What is CVE-2021-41267?

CVE-2021-41267 is a medium-severity vulnerability in Sensiolabs Symfony, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 6.5/10. Published 2021-11-24.

How severe is CVE-2021-41267?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Vulnerability in Sensiolabs Symfony

CVE-2021-41267

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.

Vulnerability class: HTTP Request Smuggling

Published 2021-11-24 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.013 (68.9th percentile), scored 2026-10-01.

Low probability of exploitation in the next 30 days, but still tracked. 69th percentile — 68.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.013 · change: +0.000

Affected products

Sensiolabs Symfony
Symfony — versions >= 5.2.0, < 5.3.12

Weakness classification (CWE)

CWE-444 — Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling)

References

github.com/symfony/symfony/pull/44243 (Patch, Third Party Advisory)
github.com/symfony/symfony/releases/tag/v5.3.12 (Third Party Advisory, Release Notes)
github.com/symfony/symfony/security/advisories/GHSA-q3j3-w37x-hq2q (Patch, Third Party Advisory)
github.com/symfony/symfony/commit/95dcf51682029e89450aee86267e3d553aa7c487 (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-41267?: CVE-2021-41267 is a medium-severity vulnerability in Sensiolabs Symfony, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 6.5/10. Published 2021-11-24.
How severe is CVE-2021-41267?: Medium severity. CVSS v3 base score is 6.5 out of 10.