CVE-2026-33807: fastify fastify\/express Interpretation Conflict

Vulnerability summary

CVE-2026-33807 is a critical (CVSS 9.1) Interpretation Conflict vulnerability in fastify fastify\/express. EPSS estimates a 0.53% chance of exploitation in the next 30 days, higher than 44.1% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 15-04-2026
   PublishedVulnerability first disclosed.
2. 15-04-2026
   Last modifiedVulnerability information updated.

Description

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required. Upgrade to @fastify/express v4.0.5 or later.

• CWE-436Interpretation ConflictProduct A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.