Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values which contain the LF character could allow HTTP request smugggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. The only proxy which has this behavior, as far as the Puma team is aware of, is Apache Traffic Server. 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. This vulnerability was patched in Puma 5.5.1 and 4.3.9. As a workaround, do not use Apache Traffic Server with `puma`.

Puma is a Ruby/Rack web server built for parallelism. Prior to `puma` version `5.6.2`, `puma` may not always call `close` on the response body. Rails, prior to version `7.0.2.2`, depended on the response body being closed in order for its `CurrentAttributes` implementation to work correctly. The combination of these two behaviors (Puma not closing the body + Rails' Executor implementation) causes information leakage. This problem is fixed in Puma versions 5.6.2 and 4.3.11. This problem is fixed in Rails versions 7.02.2, 6.1.4.6, 6.0.4.6, and 5.2.6.2. Upgrading to a patched Rails _or_ Puma version fixes the vulnerability.