What’s Vulnerable › Vulnerabilities

CVE-2021-41267: Webcache Poisoning in Symfony

• Severity: medium (CVSS 4.3 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-11-24 · updated 2026-06-17
• Weakness: CWE-444

Description

Symfony/Http-Kernel is the HTTP kernel component for Symfony, a PHP framework for web and console applications and a set of reusable PHP components. Headers that are not part of the "trusted_headers" allowed list are ignored and protect users from "Cache poisoning" attacks. In Symfony 5.2, maintainers added support for the `X-Forwarded-Prefix` headers, but this header was accessible in SubRequest, 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` header, leading to a web cache poisoning issue. Versions 5.3.12 and later have a patch to ensure that the `X-Forwarded-Prefix` header is not forwarded to subrequests when it is not trusted.

Affected products

• symfony: from 5.2.0 before 5.3.12 — fixed in 5.3.12

How to fix

Upgrade to a fixed version: symfony 5.3.12. Check the vendor advisory for mitigations.

References

• https://github.com/symfony/symfony/commit/95dcf51682029e89450aee86267e3d553aa7c487
• https://github.com/symfony/symfony/pull/44243
• https://github.com/symfony/symfony/releases/tag/v5.3.12
• https://github.com/symfony/symfony/security/advisories/GHSA-q3j3-w37x-hq2q
• https://github.com/symfony/symfony/commit/95dcf51682029e89450aee86267e3d553aa7c487
• https://github.com/symfony/symfony/pull/44243
• https://github.com/symfony/symfony/releases/tag/v5.3.12
• https://github.com/symfony/symfony/security/advisories/GHSA-q3j3-w37x-hq2q

Get alerted when CVE-2021-41267 or the next vulnerability affects your stack