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High, CVSS 8.2amended

@fastify/middie has Improper Path Normalization when Using Path-Scoped Middleware

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

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (@fastify/middie 9.2.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
8.2 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-2880: 0.43% estimated exploitation activity in the next 30 days; percentile 37.25 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary A path normalization inconsistency in @fastify/middie can result in authentication/authorization bypass when using path-scoped middleware (for example, app.use('/secret', auth)). When Fastify router normalization options are enabled (such as ignoreDuplicateSlashes, useSemicolonDelimiter, and related trailing-slash behavior), crafted request paths may bypass middleware checks while still being routed to protected handlers. Impact An unauthenticated remote attacker can access endpoints intended to be protected by middleware-based auth/authorization controls by sending specially crafted URL paths (for example, //secret or /secret;foo=bar), depending on router option configuration. This may lead to unauthorized access to protected functionality and data exposure. Affected versions - Confirmed affected: @fastify/middie@9.1.0 - All versions prior to the patch are affected. Patched versions - Fixed in: 9.2.0 Details The issue is caused by canonicalization drift between: 1.…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-8p85-9qpw-fwgw

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-8p85-9qpw-fwgw

Agent brief

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# Patch Next advisory brief: CVE-2026-2880

Advisory: @fastify/middie has Improper Path Normalization when Using Path-Scoped Middleware
IDs: CVE-2026-2880, GHSA-8p85-9qpw-fwgw
Record: https://patchnext.com/advisories/cve-2026-2880 (revision 3, source checked 26 Oct 2026, 12:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @fastify/middie 9.2.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @fastify/middie: affected < 9.2.0; fixed in 9.2.0

## Suggested inspection (read-only)
- `npm ls @fastify/middie --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @fastify/middie.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-8p85-9qpw-fwgw (OSV record GHSA-8p85-9qpw-fwgw lists affected ranges: npm @fastify/middie >=0 <9.2.0 (fixed))
- Verified: https://github.com/advisories/GHSA-8p85-9qpw-fwgw (GitHub-reviewed advisory GHSA-8p85-9qpw-fwgw lists affected ranges: npm @fastify/middie >=0 <9.2.0)

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