Top API Vulnerabilities Every Tester Should Know

Understanding common API vulnerabilities helps you think of more testing ideas.

Information Disclosure

Sensitive data may be exposed in API responses or public sources such as code repositories, search results, news, social media, target sites, and public API directories. Attackers can use disclosed user information, for example, a WordPress site that returns user details at GET https://www.sitename.org/wp-json/wp/v2/users:

GET https://www.sitename.org/wp-json/wp/v2/users

[{"id":1,"name":"Administrator","slug":"admin"},
{"id":2,"name":"Vincent Valentine","slug":"Vincent"}]

These slugs can be used for brute‑force, credential stuffing, or password‑spraying attacks.

Broken Object Level Authorization (BOLA)

BOLA occurs when an API provider lets a user access resources they are not authorized to view. For instance, a request to GET https://bestgame.com/api/v3/users?id=5501 returns:

{
  "id": "5501",
  "first_name": "Cloud",
  "last_name": "Strife",
  "link": "https://www.bestgame.com/user/strife.buster.97",
  "name": "Cloud Strife",
  "dob": "1997-01-31",
  "username": "strife.buster.97"
}

By guessing nearby IDs, an attacker can enumerate other users' data.

User Authentication Break

Weak or missing authentication mechanisms allow attackers to bypass login controls. Tokens that lack sufficient randomness can be guessed or forged, and hard‑coded tokens in JavaScript can be harvested and reused to access privileged endpoints.

Examples include password‑reset endpoints that accept unlimited attempts, enabling brute‑force of short verification codes.

Over‑exposure of Data

APIs sometimes return more information than required. A request such as GET /api/v3/account?name=Cloud+Strife might return:

{
  "id": "5501",
  "first_name": "Cloud",
  "last_name": "Strife",
  "privilege": "user",
  "representative": [
    {"name": "Don Corneo", "id": "2203", "email": "[email protected]", "privilege": "super-admin", "admin": true, "two_factor_auth": false}
  ]
}

Testing the response reveals unnecessary personal details.

Lack of Resource and Rate Limiting

Without proper rate limits, APIs can be flooded with requests, leading to denial‑of‑service conditions. Attackers may try to bypass limits by changing parameters, using different clients, or rotating IP addresses.

Broken Function Level Authorization (BFLA)

BFLA is similar to BOLA but concerns the ability to invoke actions rather than read resources. An API that permits privileged HTTP methods (e.g., POST, PUT, DELETE) to non‑admin users can be abused to modify or delete data.

Mass Assignment

When an API accepts extra parameters that are directly merged into internal objects, attackers can set fields they should not control, such as elevating a user to admin:

// Example request to create an account
{
  "User": "scuttleph1sh",
  "Password": "GreatPassword123",
  "isAdmin": true
}

Discovering such flaws involves inspecting API documentation for undocumented parameters and fuzzing requests.

Security Misconfiguration

Misconfigurations include exposed server headers, missing TLS, default credentials, and unnecessary HTTP methods. For example, the following response headers reveal backend technology and timing information:

HTTP/ 200 OK
--snip--
X-Powered-By: VulnService 1.11 // reveals backend tech
X-XSS-Protection: 0 // should be 1
X-Response-Time: 566.43
--snip--
X-Response-Time: 25.5 (for non‑existent records)
X-Response-Time: 510.00 (for existing record)

Such details can aid enumeration and exploitation.

Injection

Improper input sanitization leads to injection flaws. An example SQL injection payload:

POST /api/v1/register HTTP/1.1
Host: example.com
--snip--
{
  "Fname": "hAPI",
  "Lname": "Hacker",
  "Address": "' OR 1=0--"
}

Another example shows a file‑read attack retrieving /etc/passwd via a query parameter:

GET http://10.10.78.181:5000/api/v1/resources/books?show=/etc/passwd

root:x:0:0:root:/root:/bin/bash
... (rest of passwd file)

Improper Asset Management

Exposing deprecated or in‑development APIs can introduce many of the above vulnerabilities. Monitoring versioned endpoints, changelogs, and repository history helps identify unmanaged assets.

Business Logic Vulnerabilities

These arise when an API’s intended functionality can be abused, such as bypassing multi‑factor authentication by altering request parameters, or uploading arbitrary files without validation. Detecting them requires understanding the application’s workflow and testing assumptions.

Summary

Familiarity with these API weaknesses enables security professionals to spot, exploit, and report them during penetration testing, helping organizations protect their customers and avoid headline‑making breaches.