Prevent cache poisoning via a Response Content-Type header in Symfony

Exploitation

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

CVSS

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

EPSS

• CVE-2020-5255: 1.30% estimated exploitation activity in the next 30 days; percentile 70.42 (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 Response does not contain a Content-Type header, Symfony falls back to the format defined in the Accept header of the request, leading to a possible mismatch between the response's content and Content-Type header. When the response is cached, this can lead to a corrupted cache where the cached format is not the right one. Resolution ---------- Symfony does not use the Accept header anymore to guess the Content-Type. The patch for this issue is available here for the 4.4 branch. Credits ------- I would like to thank Xavier Lacot from JoliCode for reporting & Yonel Ceruto and Tobias Schultze for fixing the issue.

Published by the source

30 Mar 2020, 20:09 UTC

Last modified by the source

5 Feb 2024, 11:10 UTC

Source last checked

26 Oct 2026, 14:15 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mcx4-f5f5-4859

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# Patch Next advisory brief: CVE-2020-5255

Advisory: Prevent cache poisoning via a Response Content-Type header in Symfony
IDs: CVE-2020-5255, GHSA-mcx4-f5f5-4859
Record: https://patchnext.com/advisories/cve-2020-5255 (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: Low (CVSS 2.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: symfony/symfony 5.0.7, 4.4.7; symfony/http-foundation 5.0.7, 4.4.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist symfony/symfony: affected >= 4.4.0, < 4.4.7; >= 5.0.0, < 5.0.7; fixed in 4.4.7, 5.0.7
- Packagist symfony/http-foundation: affected >= 4.4.0, < 4.4.7; >= 5.0.0, < 5.0.7; fixed in 4.4.7, 5.0.7

## Suggested inspection (read-only)
- Packagist symfony/symfony: check the installed version with your package manager.
- Packagist symfony/http-foundation: 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-foundation.
- 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-mcx4-f5f5-4859 (GitHub-reviewed advisory GHSA-mcx4-f5f5-4859 lists affected ranges: Packagist symfony/http-foundation >=4.4.0 =5.0.0 =4.4.0 =5.0.0 <5.0.7)

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