CVE-2021-41136: 
Ruby vulnerability analysis and mitigation

CVE-2021-41136 is a vulnerability discovered in Puma, a HTTP 1.1 server for Ruby/Rack applications. The vulnerability was disclosed on October 12, 2021, affecting versions prior to 5.5.1 and 4.3.9. The issue involves HTTP request smuggling when Puma is used with a proxy that forwards HTTP header values containing LF (Line Feed) characters (GitHub Advisory).

The vulnerability occurs when Puma is used with a proxy that forwards HTTP header values containing LF characters as line endings. This behavior can lead to inconsistent interpretation of HTTP requests between Puma and the proxy server. The severity of this vulnerability is considered Low with a CVSS v3 score of 3.7 (Red Hat CVE).

If exploited, this vulnerability could allow a client to smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. If the proxy uses persistent connections and the client adds another request via HTTP pipelining, the proxy may mistake it as the first request's body, while Puma would interpret it as two separate requests. This could lead to responses being sent to unintended clients (GitHub Advisory).

The vulnerability has been patched in Puma versions 5.5.1 and 4.3.9. For users unable to upgrade immediately, a workaround is to use a proxy that does not forward LF characters as line endings. Compatible proxies include Nginx, Apache (>2.4.25), Haproxy, Caddy, and Traefik. Note that implementing the fix may cause legacy clients using LF as line endings in HTTP headers to receive 400 errors (GitHub Advisory).