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CriticalVulnerabilityCVE-2026-112362updated 27 Oct 2026, 12:18 UTC

Fastify Express plugin can skip security middleware on some URLs

Under a specific non-default setting, a crafted web address can make the server run an admin page while the login and permission checks are skipped. Anyone on the internet could reach protected pages without logging in.

Fixed in @fastify/express 4.0.9Official source
SeverityCriticalCVSS 9.1 of 10
Being exploited?UnclearNo reports either way yet
Fix available?Yes@fastify/express 4.0.9 · since 27 Oct 2026
How sure is this?Official sourceBacked by an official source

How the attack works simplified

  1. 1Attacker sends requestUses an absolute-form URL target
  2. 2Slashes collapsedignoreDuplicateSlashes alters the path
  3. 3Router matches admin routeHandler is selected for the scoped path
  4. 4Auth middleware skippedExpress layer no longer sees the path
  5. ResultProtected admin handler runs without checks

What happened

In @fastify/express versions 4.0.5 through 4.0.8, when Fastify runs with ignoreDuplicateSlashes enabled, absolute-form request targets can cause path-scoped Express middleware to be bypassed while the matching route handler still runs.

Why it is dangerous: Authentication, authorization, or rate-limiting middleware can be skipped, letting unauthenticated attackers reach scoped routes such as admin paths.

Are you affected?

You are affected if you use @fastify/express 4.0.5 through 4.0.8 with Fastify's non-default ignoreDuplicateSlashes option enabled.

@fastify/express 4.0.5 through 4.0.8 (only when Fastify ignoreDuplicateSlashes is enabled)

What to do

  1. Update @fastify/express to version 4.0.9 or later
  2. If you cannot update now, enforce authentication in a Fastify preHandler hook that runs after routing
  3. Or turn off ignoreDuplicateSlashes if you do not need it

Timeline

  1. 27 Oct 2026, 12:17 UTC
    Publicly disclosed
  2. 27 Oct 2026, 12:17 UTC
    Reported by NVD · NVD
  3. 27 Oct 2026, 12:18 UTC
    Picked up by Zero Radar

Sources 1 report, oldest first

Official vendor, government CERT or advisory database

Technical details

ID
CVE-2026-112362
CVSS
9.1 / 10
EPSS
0.4% chance of exploitation within 30 days
Disclosed
27 Oct 2026, 12:17 UTC
On Zero Radar
27 Oct 2026, 12:18 UTC (1 min after disclosure)
Last change
27 Oct 2026, 12:18 UTC

The plain-language parts and the attack diagram were written by an AI model (anthropic/claude-haiku-5.5) from the sources above. Facts like versions, exploitation and dates come from the sources themselves. How Zero Radar works

Fastify Express plugin can skip security middleware on some URLs · Zero Radar