpoints or functions that are not intended for their role or privilege level. This can be done by manipulating API calls, exploiting unprotected internal endpoints, or bypassing access controls that are implemented on the client side but not sufficiently enforced on the server side.
Cause: The main causes of this vulnerability are missing or insufficient access controls at the level of individual API functions. Permissions are often not defined with sufficient granularity, or checking whether a user is authorised to call a particular function is poorly implemented or forgotten altogether. Complex and poorly managed access control policies that do not follow the principle of least privilege can also lead to this vulnerability.
Possible consequences: Attackers could perform administrative actions, change sensitive configurations, manipulate user rights or even gain complete control of the system. This could lead to data manipulation, business interruption, financial loss and serious security breaches.
Example attack scenario: An API provides various functions to both standard users and administrators. A standard user discovers an inadequately protected API endpoint that allows user roles to be changed - a function that should only be available to administrators. By calling this endpoint directly and manipulating the parameters, the attacker can elevate his own privileges to administrator level and gain access to all administrative functions of the API.

Improper Asset Management

Inadequate inventory management of APIs is an often underestimated security issue. In the dynamic world of software development, APIs are constantly being created and modified. If these API assets and their associated security controls are not carefully recorded and managed, significant security gaps can result. Imagine that an organisation has multiple APIs, some of which are outdated or no longer in active use, but remain online and potentially unsecured. Inadequate inventory of these assets prevents necessary security measures from being applied or obsolete interfaces from being decommissioned in a timely manner. This creates unnecessary attack surfaces and can unnecessarily put sensitive data at risk.

Attack Description: A poor asset management attack often does not directly target active, well-protected APIs. Instead, attackers look for forgotten, poorly documented, or outdated API endpoints that may be less closely monitored or not monitored at all. These 'ghost APIs' can inadvertently expose sensitive data or act as a gateway into the organisation's systems.
Cause: The main cause of poor inventory management is a lack of, or inadequate processes to capture and track all APIs in use. This can be due to the rapid growth of the API landscape, lack of documentation, lack of automation in the inventory process, or simply forgetting about older APIs after they have been replaced. Failure to keep track of the security controls associated with each API also contributes to this vulnerability.
Possible consequences: The consequences of a successful attack on a poorly managed API can be similar to other vulnerabilities: unauthorised access to sensitive data, data leakage or compromise of back-end systems. Because these APIs are often not the focus of current security efforts, attacks may go undetected for longer.
Example attack scenario: Some time ago, a company provided a public API for a specific feature. After this feature was replaced by a newer API, the old API was forgotten and not disabled. Since the associated security updates were also missing, an attacker discovers a known vulnerability in this old API that allows him to access internal databases, even though the company's current API is well secured. Poor inventory management has left a back door open.

Excessive Data Exposure

This vulnerability occurs when an API returns more data in its responses than the calling client actually needs. Even if this sensitive information is not displayed directly to the user, it can still be included in the response and thus made available to potential attackers who can intercept and analyse the traffic. Assuming that the client has already filtered out the data it needs is dangerous and goes against the principle of data minimisation. A well-designed API should provide only the information that is absolutely necessary to minimise the attack surface and reduce the risk of data leakage.

Attack description: In an attack that exploits overexposure, attackers analyse the full API responses to potentially discover additional sensitive data that is not necessary for the intended function of the API call. This information can then be used for further attacks or to compromise systems.
Cause: The root cause of this vulnerability is often poor data filtering on the server side. Developers may implement simple queries that return all available fields of a database record, rather than limiting the response to the required information. Missing or inadequate specification of required data formats by client applications can also contribute to APIs returning unnecessarily large amounts of data. Sometimes debugging or testing fields are inadvertently left in production environments, exposing them to the outside world.
Possible consequences: Even if the additional data disclosed is not directly considered highly sensitive, in combination with other information or after further analysis, it can provide valuable clues for attackers to exploit other vulnerabilities or to gain a more complete picture of systems and users. In the worst case scenario, sensitive personal data, financial information or trade secrets may be unintentionally exposed.
Example attack scenario: A mobile application queries user profiles via an API to display the display name and a profile picture. However, in addition to the display, the API response also contains the user's private email address, phone number and date of birth. An attacker intercepting the application's network traffic or directly accessing the API could gain access to this additional, unwanted, sensitive data and use it for phishing or other malicious purposes.

