Application Programming Interfaces (APIs) sit at the heart of modern digital infrastructure, serving as the connective tissue between services, devices, and data. As organizations expose more functionality through APIs — from payment processing to healthcare records to AI agents — the attack surface expands dramatically. APIs now represent the dominant vector for data breaches, accounting for over 60% of malicious traffic in 2026. Unlike traditional web applications, APIs expose structured data and business logic directly, making authorization flaws, injection attacks, and excessive data exposure far more impactful. The key insight: API security isn't web security. APIs require fundamentally different defenses — property-level authorization, schema validation, behavior monitoring, and comprehensive inventory management — because attackers exploit the machine-readable nature of APIs to automate data exfiltration and privilege escalation at scale.
What This Cheat Sheet Covers
This topic spans 12 focused tables and 96 indexed concepts. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.
Table 1: OWASP API Security Top 10 Vulnerabilities
The OWASP API Security Top 10 is the canonical risk list every API team measures itself against, and what stands out is how many of the entries are authorization failures rather than classic injection bugs. Broken object-level authorization (BOLA) sits at number one because the attack is trivial — swap an ID in the URL and read someone else's data — yet APIs routinely forget to check that the caller actually owns the resource. Read these as the flaws attackers reach for first, in roughly the order they pay off.
| Vulnerability | Example | Description |
|---|---|---|
GET /api/users/123/ordersGET /api/users/456/orders | • Attackers change object IDs in requests to access data belonging to other users • the #1 API risk in 2026. APIs fail to verify the authenticated user owns the requested resource | |
Weak password policy, no MFA, JWT without signature verification | • Most common vulnerability found in 23.5% of production APIs • includes weak credential policies, missing token expiration, and flawed session management | |
{"email":"user@example.com","isAdmin":false} | • Combines mass assignment and excessive data exposure • API returns or accepts more fields than needed, allowing attackers to read sensitive properties or escalate privileges by modifying fields like isAdmin. | |
No rate limit → for i in range(10000): requests.get(api_url) | • Missing or inadequate rate limiting, pagination, and resource quotas enable DoS attacks and resource exhaustion • attackers drain APIs through mass requests | |
Regular user calls DELETE /api/admin/users/123 | • APIs expose administrative or privileged functions to unauthorized roles • fails to enforce role-based access control (RBAC) at the endpoint level |