As cloud and AI integrations have become a part of today’s digital environment, application programming interface (API) penetration testing has become critical for effective cybersecurity. API calls made up 71% of all web traffic at the start of 2024, setting the stage for a 32% increase in API security incidents that year.
API pentesting requires a different methodology than its web app counterpart, focusing on specific vulnerabilities exposed by backend communication between clients and endpoints. Here we’ll take a deep dive into what API pentesting is, what it’s for, how it works, and why you need it.
Defining API Penetration Testing
API penetration testing is an offensive security assessment that simulates attacks on backend communication to identify vulnerabilities in server services and data exchange. Unlike traditional pentesting that targets human-facing interfaces, this practice focuses on machine-to-machine logic and the integrity of software interactions.
Security professionals probe for flaws in areas such as authentication, access control, input validation, and business logic. The process follows a structured workflow:
- Reconnaissance: Mapping endpoints, session management, and authentication flows.
- Analysis & Simulation: Executing targeted attacks to expose potential exploits.
- Reporting: Providing prioritized remediation advice based on vulnerability severity.
Pentesters utilize a specialized toolkit—including interception proxies, automated scanners, and protocol inspectors—to manipulate traffic and verify that fixes are effective. This proactive approach ensures that the underlying architecture remains secure against sophisticated breaches.
Why API Testing Is Different from Application Pentesting
API pentesting differs significantly from testing processes used to probe vulnerabilities in web apps. Web pentests focus on browser-based vulnerabilities that manifest through the user interface. In contrast, because the headless architecture of APIs separates frontend interfaces from backend processes, API pentesting focuses on backend vulnerabilities.
This requires API pentesting to probe endpoints that connect multiple software services and the structured data formats used to exchange information between services, such as JSON and XML. For example, a common vulnerability involves attackers misusing API endpoints to access unauthorized objects, an API-specific counterpart to a vulnerability that occurs in a web application context as broken access control.
The OWASP API Security Top 10
The Open Web Application Security Project (OWASP) maintains an API Security Top 10 list that tracks the most common vulnerabilities and recommends mitigations. The current list includes:
- Broken Object Level Authorization (BOLA)
- Broken Authentication
- Broken Object Property Level Authorization (BOPLA)
- Unrestricted Resource Consumption
- Broken Function Level Authorization
- Unrestricted Access to Sensitive Business Flows
- Server Side Request Forgery (SSRF)
- Security Misconfiguration
- Improper Inventory Management
- Unsafe Consumption of APIs
Here’s a brief breakdown of what these vulnerabilities are, what causes them, and how to fix them:
1. Broken Object Level Authorization (BOLA)
A BOLA vulnerability occurs when weak authorization controls allow an attacker to manipulate an object ID to access unauthorized objects. For example, browser requests may display data revealing endpoint resource path structures, which can be manipulated by replacing identifiers to access user data.
BOLA vulnerabilities typically reflect failure to validate that user-supplied input comes from authorized users. They can be mitigated by:
- Using authorization mechanisms that rely on user policies and hierarchy
- Verifying that users have access to perform requested actions whenever a function uses client input to access database records
- Using random and unpredictable values as GUIDs for record IDs
- Testing vulnerability of authorization mechanisms and refraining from changes that make tests fail
2. Broken Authentication
This risk happens when authentication mechanisms allow hackers to compromise authentication tokens or impersonate other users. For example, if a six-digit SMS password recovery code lacks rate limiting, a hacker can brute-force the code.
This vulnerability can stem from various authentication weaknesses, including failure to limit rate limits, lack of multi-factor authentication, weak password policies, token and session mismanagement, unencrypted credential transmission, and insecure password reset workflows. It can be mitigated by steps such as:
- Testing all possible authentication flows
- Using authentication standards
- Treating credential recovery and forgotten password endpoints as login endpoints in terms of brute force, rate limiting, and lockout protections
- Requiring re-authentication for sensitive operations like changing contact information
- Requiring multi-factor authentication
- Implementing strong mechanisms to counter brute force attacks, credential stuffing, and dictionary attacks
- Using account lockout/captcha mechanisms to prevent brute force attacks against specific users
- Checking for weak passwords
- Avoiding the use of API keys for user authentication, restricting them to API clients' authentication
3. Broken Object Property Level Authorization (BOPLA)
BOPLA vulnerabilities occur when API authorization mechanisms allow unauthorized users to view or alter fields within objects (making it more granular than BOLA, which deals with access to objects themselves). For example, an attacker might be able to add a hidden field to an API request, allowing them to modify data.
This vulnerability can be caused by overreliance on client data, direct backend model mapping, and failure to validate at the field level. Mitigations include:
- Validating that users should have access to properties of objects exposed by API endpoints
- Avoiding generic methods such as to_json() and to_string(), instead selecting specific object properties to return
- Refraining from mass assignment functions that automatically bind a client's input into code variables, internal objects, or object properties
- Allowing changes only to properties that clients should be able to update
- Deploying a schema-based response validation mechanism, which includes defining and enforcing the data returned by all API methods
- Minimizing returned data structures by limiting them to business and functional requirements for endpoints
4. Unrestricted Resource Consumption
This vulnerability occurs when APIs fail to limit client use of system or financial resources. For instance, if a hacker alters a password-recovery call to a third-party service that charges per recovery message, rewriting the script to send thousands of recovery messages at once, the client company can incur excessive charges.