Lack of Resources & Rate Limiting

APIs without mechanisms to control resource usage (computing power, bandwidth or memory) or to limit the number of requests in a given time period are vulnerable to abuse and congestion. Without such safeguards, malicious actors or even unintentional overuse by legitimate clients can seriously compromise the stability and availability of the API and the underlying systems.

Attack description: An attack that exploits lack of resources and rate limiting aims to overload an API with an excessive number of requests, exhausting the server's resources and making the API inaccessible to legitimate users. This can take the form of Denial of Service (DoS) or Distributed Denial of Service (DDoS) attacks. Attackers may also attempt to extract sensitive data (data scraping) or perform brute force attacks on authentication mechanisms by making repeated requests in a short period of time.
Cause: The main causes of this vulnerability are missing or poorly configured rate limiting mechanisms and poor capacity planning of server resources. Often no limits are set for the number of requests per user, API key or IP address. The lack of throttling mechanisms that can dynamically adjust the response rate also contributes to the vulnerability. In addition, inadequate monitoring of API usage can result in unusual or abusive activity not being detected in a timely manner.
Possible consequences: The consequences of a successful attack can include significant service outages, performance degradation for legitimate users, and financial losses due to lost business or increased operating costs to handle the load. In addition, an overloaded system may become more vulnerable to other attacks. Loss of availability can also damage an organisation's reputation.
Example attack scenario: An attacker uses a botnet to send an enormous number of requests to the login endpoint of an API in a short period of time. As the API does not have a rate limit implemented, the server resources are quickly overloaded. As a result, legitimate users are unable to log in and the entire application that relies on that API stops working. At the same time, if not monitored, the repeated failed login attempts could go undetected and possibly even mask successful brute force attacks.

Mass Assignment

This vulnerability occurs when an API allows clients to send more data than is intended or necessary when updating a resource. This could allow an attacker to manipulate unauthorised fields or attributes of an object to which they should not have access.

Attack description: In an attack exploiting mass mapping, an attacker sends additional data fields in an API request to update a resource. The goal is to manipulate or set fields that are not explicitly intended to be updated by the client and may contain sensitive or security-critical information.
Cause: The primary cause of mass mapping vulnerabilities is insufficient or no server-side control of incoming data during update operations. Developers may rely on the client to send only the intended fields, or they may not implement strict filtering and validation of the parameters sent. This allows attackers to inject additional data fields in the hope that they will be processed and accepted by the server.
Possible consequences: Attackers could escalate user privileges, change sensitive data without permission (e.g. passwords, email addresses, account balances), bypass security policies, or even take control of a system by manipulating administrative fields, for example.
Example attack scenario: An API allows users to update their profile description. The request only expects the Description field. However, an attacker also sends the 'isAdmin:true' field in the request. If the API does not ignore these additional fields and does not explicitly check the allowed fields, the attacker could set the permissions to administrator without authorisation, gaining access to administrative functions and sensitive data.

Injection

Injection attacks are a particularly dangerous category of vulnerability in APIs. They occur when an API does not adequately validate or sanitise the data it receives from clients. This allows attackers to inject malicious code into API requests, which is then interpreted and executed by the server.

Attack description: In an injection attack, an attacker attempts to inject malicious code or commands via input fields in API requests. The goal is to disrupt the normal execution of the application and gain unauthorised access to data or system functions. The most common types include SQL injection, where malicious SQL code is injected into database queries, and command injection, where operating system commands are attempted to be executed via the API.
Cause: The main cause of injection vulnerabilities is insufficient or missing validation and sanitisation of input data on the server. If the API does not ensure that the data it receives is in the expected format and does not contain malicious characters or commands, the injected code can be processed undetected. This affects various parts of an API request, such as headers, cookies and the message body. User input is often integrated directly into database queries or system commands without sufficient checking or masking. The principle of zero trust is ignored by blindly trusting the incoming data. Front-end validation alone is not sufficient, as it can be easily bypassed.
Possible consequences: The consequences of successful injection attacks can be significant security breaches. These range from the theft or manipulation of sensitive data, to the compromise of the entire system or database, to the complete failure of the application. Attackers can, for example, take over user accounts, gain administrator rights or introduce malware to the server.
Example attack scenario: An API has an endpoint to search for products by name. The underlying database query is dynamically generated from user input. An attacker enters OR '1'='1' as the search term. If the input is not sufficiently validated, the resulting SQL query could return all records in the products table because the condition '1'='1' is always true. In more complex scenarios, the attacker could also modify or delete data in this way. Another example would be an API that provides a function for processing uploaded files. An attacker could inject a filename containing operating system commands that are then executed on the server when the file is processed (command injection).

