API Penetration Testing

APIs tend to expose endpoints that handle object identifiers, creating a wide attack surface of Object Level Access Control issues. Object level authorization checks should be considered in every function that accesses a data source using an ID from the user.

• Test: Can User A access User B's resource ID? (e.g., GET /invoices/1001 → GET /invoices/1002).

Complex access control policies with different hierarchies, groups, and roles, and an unclear separation between administrative and regular functions, tend to lead to authorization flaws. By exploiting these issues, attackers can gain access to other users’ resources and/or administrative functions.

• Test: Can a regular user access administrative functions? (e.g., POST /api/users/delete or changing method from GET to DELETE).

Authentication mechanisms are often implemented incorrectly, allowing attackers to compromise authentication tokens or to exploit implementation flaws to assume other user's identities temporarily or permanently. Compromising a system’s ability to identify the client/user, compromises API security overall.

• Tokens: Check for weak JWT signing secrets, "None" algorithm, or lack of expiration.
• Mechanisms: Test for weak password recovery, lack of brute-force protection on login, or credential stuffing susceptibilit

This category combines API3:2019 Excessive Data Exposure and API6:2019 - Mass Assignment, focusing on the root cause: the lack of or improper authorization validation at the object property level. This leads to information exposure or manipulation by unauthorized parties.

• Concept: This merges the 2019 "Mass Assignment" and "Excessive Data Exposure" categories. It focuses on the user's ability to access or modify specific properties of an object they shouldn't.
• Excessive Exposure Test: Inspect JSON responses. Does GET /user/profile return sensitive backend fields like is_admin, password_hash, or pii_ssn that the UI hides?
• Mass Assignment Test: Try to inject unauthorized fields in a POST or PUT request.
• Payload: {"username": "test", "role": "admin", "balance": 999999}.

• Satisfying API requests requires resources such as network bandwidth, CPU, memory, and storage. Other resources such as emails/SMS/phone calls or biometrics validation are made available by service providers via API integrations, and paid for per request. Successful attacks can lead to Denial of Service or an increase of operational costs.
• Test: Send huge payloads, deeply nested GraphQL queries, or thousands of requests (DoS) to exhaust CPU, memory, or storage.
• Check: Are there timeouts? Are there strict rate limits?

• APIs vulnerable to this risk expose a business flow - such as buying a ticket, or posting a comment - without compensating for how the functionality could harm the business if used excessively in an automated manner. This doesn't necessarily come from implementation bugs.
• Concept: Automation abusing legitimate logic.
• Test: Script a bot to buy all tickets (Scalping) or create 1000 spam accounts. The API technically "works," but the business flow is damaged.

• Server-Side Request Forgery (SSRF) flaws can occur when an API is fetching a remote resource without validating the user-supplied URI. This enables an attacker to coerce the application to send a crafted request to an unexpected destination, even when protected by a firewall or a VPN.
• Test: If an API endpoint accepts a URL (e.g., "Upload via URL" or "Webhook"), try to force the server to connect to internal resources (e.g., http://169.254.169.254/latest/meta-data on cloud, or http://localhost:8080).

• APIs and the systems supporting them typically contain complex configurations, meant to make the APIs more customizable. Software and DevOps engineers can miss these configurations, or don't follow security best practices when it comes to configuration, opening the door for different types of attacks.
• Test: Check for verbose error messages (stack traces), CORS misconfigurations (wildcard * origins), or unpatched servers.

• APIs tend to expose more endpoints than traditional web applications, making proper and updated documentation highly important. A proper inventory of hosts and deployed API versions also are important to mitigate issues such as deprecated API versions and exposed debug endpoints.
• Test: Look for Zombie APIs (deprecated versions like /v1 still active) or Shadow APIs (undocumented endpoints).

• Developers tend to trust data received from third-party APIs more than user input, and so tend to adopt weaker security standards. In order to compromise APIs, attackers go after integrated third-party services instead of trying to compromise the target API directly.
• Concept: The target API trusts data from another 3rd party API blindly.
• Test: If the target integrates with a payment provider or external service, can you spoof the response from that service to trick the API?