CVE-2021-41136 Explained : Impact and Mitigation

Puma is an HTTP 1.1 server for Ruby/Rack applications. Versions prior to 5.5.1 and 4.3.9 are affected by a vulnerability allowing HTTP request smuggling.

Understanding CVE-2021-41136

Puma's versions < 5.5.1 and < 4.3.9 are susceptible to HTTP request smuggling due to LF character in forwarded HTTP headers.

What is CVE-2021-41136?

Puma versions < 5.5.1 and < 4.3.9 are vulnerable to HTTP request smuggling if used with a proxy forwarding headers with LF character, potentially leading to response misdirection.

The Impact of CVE-2021-41136

This vulnerability allows a client to sneak in a request through a proxy, possibly causing the proxy to send a response to an unintended client, especially when using Apache Traffic Server.

Technical Details of CVE-2021-41136

Puma's vulnerability arises from misinterpreting HTTP requests due to LF characters in forwarded headers.

Vulnerability Description

The issue enables a client to manipulate a request through a proxy, potentially causing the proxy to send response data to the wrong client.

Affected Systems and Versions

Affected Versions: Puma >= 5.0.0, < 5.5.1 and < 4.3.9

Exploitation Mechanism

A client can exploit the LF character in forwarded headers to smuggle a request through a proxy, causing response redirection.

Mitigation and Prevention

Mitigate the vulnerability in Puma versions < 5.5.1 and < 4.3.9 by following the steps below:

Immediate Steps to Take

Update Puma to version 5.5.1 or 4.3.9 to patch the vulnerability.
Avoid using Apache Traffic Server with the vulnerable Puma versions.

Long-Term Security Practices

Regularly update all software components to the latest secure versions.

Patching and Updates

Stay informed about security advisories and promptly apply relevant patches.