Insufficient Authentication & Session Management

If an API does not use robust procedures to identify and prove a user's identity, or does not handle sessions securely, attackers can gain unauthorised access and perform sensitive actions on behalf of other users.

Attack description: A weak authentication and session management attack aims to gain unauthorised access to the API and its resources by exploiting weaknesses in the mechanisms for verifying identity and maintaining user status. This may include bypassing authentication checks, hijacking third-party user accounts, or exploiting active or inactive sessions. Brute force attacks on weak credentials or intercepting session tokens are common methods.
Cause: The root causes of this vulnerability are varied. These include the use of weak or poorly protected authentication mechanisms, such as simple API keys without further security measures. The lack of multi-factor authentication (MFA), inadequate password policies or errors in token management also contribute. Session management issues, such as failure to validate session tokens after logout or insecure storage of session data, are other gateways. Sometimes default settings are retained or inadequate protection against brute force attacks (e.g. account locking) is implemented.
Possible consequences: Attackers can gain full access to user accounts and sensitive data, perform unauthorised transactions, manipulate or delete data, and perform malicious actions on behalf of the compromised user. This can result in significant financial loss, data breaches and serious reputational damage to the organisation.
Example attack scenario: In the following scenario, a user logs into an application that uses an API in the background. The API issues a session token. However, after the user logs out, the session token is not invalidated on the server side. An attacker who has previously obtained this token can continue to access the API as if the user were still logged in.

Best practices for protecting APIs

Today, a comprehensive approach to security is required that goes beyond simply avoiding known vulnerabilities. The best practices presented here cover various aspects of API development and maintenance and aim to significantly reduce the risk of security breaches.

Basic Security Principles

A solid foundation for the security of your APIs is provided by some basic security principles that should be considered at all stages of the development and operations cycle. These principles are not isolated measures; they are interrelated and together form a comprehensive security strategy.

Organisational measures:

Establish a culture of security within your team and organisation. Make developers and other stakeholders aware of the importance of API security and train them regularly in secure development practices.
Define clear security policies and processes for developing, deploying and maintaining APIs. This includes, for example, policies on password assignment, handling API keys and responding to security incidents.
Maintain a comprehensive inventory of your APIs and associated resources. You can only protect what you know.
Ensure that API security responsibilities and accountabilities are clearly defined.
Consider compliance requirements such as GDPR, HIPAA or PCI DSS from the start of the development process.

Technical measures:

Consider security when planning and designing your APIs (security by design).
Never trust the data sent by clients (zero trust policy).
Apply the principle of least privilege at the system level, for example by giving API applications only the minimum privileges needed to access databases or other resources.
Use HTTPS (TLS/SSL) for all communication with the API.
Implement comprehensive logging and monitoring of API calls to detect suspicious activity and respond in the event of a security incident. Ensure that sensitive data is not exposed in the logs.
Implement secure error handling that does not reveal unnecessary details about the internal workings of the API. Return generic error messages to the client and log detailed errors internally.
Perform code reviews where security experts check the code for potential vulnerabilities.
Automate security testing in your CI/CD pipeline to ensure that security aspects are continuously checked.
Conduct regular security audits and penetration tests to identify and remediate vulnerabilities early. Use API security tools to automate and support this process.
Stay aware of the constantly evolving threat landscape and continuously adapt your security measures to new risks.

Implementing strong Authentication Mechanisms

Implementing strong authentication is a fundamental building block for the security of any API. It ensures that only legitimate users and applications have access to your valuable resources and functionality.

Organisational measures:

Define clear authentication policies, including acceptable authentication methods, minimum credential requirements (if applicable), and frequency of password changes.
Train your developers in the basics of secure authentication practices and the risks of weak authentication methods.
Conduct regular reviews of implemented authentication mechanisms to ensure they continue to meet current security standards.

