Webcache Poisoning in symfony/http-kernel

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

EPSS

• CVE-2021-41267: 1.28% estimated exploitation activity in the next 30 days; percentile 70.04 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Description ----------- When a Symfony application is running behind a proxy or a load-balancer, you can tell Symfony to look for the X-Forwarded- HTTP headers. HTTP headers that are not part of the "trustedheaders" allowed list are ignored and protect you from "Cache poisoning" attacks. In Symfony 5.2, we've added support for the X-Forwarded-Prefix header, but this header was accessible in sub-requests, even if it was not part of the "trustedheaders" allowed list. An attacker could leverage this opportunity to forge requests containing a X-Forwarded-Prefix HTTP header, leading to a web cache poisoning issue. Resolution ---------- Symfony now ensures that the X-Forwarded-Prefix HTTP header is not forwarded to sub-requests when it is not trusted. The patch for this issue is available here for branch 5.3. Credits ------- We would like to thank Soner Sayakci for reporting the issue and Jérémy Derussé for fixing the issue.

Published by the source

24 Nov 2021, 20:04 UTC

Last modified by the source

5 Feb 2024, 11:13 UTC

Source last checked

26 Oct 2026, 14:15 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-q3j3-w37x-hq2q

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# Patch Next advisory brief: CVE-2021-41267

Advisory: Webcache Poisoning in symfony/http-kernel
IDs: CVE-2021-41267, GHSA-q3j3-w37x-hq2q
Record: https://patchnext.com/advisories/cve-2021-41267 (revision 1, source checked 26 Oct 2026, 14:15 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: symfony/symfony 5.3.12; symfony/http-kernel 5.3.12. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist symfony/symfony: affected >= 5.2.0, < 5.3.12; fixed in 5.3.12
- Packagist symfony/http-kernel: affected >= 5.2.0, < 5.3.12; fixed in 5.3.12

## Suggested inspection (read-only)
- Packagist symfony/symfony: check the installed version with your package manager.
- Packagist symfony/http-kernel: check the installed version with your package manager.

## 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 symfony/symfony, symfony/http-kernel.
- 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-q3j3-w37x-hq2q (GitHub-reviewed advisory GHSA-q3j3-w37x-hq2q lists affected ranges: Packagist symfony/http-kernel >=5.2.0 =5.2.0 <5.3.12)

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