CVE-2020-11077: 
Ruby vulnerability analysis and mitigation

CVE-2020-11077 is a HTTP request smuggling vulnerability affecting Puma (RubyGem) versions before 4.3.5 and 3.12.6. The vulnerability was discovered in May 2020 and affects the way Puma handles Transfer-Encoding headers (Debian LTS, GitHub Advisory).

The vulnerability occurs when a client could smuggle a request through a proxy using the Transfer-Encoding header. 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. Puma would interpret it as two separate requests, and when processing the second request, send back a response that the proxy does not expect. This mismatch in request/response handling between Puma and the proxy creates the smuggling condition (GitHub Advisory).

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. This could lead to response data being sent to unintended recipients, potentially exposing sensitive information (GitHub Advisory).

The vulnerability requires the attacker to be able to send specially crafted requests through a proxy server that uses persistent connections to the Puma server. The attack is made possible through HTTP pipelining and manipulation of the Transfer-Encoding header (GitHub Advisory).

The vulnerability has been patched in Puma versions 3.12.6 and 4.3.5. Users are recommended to upgrade to these or later versions to address the security issue. Various Linux distributions have also released security updates to address this vulnerability (Debian LTS, OpenSUSE).