Technical measures:

Where possible, use proven and standardised authentication protocols such as OpenID Connect.
Implement secure credential management and storage practices (e.g. secure password hashing with Salt).
Consider implementing multi-factor authentication (MFA) for critical API endpoints or user accounts with elevated privileges.
Use API keys only as a supplemental or basic form of authentication, not as the sole security measure for sensitive operations. API keys should be securely generated, transmitted and stored.
Protect against brute force attacks on credentials with measures such as limiting the number of failed login attempts and temporary account lockouts.
Rely on token-based authentication using JSON Web Tokens (JWT). JWTs are self-describing, lightweight, and enable stateless authentication. Ensure that strong cryptographic algorithms are used to sign tokens.
Ensure that session tokens are securely generated, transmitted (e.g. as HTTP-only cookies over HTTPS) and managed on the server side. Invalidate session tokens after the user logs out and set appropriate session expiration times.

Implementation notes:

Integrate authentication logic into security-relevant layers of your application, not directly into the API endpoints. For example, use middleware or filters to perform authentication before the request is actually processed.
Use established libraries and frameworks to implement authentication protocols, rather than developing your own, potentially flawed solutions.
Ensure that each API call is authenticated and that authentication information is verified for each request.
Avoid sending sensitive authentication information in the URL. Instead, use HTTP headers or the request body (for POST requests).
Implement secure logout functionality that terminates the user's current session and invalidates any associated tokens.
Perform extensive testing of the implemented authentication mechanisms to ensure that they work correctly and do not contain any vulnerabilities.
Log authentication attempts (both successful and failed) for monitoring and analysis purposes without revealing sensitive information.

Implementing fine-grained Authorisation

Fine-grained authorisation goes beyond simple all-or-nothing decisions and enables the definition and enforcement of very granular access rights. This is critical for effectively protecting sensitive data and functionality, and for enforcing the principle of least privilege.

Organisational measures:

Define clear roles and responsibilities within your organisation and assign specific permissions to those roles to access API resources and functionality.
Create a detailed permissions model that accurately describes the different levels of access and associated actions. This model should be regularly reviewed and adapted to changing requirements.
Implement processes for granting and revoking privileges to ensure that users are only granted the privileges they currently need, and that privileges that are no longer required are removed in a timely manner.
Conduct regular audits of access rights to ensure that they have been assigned correctly and according to defined policies.

Technical measures:

Implement mechanisms to enforce defined permissions on all relevant API endpoints and for all security-critical operations. This can be done through Role-Based Access Control (RBAC), where permissions are tied to roles, or Attribute-Based Access Control (ABAC), where access is decided based on attributes of the user, resource and context.
Use claims within JWTs or other security tokens to transport the user's permissions. These claims can then be checked on the server side to make access control decisions.
Ensure that the authorisation check occurs after authentication and before the requested action is performed.
Avoid implicit authorisation, where access is assumed solely on the basis of authentication or the existence of an object. Instead, perform explicit authorisation checks.
Consider function-level authorisation. Ensure that users can only access the functions they are authorised to use, especially for sensitive operations.
Implement security measures against Broken Object Level Authorisation (BOLA). Ensure that users can only access the data objects for which they are authorised, even if they know the object ID. Check the access permission for each object accessed by a user ID.
Consider Broken Object Property Level Authorization. Restrict access to individual attributes or properties of data objects to avoid excessive data exposure.

Implementation notes:

Integrate authorisation logic into your back-end services and don't just rely on client-side controls.
Use centralised authorisation policy enforcement mechanisms to ensure consistency and maintainability.
Thoroughly test authorisation mechanisms to ensure that they work correctly and that there are no unwanted access options. Write unit and integration tests for different authorisation scenarios.
Log authorisation decisions (especially denied access attempts) for monitoring and analysis.
Simplify complex access control policies to minimise the risk of misconfiguration and broken function level authorisation.
Consistently apply the principle of least privilege, granting users and applications only the privileges they need to do their jobs.
Be careful when implementing mass assignment and ensure that users can only update the fields for which they are authorised. Implement strict controls and validations for incoming data.

Data Encryption

Data encryption is an essential part of a comprehensive API security strategy. It ensures that sensitive information is rendered unreadable if it falls into the wrong hands. This is especially important when data is transmitted over insecure networks and stored on different systems. Effective encryption minimises the risk of data leakage and ensures that your data remains confidential even when other security measures fail.

