Summary
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.
Impact
CVE-2021-41267 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.3.12); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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symfony/http-kernel to 5.3.12 or later; symfony/symfony to 5.3.12 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2021-41267? CVE-2021-41267 is a medium-severity security vulnerability in symfony/http-kernel (composer), affecting versions >= 5.2.0, < 5.3.12. It is fixed in 5.3.12.
- How severe is CVE-2021-41267? CVE-2021-41267 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2021-41267?
symfony/http-kernel(composer) (versions >= 5.2.0, < 5.3.12)symfony/symfony(composer) (versions >= 5.2.0, < 5.3.12)
- Is there a fix for CVE-2021-41267? Yes. CVE-2021-41267 is fixed in 5.3.12. Upgrade to this version or later.
- Is CVE-2021-41267 exploitable, and should I be worried? Whether CVE-2021-41267 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-41267 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-41267?
- Upgrade
symfony/http-kernelto 5.3.12 or later - Upgrade
symfony/symfonyto 5.3.12 or later
- Upgrade