Unrestricted resource consumption vulnerabilities typically result from failing to enforce boundaries such as rate limiting, throttling, and file size restrictions. They can be mitigated by:
- Use a solution such as AWS Lambda that enables limiting of memory, CPU, restarts, file descriptors, and processes
- Impose a maximum size of data on all incoming parameters and payloads, such as string lengths, array elements, and upload file size
- Implement a limit on how often a client can interact with the API within a defined timeframe (rate limiting).
- Customize rate limiting to business needs
- Limit or throttle how many times or how often a single API client or user can execute an operation
- Use server-side validation for query string and request body parameters to control how many records are returned in responses
- Set spending limits or set up billing alerts for all service providers and API integrations
5. Broken Function Level Authorization
This risk emerges when authorization checks fail to prevent users from performing unauthorized administrative actions. For example, a hacker might rewrite an app invitation request to send themselves an invitation to create an account with administrative privileges.
Broken function-level authorization typically reflects a failure to check user privileges, misconfiguration of role permissions, reliance on predictable endpoints, the assumption that hidden endpoints are secure, or mishandling of HTTP methods. This vulnerability can be mitigated by:
- Using a consistent authorization module invoked from all business functions
- Denying access by default and requiring explicit grants to specific roles for access to all functions
- Assessing API endpoints against function-level authorization flaws in light of business logic and group hierarchy
- Verify that all administrative controllers inherit from an administrative abstract controller that runs authorization checks based on groups and roles
- Ensure all administrative functions inside a regular controller apply authorization checks based on groups and roles
6. Unrestricted Access to Sensitive Business Flows
This risk emerges when APIs expose a business flow that can cause damage when used excessively. For example, a user might exploit a referral program cash reward offer by writing a script to automate the creation of a large number of referral registrations.
This category of vulnerability can stem from causes such as a lack of rate and throttling limits, lax authentication, or failure to validate business logic constraints. Mitigations include:
- Identifying business flows that can cause harm when used excessively
- Denying service to unexpected client devices, such as headless browsers
- Using CAPTCHA or advanced biometric solutions to detect human patterns
- Analyze user flow to detect non-human patterns
- Block IP addresses of Tor exit nodes and well-known proxies
- Secure and restrict access to APIs consumed directly by machines, such as APIs for developers and B2B applications
7. Server Side Request Forgery (SSRF)
SSRF vulnerabilities occur when attackers can abuse API endpoints to issue unexpected requests. For example, an API endpoint that allows users to upload profile images via a URL can enable attackers to submit malicious URLs and trigger port scans.
This risk may reflect lax validation and sanitization procedures, acceptance of user-defined URLs, uncritical following of redirects, mishandling of partial URLs, or enabling of access to cloud metadata. Mitigations include:
- Isolating network resource fetching mechanisms in your network to verify that they retrieve remote resources and not internal ones
- Use allow lists of remote resource origins, URL schemes, and ports, and accepted media types for specific functions
- Disable HTTP redirections.
- Use an up-to-date URL parser
- Validate and sanitize all client-supplied inputs
- Avoid sending clients raw responses
8. Security Misconfiguration
This risk arises when attackers exploit configuration flaws to gain unauthorized access to or knowledge of systems. For instance, failing to change default credentials can allow unauthorized access.
Security misconfigurations may follow from leaving insecure defaults intact, failing to update configurations after system updates, using poor encryption, exposing insecure error messages or logs, or using outdated software. Mitigations include:
- Designing an API life cycle with a secure deployment process, a configuration review and update process across the entire API stack, and an automated continuous configuration assessment process
- Verifying that all API communications from clients to API servers and any downstream or upstream components use encrypted communication
- Specifying which HTTP verbs APIs can be accessed by and disallowing others
- Checking that APIs accessible from browser-based clients use Cross-Origin Resource Sharing (CORS) policy and Security Headers
- Limiting incoming content types and data formats to necessary business and functional requirements
- Verifying that all servers in the HTTP server chain follow uniform rules when processing incoming requests
- Defining and enforcing API response payload schemas to keep sensitive information from being transmitted to attackers
9. Improper Inventory Management
This risk occurs when organizations lose track of APIs that contain vulnerabilities. For example, discrepancies between production APIs and beta versions may allow hackers to use beta-version vulnerabilities to discover sensitive information such as passwords.
