Puma CVE-2026-47736

Requires that the target Puma server (version 5.5.0 through 7.2.0, or 8.0.0/8.0.1) has the non-default set_remote_address proxy_protocol: :v1 directive set in its Puma DSL configuration, and that the attacker can open a raw TCP connection directly to the Puma listener port. No authentication, no user interaction, and no specific OS or Ruby version are required. Exploitation is limited by the fact that PROXY v1 parsing is off by default and that production deployments typically place Puma behind a load balancer that terminates client TCP, so an attacker reaching the listener directly (e.g. exposed port, flat network, or compromised intermediate host) is the principal gating condition.

The CVSS v3.1 vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H correctly characterises an unauthenticated network-reachable availability-only flaw, scoring 7.5 (High). The realistic risk is more nuanced: exploitation is trivial (a single TCP connection with no CRLF), automatable, and pre-authentication, but the impact is bounded to denial of service of the Puma process or container, with no path to code execution or data disclosure. The advisory itself notes that only deployments enabling the non-default set_remote_address proxy_protocol: :v1 directive are affected, which materially narrows the exposed population. There is no CISA KEV listing, no EPSS score supplied, and no public exploit identified at time of analysis, so this should be treated as a real but targeted DoS risk: high priority for shops that have explicitly enabled PROXY v1 and expose Puma directly to untrusted networks, lower priority for the typical deployment behind a load balancer that strips or never speaks PROXY v1.

An attacker identifies an internet-reachable Puma listener that has PROXY protocol v1 enabled (for example, a misconfigured deployment where the listener is exposed alongside, rather than behind, the load balancer). The attacker opens a TCP socket and slowly trickles bytes that never contain '\r\n', causing Puma to accumulate the data in its pre-parse buffer and repeatedly scan it for CRLF; a handful of such connections from a single host can drive the worker process into out-of-memory termination or pin CPU, taking the service offline. No public exploit identified at time of analysis, but a working proof of concept is on the order of a few lines of netcat or Python.