MediumGHSA-q3j3-w37x-hq2qBIT-symfony-2021-41267CVE-2021-41267

Webcache Poisoning in symfony/http-kernel

Published 24 Nov 2021 · Updated 8 Jul 2026

Affected packages

symfony/http-kernelComposer

Affected: >=5.2.0 <5.3.12
Fixed in: 5.3.12

Update to the fixed version for your release line, for example:composer require symfony/http-kernel:^5.3.12

symfony/symfonyComposer

Affected: >=5.2.0 <5.3.12
Fixed in: 5.3.12

Update to the fixed version for your release line, for example:composer require symfony/symfony:^5.3.12

Details

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 "trusted_headers" 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 "trusted_headers" 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.

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References

Advisory data from OSV.dev (opens OSV.dev in a new tab), refreshed daily.