CVE-2021-41136

Puma with proxy which forwards LF characters as line endings could allow HTTP request smuggling

Workarounds

This vulnerability only affects Puma installations without any proxy in front.

Use a proxy which does not forward LF characters as line endings.

Proxies which do not forward LF characters as line endings:

• Nginx
• Apache (>2.4.25)
• Haproxy
• Caddy
• Traefik

Possible Breakage

If you are dealing with legacy clients that want to send LF as a line ending in an HTTP header, this will cause those clients to receive a 400 error.

References

• HTTP Request Smuggling

For more information

If you have any questions or comments about this advisory:

• Open an issue in Puma
• See our security policy

Prior to puma version 5.5.0, using puma with a proxy which forwards LF characters as line endings could allow HTTP request smuggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client.

This behavior (forwarding LF characters as line endings) is very uncommon amongst proxy servers, so we have graded the impact here as "low". Puma is only aware of a single proxy server which has this behavior.

If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mistake it as the first request's body. Puma, however, would see it as two requests, and when processing the second request, send back a response that the proxy does not expect. If the proxy has reused the persistent connection to Puma to send another request for a different client, the second response from the first client will be sent to the second client.

CVE-2021-41136 has a CVSS score of 3.7 (Low). The vector is network-reachable, low 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 (5.5.1, 4.3.9); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

This vulnerability was patched in Puma 5.5.1 and 4.3.9.

1. What is CVE-2021-41136? CVE-2021-41136 is a low-severity security vulnerability in puma (rubygems), affecting versions >= 5.0.0, < 5.5.1. It is fixed in 5.5.1, 4.3.9.
2. How severe is CVE-2021-41136? CVE-2021-41136 has a CVSS score of 3.7 (Low). 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.
3. Which versions of puma are affected by CVE-2021-41136? puma (rubygems) versions >= 5.0.0, < 5.5.1 is affected.
4. Is there a fix for CVE-2021-41136? Yes. CVE-2021-41136 is fixed in 5.5.1, 4.3.9. Upgrade to this version or later.
5. Is CVE-2021-41136 exploitable, and should I be worried? Whether CVE-2021-41136 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
6. What actually determines whether CVE-2021-41136 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.
7. How do I fix CVE-2021-41136?
   • Upgrade puma to 5.5.1 or later
   • Upgrade puma to 4.3.9 or later

Other vulnerabilities in puma