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Critical, CVSS 9.1amended

@fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

CVE-2026-33808GHSA-6hw5-45gm-fj88

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (@fastify/express 4.0.5), 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
9.1 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-33808: 0.48% estimated exploitation activity in the next 30 days; percentile 41.70 (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 @fastify/express v4.0.4 fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via two vectors: 1. Duplicate slashes (//admin/dashboard) when ignoreDuplicateSlashes: true is configured 2. Semicolon delimiters (/admin;bypass) when useSemicolonDelimiter: true is configured In both cases, Fastify's router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. Note: This is distinct from GHSA-g6q3-96cp-5r5m (CVE-2026-22037), which addressed URL percent-encoding bypass and was patched in v4.0.3. These normalization gaps remain in v4.0.4. A similar class of normalization issue was addressed in @fastify/middie via GHSA-8p85-9qpw-fwgw (CVE-2026-2880), but @fastify/express does not include the equivalent fixes. Details The vulnerability exists in…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from ghsa record GHSA-6hw5-45gm-fj88

  2. Revision 3 · amended ·

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

  3. Revision 2 · amended ·

    Amended from osv record GHSA-6hw5-45gm-fj88

  4. Revision 1 · created ·

    Published from ghsa record GHSA-6hw5-45gm-fj88

Agent brief

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# Patch Next advisory brief: CVE-2026-33808

Advisory: @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)
IDs: CVE-2026-33808, GHSA-6hw5-45gm-fj88
Record: https://patchnext.com/advisories/cve-2026-33808 (revision 4, source checked 26 Oct 2026, 12:53 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @fastify/express 4.0.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @fastify/express: affected <= 4.0.4; fixed in 4.0.5

## Suggested inspection (read-only)
- `npm ls @fastify/express --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/express.
- 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://github.com/advisories/GHSA-6hw5-45gm-fj88 (GitHub-reviewed advisory GHSA-6hw5-45gm-fj88 lists affected ranges: npm @fastify/express >=0 <=4.0.4)
- Verified: https://osv.dev/vulnerability/GHSA-6hw5-45gm-fj88 (OSV record GHSA-6hw5-45gm-fj88 lists affected ranges: npm @fastify/express >=0 <4.0.5 (fixed))

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