Machine-written summary
Summary written by Claude Haiku from the source advisory. Not reviewed by a person.
Symfony's symfony/http-kernel package, versions 5.2.0 up to but not including 5.3.12, and the symfony/symfony package in the same range, are affected. The X-Forwarded-Prefix header could reach sub-requests even when it was not in the trusted headers list, so an attacker could send forged requests with that header and poison a web cache. Upgrade to version 5.3.12.
Not analysed yet
We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.
Severity and exploitation
| Score | Rating | CVSS | Vector | Source |
|---|---|---|---|---|
| 6.5 | Medium | 3.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N | CNA (GitHub_M) |
- CISA KEV Not listed.
- EPSS 0.013, the 70th percentile, on 7 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.
Timeline
- 31 Oct 2026, 14:25 UTC First seen by agentprime
- 31 Oct 2026, 14:25 UTC Confirmed by OSV
Sources
- GitHub Advisory Database: GHSA-q3j3-w37x-hq2q
- OSV: GHSA-q3j3-w37x-hq2q
- github.com/symfony/symfony/security/advisories/GHSA-q3j3-w37x-hq2q
- nvd.nist.gov/vuln/detail/CVE-2021-41267
- github.com/symfony/symfony/pull/44243
- github.com/symfony/symfony/commit/95dcf51682029e89450aee86267e3d553aa7c487
- github.com/FriendsOfPHP/security-advisories/blob/master/symfony/http-kernel/CVE-2021-41267.yaml
- github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2021-41267.yaml
- github.com/symfony/symfony/releases/tag/v5.3.12
- symfony.com/cve-2021-41267