Organisational measures:

Define clear policies for encrypting sensitive data, both in transit and at rest. Determine what data is considered sensitive and therefore must be encrypted.
Train your developers in encryption technologies and key management best practices. Make them aware of the risks of unencrypted data.
Conduct regular reviews of the encryption mechanisms and key management processes in place to ensure they remain effective and secure.
Consider compliance requirements such as GDPR, HIPAA or PCI DSS, which may have specific requirements for encrypting personal or financial data.

Technical measures:

Use HTTPS (TLS/SSL) for all communication with the API to encrypt data in transit and protect against man-in-the-middle attacks. Ensure that up-to-date and secure versions of TLS (at least TLS 1.2) are used.
Encrypt sensitive data at rest (e.g. in databases, file systems or log files) using strong cryptographic algorithms such as AES.
Use secure key generation, storage and management practices. Key management is critical because the security of the encrypted data depends directly on the security of the keys used. Avoid storing keys in code.
Consider using homomorphic encryption or other advanced encryption techniques for specific use cases where data needs to be processed without decryption.

Implementation notes:

Encrypt sensitive data as early as possible in the process and decrypt it as late as necessary.
Use established and well-tested encryption libraries and frameworks rather than developing your own encryption algorithms.
Ensure that all endpoints that send or receive sensitive data use HTTPS. Enforce HTTPS and redirect HTTP requests accordingly.
Regularly review the configuration of your TLS/SSL implementation to ensure that there are no known vulnerabilities (e.g. by using appropriate cipher suites).
Consider the performance impact of encryption and optimise your implementation accordingly.
Perform tests to ensure that encryption and decryption work correctly and that data is effectively protected.
Log the use of encryption mechanisms for auditing purposes without exposing sensitive keys.

Input Validation and Sanitisation

Input validation is the process of verifying that incoming data conforms to pre-defined rules and expectations. This includes checking the data type, format (e.g. email address, phone number), length and value range. Input sanitisation, on the other hand, aims to remove or mitigate potentially harmful or unwanted characters and patterns from the input data before it is further processed. Both processes are essential to prevent common types of attacks such as injection attacks (e.g. SQL injection, NoSQL injection), cross-site scripting (XSS) and data manipulation.

Organisational measures:

Define clear and detailed specifications for all expected input data from your API endpoints. Document the required formats, data types, length constraints, and permitted value ranges.
Train your developers on the principles of secure input processing and the common attack vectors that are enabled by inadequate validation and sanitising.
Implement policies and processes for reviewing and updating input validation rules to keep pace with changing requirements and potential new threats.
Conduct regular code reviews with a particular focus on the correct implementation of input validation and sanitising.

Technical measures:

Always perform server-side input validation. Don't rely on client-side validation alone, as it can be easily bypassed.
Use strict validation rules (allow-listing), where you explicitly define which inputs are allowed, rather than trying to block all possible invalid inputs (denying).
Validate all types of input, including parameters in the URL, headers, cookies and the body of the request. Also consider the structure of data formats such as JSON or XML.
Clean up input data by removing or rendering harmless potentially malicious characters or scripts (e.g. by encoding or escaping). Be careful with automatic cleanup, however, as it can cause undesired behaviour in some cases. In many cases it is safer to reject invalid entries.
Use the appropriate libraries and frameworks for validation and cleanup in your programming language and framework. These often provide pre-built functionality and help to avoid common errors.
Implement content type validation to ensure that requests match the expected data formats.

Rate Limiting and Throttling

Without rate limiting and throttling, APIs can become vulnerable to denial of service (DoS) attacks, where malicious actors overload the API with a flood of requests, rendering it unusable for legitimate users. In addition, rate limiting and throttling prevent data scraping, brute force attacks, and excessive resource usage that can lead to poor quality of service. At its core, it's about ensuring fair terms of use and maintaining the availability of the API for all users.

Rate limiting places a cap on the number of requests a client can make to the API in a given period of time. Throttling, on the other hand, can dynamically adjust the rate of incoming requests based on pre-defined thresholds or current system load.

