CVE-2025-31135: phires go-guerrilla Improper Input Validation

Vulnerability summary

CVE-2025-31135 is a medium (CVSS 5.3) Improper Input Validation vulnerability in phires go-guerrilla. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 30.2% of scored vulnerabilities.

Timeline

1. 01-04-2025
   PublishedVulnerability first disclosed.
2. 28-01-2026
   Last modifiedVulnerability information updated.

Description

Go-Guerrilla SMTP Daemon is a lightweight SMTP server written in Go. Prior to 1.6.7, when ProxyOn is enabled, the PROXY command will be accepted multiple times, with later invocations overriding earlier ones. The proxy protocol only supports one initial PROXY header; anything after that is considered part of the exchange between client and server, so the client is free to send further PROXY commands with whatever data it pleases. go-guerrilla will treat these as coming from the reverse proxy, allowing a client to spoof its IP address. This vulnerability is fixed in 1.6.7.

• CWE-20Improper Input ValidationThe product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.