Improper inventory management may be caused by overly rapid versioning or migration, unsecured developer environments, outdated documentation, or inefficient governance. Mitigations include:
- Inventory all API hosts and document who should have access to hosts and versions
- Inventory integrated services and document their roles, data flow, and data sensitivity
- Documenting API features such as authentication, errors, redirects, rate limiting, cross-origin resource sharing (CORS) policies, and endpoints
- Automating documentation builds in CI/CD pipelines
- Only providing API documentation to authorized personnel
- Deploying external protection measures such as API security-specific solutions for all exposed API versions, not just current productions
- Avoiding the use of production data in non-production environments
- Securing any non-production API deployments
- Running risk analysis to identify mitigation actions for older versions when updates are deployed
10. Unsafe Consumption of APIs
Unsafe API consumption occurs when applications process third-party data without adequate security checks. By trusting unvetted sources or failing to sanitize inputs, apps remain vulnerable to exploits like malicious SQL queries from AI services. These risks typically stem from using insecure protocols, mishandling unexpected responses, or failing to validate redirects.
Mitigations include:
- Vetting service providers for their security postures
- Restricting API interactions to secured channels
- Validating and sanitizing data received from APIs
- Using an allowlist to manage redirects
Anatomy of an API Attack Surface
Effective API pentesting must cover all critical target areas of the API attack surface. Some of the most important elements can include:
- REST (Representational State Transfer) API architecture, including endpoints, HTTP method verbs, headers, parameters, and response codes
- SOAP (Simple Object Access Protocol) data packet architecture, including envelopes, headers, bodies, and faults
- GraphQL architectures, including schema, resolvers, queries, mutations, and subscriptions
- JSON Web Tokens (JWTs), API keys, or Rate-limiting headers
The API reconnaissance and discovery processes use automated tools to help identify other important elements and provide a comprehensive map of attack surfaces.
The API Pentesting Lifecycle
The API pentesting process can be simplified into four stages:
- Discovery: Pentesting begins by finding documented and undocumented (shadow) APIs, identifying authentication and session workflows, charting business logic, classifying data sensitivity, validating endpoint accessibility, and assessing filtering and throttling controls.
- Analysis: After identifying the attack surface, pentesters analyze endpoints, authentication, authorization, HTML methods, data flows, and other components.
- Fuzzing: To deepen analysis, pentesters send unexpected data to test input validation by seeing how the API handles errors.
- Exploitation: Based on the results of analysis and fuzzing, pentesters probe specific vulnerabilities and attempt to bypass authorization or manipulate data.
The pentesting process culminates in the production of a report listing vulnerabilities, ranking their severity, and recommending remediations. After fixes have been implemented, retesting may be done to ensure their resiliency. Periodic pentesting and repeat testing yield ongoing, continuous hardening of API defenses.
Common Vulnerabilities: Three Frequent Fails
What kind of vulnerabilities do API pentests uncover? Some of the most frequent fails are basic mistakes that can easily be prevented. Here are a few common errors to avoid:
- Exposing data in URI parameters: BOLA vulnerabilities and other OWASP categories often involve exposure of path parameters due to lax authentication and validation procedures
- Unmanaged resource limiting and throttling: Unrestricted resource consumption and unrestricted access to sensitive business flows can stem from a failure to impose limits on the amount and frequency of resource usage, setting the stage for attacks such as denial of service.
- Improper input validation: BOLA, BOPLA, unrestricted resource consumption, SSRF, and unsafe API consumption vulnerabilities can all result from failure to use input validation and sanitization constraints.
Running API pentests can help you avoid these unforced errors and other common mistakes.
Tools of the Trade
API pentesters use a variety of specialized tools. Some of the most important include:
- Postman: A collaborative API development platform that has proven useful to pentesters as a browser and client application layer that can provide a clean version of API calls for tampering and fuzzing through an interception proxy.
- Burp Suite: An integrated pentesting suite that can serve as a proxy server to intercept, analyze, and alter real-time traffic between clients and API endpoints.
- Zed Attack Proxy (ZAP, formerly OWASP ZAP): A free, open-source Dynamic Application Security Testing (DAST) tool that can serve as a proxy server between clients and API endpoints.
- Ffuf (Fuzz Faster U Fool): An open-source fuzzing tool that uses wordlists to send automated HTTP requests and expose hidden directories, files, parameters, and virtual hosts.
- SSLscan: An open-source command-line tool for analyzing protocol vulnerabilities.
Pentesters may use many other specialized tools depending on the specific application and API environment being probed.
The Value of API Security for Modern Business
API pentesting plays a vital role for modern businesses. Today’s business environment is characterized by the prevalence of mobile-first and cloud environments, the growing importance of AI, automated workflows, and reliance on third-party software supply chains. While these trends offer benefits, they also increase the risk of data breaches, ransomware attacks, and other vulnerabilities through integration with cloud and AI apps and third-party dependencies.
Pentesting reduces this risk by defending businesses against the most common causes of API security incidents.
Learn More about Penetration Testing from the Cobalt Education Center
API pentesting is just one facet of today’s offensive security landscape. Pentesting specializations today include tests for internal and external networks, web applications, cloud environments, and AI and LLM tools. Learn more about the latest security trends by visiting the Cobalt Offensive Security Learning Center.