Modern applications evolve quickly. APIs get upgraded, routes get deprecated, and new protections are rolled out on newer endpoints. But what if the old versions are never turned off?

Versioned APIs — like /api/v1/, /api/v2/, or /graphql/v3/ — are supposed to represent progress, better structure, and stronger security. Yet, many developers forget or ignore the security posture of older versions. And that’s where the real danger begins.

As bug bounty hunters, security researchers, or curious developers, this presents a juicy opportunity: API downgrade attacks. By identifying older, undocumented, or deprecated API versions still accessible in production, you can often bypass modern authentication checks, exploit unpatched logic, or trigger insecure deserialization flows.

This article dives deep into the methodology, real-world examples, and tools you can use to exploit version mismatches. You’ll learn how a single /v1/ endpoint can become your gateway to significant vulnerabilities.