Webcache Poisoning in symfony/http-kernel
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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | symfony/http-kernel | >= 5.2.0, < 5.3.12 | 5.3.12 |
| Composer package | symfony/symfony | >= 5.2.0, < 5.3.12 | 5.3.12 |
References
- https://github.com/symfony/symfony/security/advisories/GHSA-q3j3-w37x-hq2q
- https://nvd.nist.gov/vuln/detail/CVE-2021-41267
- https://github.com/symfony/symfony/pull/44243
- https://github.com/symfony/symfony/commit/95dcf51682029e89450aee86267e3d553aa7c487
- https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/http-kernel/CVE-2021-41267.yaml
- https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2021-41267.yaml
- https://github.com/symfony/symfony/releases/tag/v5.3.12
- https://symfony.com/cve-2021-41267
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N- Weakness (CWE)
CWE-444- Platform
- Composer
- Package
symfony/http-kernel, symfony/symfony- Disclosed
- 24.11.2021
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-q3j3-w37x-hq2q - CC BY 4.0
Vulnerability history: symfony/http-kernel
All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 8
- First → latest
- 09.2020 05.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 4
- Medium 4
Disclosures per year
- 2020: 1
- 2021: 1
- 2022: 3
- 2023: 1
- 2024: 1
- 2025: 0
- 2026: 1
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