Unlocking API Security: 10 Common Vulnerabilities Every Tester Must Know

Common API Interface Vulnerabilities

Understanding typical API flaws gives testers more ideas for probing interfaces.

Information Disclosure

Sensitive data may appear in API responses or public sources such as code repositories, search results, news, social media, target sites, and public API directories. For example, a WordPress site may expose user information via GET https://www.sitename.org/wp-json/wp/v2/users, returning user IDs and slugs that can be used for credential‑stuffing attacks.

Broken Object Level Authorization (BOLA)

When an API allows access to resources a user is not authorized for, a BOLA vulnerability exists. By guessing nearby object IDs (e.g., {"id":"5501","first_name":"Cloud",...}) an attacker can retrieve data from other users. Detect BOLA by mapping the API’s resource structure and attempting unauthorized accesses.

Broken User Authentication

Weak or missing authentication mechanisms let attackers bypass login controls. Tokens that lack sufficient entropy can be guessed or forged, and insecure password‑reset flows enable brute‑force attacks on short verification codes.

Excessive Data Exposure

Endpoints that return more information than required can leak personal details, such as name, birthdate, email, and phone numbers. Testing involves sending normal requests and inspecting the response payload for unnecessary fields.

Lack of Resource and Rate Limiting

Without proper rate limits, an API can be overwhelmed, leading to denial‑of‑service conditions. Testers should try to bypass limits by altering parameters, switching clients, or changing IP addresses.

Broken Function Level Authorization (BFLA)

BFLA occurs when a user can invoke API functions reserved for other roles or groups. Unlike BOLA, it concerns operation permissions rather than resource access. Detect it by examining API documentation and sending privileged‑operation requests as a non‑privileged user.

Mass Assignment

If an API accepts unexpected parameters and assigns them directly to internal objects, attackers can modify fields like account privileges or balances. Example payloads demonstrate adding an isAdmin flag to gain administrator rights.

Security Misconfiguration

Misconfigured headers, missing TLS, default accounts, and unnecessary HTTP methods expose sensitive data. Sample responses show how headers such as X-Powered-By or timing differences can reveal backend technologies and resource existence.

Improper Asset Management

Exposing deprecated or in‑development APIs leads to additional vulnerabilities such as data over‑exposure, BOLA, BFLA, and rate‑limit bypasses. Detect by reviewing versioned endpoint patterns and change logs.

Business Logic Vulnerabilities

These arise from flaws in the intended application workflow, such as allowing arbitrary file uploads, insecure password‑reset flows, or bypassing multi‑factor authentication by altering request parameters. Detect by reading API specifications for statements like “only admins can perform X” and testing violations.

Summary

Familiarity with these API weaknesses enables quick identification, exploitation during penetration testing, and proper reporting to prevent attackers from compromising client systems.