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Critical, CVSS 10.0amended

Prefix escape

CVE-2021-21321GHSA-qmw8-3v4g-gwj4

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (fastify-reply-from 4.0.2), 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
10.0 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
EPSS
  • CVE-2021-21321: 1.82% estimated exploitation activity in the next 30 days; percentile 78.78 (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.

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 A patch have been submitted by Corey Farrell git@cfware.com, the reporter. All releases after v4.0.2 include the fix. Workarounds There are no workaround available. For more information If you have any questions or comments about this advisory: Open an issue in fastify-reply-from Email us at hello@matteocollina.com

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-qmw8-3v4g-gwj4

  2. Revision 1 · created ·

    Published from osv record GHSA-qmw8-3v4g-gwj4

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2021-21321

Advisory: Prefix escape
IDs: CVE-2021-21321, GHSA-qmw8-3v4g-gwj4
Record: https://patchnext.com/advisories/cve-2021-21321 (revision 2, source checked 26 Oct 2026, 12:20 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 10.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fastify-reply-from 4.0.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fastify-reply-from: affected < 4.0.2; fixed in 4.0.2

## Suggested inspection (read-only)
- `npm ls fastify-reply-from --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-reply-from.
- 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-qmw8-3v4g-gwj4 (OSV record GHSA-qmw8-3v4g-gwj4 lists affected ranges: npm fastify-reply-from >=0 <4.0.2 (fixed))
- Verified: https://github.com/advisories/GHSA-qmw8-3v4g-gwj4 (GitHub-reviewed advisory GHSA-qmw8-3v4g-gwj4 lists affected ranges: npm fastify-reply-from >=0 <4.0.2)

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