Impact Puma is vulnerable to source IP spoofing when "set_remote_address proxy_protocol: :v1" is enabled and persistent connections are used. PROXY protocol v1 is a connection-level protocol. "Support was added to Puma in v5.5.0" (https://github.com/puma/puma/issues/2651). A proxy sends one PROXY header at the beginning of a TCP connection, before any HTTP data. Puma incorrectly re-parsed PROXY protocol headers after each keep-alive request on the same connection. An attacker able to send HTTP requests through a trusted proxy could therefore inject a second PROXY header between HTTP requests. Puma would treat the injected header as authoritative for the next request and overwrite "REMOTE_ADDR". This can mislead applications or middleware that use "REMOTE_ADDR" for security decisions, rate limiting, auditing, or allow/deny lists. Only deployments that explicitly enable PROXY protocol v1 are affected, and will have set: set_remote_address proxy_protocol: :v1 Puma's default configuration is not affected. Deployments that do not use persistent connections to Puma are also not expected to be affected by this issue. Patches Users should upgrade to versions 7.2.1 or 8.0.2. Workarounds Disable PROXY protocol v1 parsing if it is not required: remove/comment this: set_remote_address proxy_protocol: :v1 Users can also disable persistent connections to Puma, for example: enable_keep_alives false References - "HAProxy PROXY protocol specification" (https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt) - "CVE-2025-31135 / GHSA-c2c3-pqw5-5p7c: go-guerrilla repeated PROXY command source IP spoofing" (https://github.com/phires/go-guerrilla/security/advisories/GHSA-c2c3-pqw5-5p7c) - "Puma "set_remote_address" documentation" (https://github.com/puma/puma/blob/master/lib/puma/dsl.rb)