Organisational measures:

Define clear rate limiting policies based on expected usage behavior, the capacity of your infrastructure, and the sensitivity of API endpoints. Consider different user groups and use cases.
Communicate rate limits transparently to your API users through clear and understandable documentation. Specify the allowed request rates, the consequences of exceeding the limits, and any retry mechanisms.
Regularly monitor and analyse API traffic patterns to assess the effectiveness of current rate limits and adjust them as necessary. Identify unusual behaviour or potential abuse.
Consider the ability to define exceptions for trusted or premium users if it makes business sense.

Technical measures:

Select appropriate algorithms to implement rate limiting, such as Fixed Window, Sliding Window, Token Bucket or Leaky Bucket, based on your specific requirements for flexibility and accuracy.
Implement rate limiting on the server side to ensure that controls cannot be bypassed by the client.
Use API gateways or middleware components that provide built-in rate limiting and throttling capabilities. These often allow centralised management and configuration of rules.
Use client identification mechanisms such as IP addresses, API keys or user accounts to track requests and enforce per-client throttling.
Ensure that your implementation is scalable, especially in distributed systems where request counts may need to be synchronised.

Implementation notes:

Handle rate limit violations caused by HTTP status codes (e.g. 429 Too Many Requests) and add notes on retry strategies or limit reset time in response headers (e.g. X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset).
Log rate limit events for monitoring and analysis to identify trends and optimise configuration.
Thoroughly test your rate limiting and throttling mechanisms to ensure they work as expected and don't unnecessarily restrict legitimate users.
Integrate API usage and rating limit effectiveness metrics into your monitoring systems to identify problems or potential abuse early.
Consider differentiated rate limits for different API endpoints or user groups based on their sensitivity and expected usage.

Error Handling and Logging

Error handling in your API should aim to provide the client with meaningful, but not too detailed, information about problems that have occurred, without revealing internal details that could help attackers. Logging, on the other hand, focuses on recording all relevant activity in the background to maintain a detailed history of API operations. This includes normal access as well as suspicious activity and error conditions.

Organisational measures:

Define clear error handling guidelines for your API. Determine which types of errors should be communicated to the client and how, and which should be logged internally.
Train your developers on how to handle error situations and the importance of meaningful logging for API maintenance and security.
Implement processes to periodically review log data to identify anomalies, potential security incidents, or recurring errors.
Ensure that there are clear responsibilities for responding to logged security events and errors.

Technical measures:

Use standardised HTTP status codes to clearly communicate the type of error that has occurred (e.g. 400 for invalid request, 401 for unauthenticated, 500 for server error).
Avoid revealing sensitive information in error messages, such as internal server paths, database details, or detailed stack traces. Instead, return generic error messages that help the user understand the problem without compromising security.
Implement a centralised logging solution to store all relevant events in a secure location and facilitate analysis.
Choose an appropriate logging format (e.g. JSON) that is both human-readable and suitable for automated analysis.
Carefully configure the logging level to ensure a balance between detail and performance. Log enough information for troubleshooting and security analysis without draining resources unnecessarily.
Implement monitoring tools that can provide real-time information about unusual API activity or error conditions (e.g. Grafana).

Implementation notes:

Log security-related events such as failed login attempts, authorisation errors, and suspicious API calls.
Integrate contextual information into your logs, such as timestamp, client IP address, user ID, and the API route involved, to make it easier to trace events.
Protect your log data from unauthorised access and tampering with appropriate access controls and encryption.
Implement log rotation and archiving mechanisms to manage storage capacity and ensure retention periods are met.
Ensure that your logging meets the requirements of relevant compliance regulations, such as HIPAA traceability of access to patient information.
Thoroughly test your error handling and logging mechanisms to ensure that errors are handled correctly and that all relevant events are logged.

Regular Security Reviews and Improvements

API security is not a one-off project, but an ongoing process that requires regular review and continuous improvement.

Vulnerability assessments and penetration tests are important components of regular security reviews. Vulnerability assessments, often performed using automated tools, scan your APIs for known vulnerabilities and configuration errors. Penetration testing goes a step further, simulating realistic attacks to exploit vulnerabilities and test the robustness of your security measures.

Another important aspect is continuous monitoring of API activity. By analysing access logs and looking for unusual patterns, suspicious activity can be detected early and appropriate countermeasures can be taken. In addition, it is imperative that APIs and their dependencies are regularly updated and patched to address known security vulnerabilities.

