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CRITICAL severity

Prefix escapeGHSA-c4qr-gmr9-v23w

CRITICALFix: fastify/fastify-http-proxy@02d9b43

GHSA-c4qr-gmr9-v23w is a critical-severity (CVSS 10) Improper Input Validation vulnerability in fastify-http-proxy. A fix is available for fastify-http-proxy — see the affected versions and patch details below.

Also known asCVE-2021-21322
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk
0.00%0.82%1.63%2.45%0.2%1.6%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-c4qr-gmr9-v23w by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.

fastify-http-proxynpm
1Kdownloads / week

Description

Impact

By crafting a specific URL, it is possible to escape the prefix of the proxied backend service. If the base url of the proxied server is /pub/, a user expect that accessing /priv on the target service would not be possible. Unfortunately, it is.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Patches

All releases after v4.3.1 include the fix.

Workarounds

There are no workaround available.

For more information

If you have any questions or comments about this advisory:

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
📦npmfastify-http-proxyall versions4.3.1npm install fastify-http-proxy@4.3.1

Affected Products

1 product · 1 configurations
Application
fastify-http-proxyfastify-http-proxy_project
< 4.3.1
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify-http-proxy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update fastify-http-proxy to 4.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c4qr-gmr9-v23w is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact By crafting a specific URL, it is possible to escape the prefix of the proxied backend service. If the base url of the proxied server is `/pub/`, a user expect that accessing `/priv` on the target service would not be possible. Unfortunately, it is. [CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N](https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N) ### Patches All releases after v4.3.1 include the fix. ### Workarounds There are no workaround available. ### For more information If you have any questions or comments about this advisory: * Open
O3 Security · Impact-Aware SCA

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Prefix escape (Critical 10)