OWASP API Security Top 10 Overview and Best Practices

Robust API security measures are necessary to protect data from unauthorized access, manipulation, or exposure to ensure privacy and maintain the trust of users and stakeholders, as well as ensure the confidentiality, integrity, and availability of APIs. Best practices for API security include the following:

• Implement strong authentication and authorization. Enforce proper authorization checks to ensure that authenticated clients have the necessary permissions to access specific resources or perform certain actions. Use granular access controls to limit access to sensitive API endpoints or data, as well as relevant objects and functions.
• Validate input and output encoding. Validate and sanitize all input received from API clients to prevent injection attacks and encode output appropriately to prevent the execution of malicious scripts.
• Use secure communication. Employ secure protocols for transmitting data between API clients and servers and encrypt sensitive information in transit and at rest to ensure the confidentiality and integrity of data. 
• Implement rate limiting and throttling. Enforce limits on the number of requests that API clients can make within a specified time frame to prevent excessive usage or unauthorized access attempts, such as Distributed Denial of Service (DDoS) and brute force attacks.
• Conduct regular security testing and auditing. Perform regular security assessments, penetration testing, and code reviews to identify and address potential vulnerabilities in your APIs, and conduct security audits to detect weaknesses and ensure compliance with industry standards and mandates. This is especially important due to the interdependencies of APIs and underlying frameworks and libraries.
• Enforce schema and protocol compliance. Automatically creating and enforcing a positive security model with OpenAPI specifications is a valuable tool to ensure a consistent security policy.
• Dynamically discover and continuously assess APIs. API proliferation has resulted in unaccounted for or unmaintained APIs, including shadow APIs. Security controls need to constantly inventory and protect APIs using zero trust and least privilege access paradigms to mitigate unforeseen risk of third-party interdependencies. 
• Comprehensive threat detection. APIs are subject to, and need to be protected from, a variety of threats including exploits, misconfiguration, bots, fraud, and abuse.

The OWASP API Security Top 10 – 2023 was formulated to increase awareness of common API security weaknesses and to help developers, designers, architects, managers, and others involved in API development and maintenance maintain a proactive approach to API security.

The OWASP API Security Top 10 risks for 2023 are:  

1. Broken object level authorization. This security vulnerability occurs when an application fails to properly enforce access controls at the object or data level, allowing an attacker to manipulate or bypass authorization checks and grant unauthorized access to specific objects or data within an application. This can happen due to improper implementation of authorization checks, lack of proper validation, or bypassing of access controls. Every API endpoint that receives an ID of an object, and performs any action on the object, should implement object-level authorization checks to validate that the logged-in user has permissions to perform the requested action on the requested object.
2. Broken authentication. Authentication mechanisms in an API are often implemented incorrectly, allowing attackers to gain unauthorized access to user accounts or sensitive data, or perform unauthorized actions. It typically arises due to improper implementation or configuration of authentication processes, weak password policies, session management flaws, or other weaknesses in the authentication workflow. 
3. Broken object property level authorization. This threat occurs when an API fails to properly enforce access controls and authorization checks at the object property level. An API endpoint is vulnerable to these attacks if it exposes properties of an object that are considered sensitive and should not be read by the user, an exploit sometimes referred to as excessive data exposure. An API endpoint is also vulnerable to these attacks if it allows a user to change, add, or delete the value of a sensitive object's property, an exploit sometimes called mass assignment.
4. Unrestricted resource consumption. This attack, also called resource exhaustion, involves exploiting weaknesses in the API implementation to intentionally consume an excessive amount of resources, such as CPU, memory, bandwidth, or other system resources. This denial of service (DoS) degrades the performance or availability of the API or the underlying system and can lead to downtime.  
5. Broken function level authorization. This threat occurs when an API fails to properly enforce authorization checks at the function or operation level, allowing attackers to access unauthorized functionality. Implementing proper authorization checks can be confusing since modern applications can define many types of functional roles and groups and involve complex user hierarchies, which attackers can manipulate. 
6. Unrestricted access to sensitive business flows. This attack occurs when an API lacks proper access controls or authorization checks, allowing attackers to automate access to sensitive business flows backed by the API. These business flows might support mass purchasing of high-value, low-inventory products such as tickets or sneakers, which can be resold at a mark-up on secondary markets. Attackers often retool their attacks using sophisticated automation toolkits and may pivot to target business logic behind APIs if the target’s web apps are adequately protected by anti-automation defenses.  
7. Server-side request forgery (SSRF). This vulnerability results when an attacker identifies a vulnerable API endpoint that accepts user-supplied URLs or performs server-side requests to external resources. The attacker crafts malicious requests that specify URLs of internal resources or systems that the attacker wishes to target. Unaware of the malicious intent, the server performs the server-side request to the specified URL, potentially exposing sensitive information or services. 
8. Security misconfiguration. Attackers attempt to find unpatched flaws, common endpoints, services running with insecure default configurations, or unprotected files and directories to gain unauthorized access to the API. This vulnerability can result when appropriate security hardening is missing across any level of the API stack, from the network to the application level, or if there are improperly configured permissions on cloud services. Misconfiguration impacts web apps and APIs, and is an increasing risk as architecture continues to decentralize and become distributed across multi-cloud environments.
9. Improper inventory management. APIs are subject to changes and updates over time, but outdated or insecure API versions may remain in production, or older endpoints may be left running unpatched or using weaker security requirements, increasing the risk of security breaches. A lack of proper inventory management makes it difficult to track which versions are in use, which ones are outdated or deprecated, and which vulnerabilities have been addressed. Shadow and Zombie APIs pose significant risks, underscoring the importance of continuous discovery and automated protections.  
10. Unsafe consumption of APIs. Developers tend to trust data received from third-party APIs, particularly from APIs offered by well-known companies, and adopt weaker security requirements for this data in terms of input validation and sanitization or transport security. Unsafe consumption can also occur when APIs are accessed over insecure protocols or when proper encryption mechanisms are not used, leading to eavesdropping, data interception, and unauthorized access to sensitive information.