Organisational measures:

Establish a fixed schedule for regular security audits, including vulnerability assessments, penetration tests and code reviews.
Define clear responsibilities for performing and tracking security audits and implementing remediation actions.
Integrate security considerations throughout the software development lifecycle (SDLC) to build security in from the start.
Promote a culture of security within the development team through training on secure coding practices and the most common API vulnerabilities.
Stay abreast of new threats and security recommendations by following resources such as the OWASP API Security Top 10 project.
Implement security incident response policies, including the creation of dedicated incident response teams.

Technical measures:

Use automated vulnerability scanning tools to quickly identify known vulnerabilities in your API and its dependencies.
Perform regular manual penetration tests to uncover complex vulnerabilities and logic flaws that may not be detected by automated tools.
Implement static and dynamic code analysis tools to identify potential security issues in the source code at an early stage.
Establish a comprehensive monitoring system to monitor API traffic, errors and suspicious activity in real time.
Automate patch management to ensure that APIs and their dependencies are updated with the latest security updates in a timely manner.
Use API gateways to monitor API traffic and centrally manage security policies.

Implementation notes:

Prioritise remediation of found vulnerabilities based on their severity and potential risk to your applications and data.
Document all security checks performed, vulnerabilities found, and actions taken to remediate them.
Thoroughly test any security enhancements you implement to ensure they have the intended effect and do not introduce new problems.
Regularly review and update your security measures to keep pace with the evolving threat landscape.
Incorporate lessons learned from security incidents into your regular security audits and improvements to learn from mistakes.
Incorporate threat intelligence findings into your security strategy to proactively address potential future threats.

API Security Tools

In the following categories, some well-known tools are mentioned as examples. We are in no way affiliated with the manufacturers and suppliers of these products. This is neither an advertisement nor a direct recommendation, but merely an indication of the direction in which research can be done.

Threat Prevention and Detection Tools

These tools continuously monitor and analyse traffic.
They automatically detect known threats in API traffic.
They use behavioural and traffic analysis to detect deviations from normal API usage and identify anomalies.
AI and machine learning can increase detection accuracy.
This enables real-time response to attacks.
Examples of tools in this category are:

Wallarm API Security Platform provides automated threat detection.
Salt Security uses behaviour-based API protection to detect anomalies.
Reblaze provides protection against multiple attack methods.

Security Policy Enforcement Tools

These tools manage user and application authentication.
They control authorisation and restrict access to resources (e.g. using the principle of least privilege).
They use encryption protocols such as TLS/SSL.
They implement rate limiting and throttling.
They validate incoming data to prevent injection attacks.
They configure and manage CORS policies.
They support compliance with regulatory compliance.
API gateways centralise security policy management.
Examples of tools in this category are:

Reblaze can enforce security policies.
API gateways such as Apigee, Kong Gateway or AWS API Gateway manage and enforce security policies.
Wallarm API Security Platform and Salt Security can help enforce security policies by detecting and mitigating threats.

Security Assessment and Testing Tools

These tools automatically scan APIs for security vulnerabilities.
They validate input and output.
They check for excessive data exposure.
They identify vulnerabilities such as Broken Object Level Authorization, Broken User Authentication or configuration errors.
They simulate attacks as part of penetration testing.
They test the rate limit.
They enable regular safety assessments.
Integration with CI/CD pipelines is possible.
They provide reports and analysis of test results.
They assess compliance with security standards.
Examples of tools in this category are:

OWASP ZAP is used to scan APIs for vulnerabilities.
StackHawk is suitable for integrating security testing into the development process.
Beagle Security provides an easy-to-use interface for API security testing.
Postman Security Scanner can be used to test APIs and their security mechanisms.

Further information and links

If you are interested, you can delve deeper into the respective topic here.

Note: The following links lead to external sites over whose content we do not control. The linked pages were checked for possible legal violations at the time of linking. Illegal contents were not recognizable at the time of linking. If we become aware of any violations of the law, we will remove such links immediately.

Information

OWASP API Security Top 10
https://owasp.org/API-Security/editions/2023/en/0x00-header/
IBM: "Cost of a Data Breach Report 2024"
https://www.ibm.com/reports/data-breach

Let us conclude

Thank you for reading this article. I hope I was able to give you some new food for thought and whet your appetite for more. Stay curious, educate yourself, and help shape the future! If we can help you, please feel free to contact us!

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