CVE-2021-41267: Webcache Poisoning via X-Forwarded-Prefix and sub-request


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.


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.


We would like to thank Soner Sayakci for reporting the issue and Jérémy Derussé for fixing the issue.

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