CVE-2026-47736

Impact

PROXY protocol support for Puma was added in version 5.5.0.

When PROXY protocol v1 support is enabled, Puma reads incoming bytes into an internal buffer. It waits for "\r\n" to determine whether a PROXY v1 line is present. If an attacker opens a TCP connection and continuously sends bytes without CRLF, Puma keeps appending to this pre-parse buffer.

This can cause unbounded in-process memory growth and additional CPU cost from repeatedly scanning the growing buffer for CRLF. A single, unauthenticated TCP connection can drive significant memory growth and may cause process/container OOM or degraded availability.

Only Puma servers using the following non-default config are affected:

   set_remote_address proxy_protocol: :v1

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

• Restrict direct network access to Puma listeners using PROXY protocol:
  • Only allow trusted load balancers/reverse proxies to connect.
  • Block arbitrary client TCP access with firewall/security group rules.

Resources

• HAProxy PROXY protocol specification
• CWE-400: Uncontrolled Resource Consumption
• CWE-770: Allocation of Resources Without Limits or Throttling
• Puma set_remote_address documentation
• Puma client PROXY protocol parsing code
• Puma constants, including PROXY_PROTOCOL_V1_REGEX