CVE-2026-2880: Fastify @fastify/middie auth bypass via improper path normalization

CVE-2026-2880 · Severity: critical · CVSS 9.1 · Published 2026-02-27

A security flaw in the @fastify/middie middleware library allows attackers to bypass security checks on protected web folders. By using specially crafted web addresses (such as adding extra slashes or semicolons), an attacker can trick the system into skipping authentication steps while still accessing private data. This could lead to unauthorized access to sensitive information or administrative functions.

Technologies: Fastify Middie, @fastify/middie (npm). Vendors: Fastify, npm.

Quick answers

Is CVE-2026-2880 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-2880 being exploited in the wild.
How severe is CVE-2026-2880?
Critical severity, with a CVSS score of 9.1, published 27 February 2026.
Which products and versions are affected?
Fastify Middie < 9.2.0; @fastify/middie (npm) SEMVER:introduced=0,fixed=9.2.0.
Is there a fix for CVE-2026-2880?
Yes. @fastify/middie (npm): upgrade to 9.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-2880 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-2880?
Check whether you run Fastify Middie or @fastify/middie (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • @fastify/middie (npm): upgrade to 9.2.0 or later
    npm install @fastify/middie@9.2.0

Technical details

A path normalization inconsistency exists between @fastify/middie's path matching logic and the Fastify router's (find-my-way) normalization. When specific router options like 'ignoreDuplicateSlashes' or 'useSemicolonDelimiter' are enabled, the middleware may fail to recognize a path as protected, while the router still resolves the request to a sensitive handler. An unauthenticated remote attacker can exploit this by sending crafted URLs (e.g., using '//' or ';') to bypass authentication or authorization guards. The issue is resolved in version 9.2.0; as a workaround, developers should implement security checks at the route level rather than relying solely on path-scoped middleware.

Affected products

  • Fastify @fastify/middie < 9.2.0

Identifiers

  • CVE-2026-2880
  • GHSA-8p85-9qpw-fwgw

Timeline

  • 2026-02-27: disclosed
  • 2026-02-27: advisory
  • 2026-02-27: patched: Fixed in version 9.2.0

References

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