Go-Guerrilla SMTP Daemon allows the PROXY command to be sent multiple times

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS

• CVE-2025-31135: 0.39% estimated exploitation activity in the next 30 days; percentile 32.70 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The PROXY command is accepted multiple times, allowing a client to spoof its IP address when the proxy protocol is being used. Details When ProxyOn is enabled, it looks like 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. Note that the format of the PROXY header is well-defined. It probably shouldn't be treated as an SMTP command; parsing it the same way is likely to result in odd behavior and could lead to other vulnerabilities. PoC I'm working on writing a PR to fix this vulnerability. It'll include a unit test that will serve as a PoC on the current version. Impact Any instance with ProxyOn…

Published by the source

1 Apr 2025, 22:23 UTC

Last modified by the source

8 Jun 2026, 15:57 UTC

Source last checked

26 Oct 2026, 15:23 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-c2c3-pqw5-5p7c

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# Patch Next advisory brief: CVE-2025-31135

Advisory: Go-Guerrilla SMTP Daemon allows the PROXY command to be sent multiple times
IDs: CVE-2025-31135, GHSA-c2c3-pqw5-5p7c
Record: https://patchnext.com/advisories/cve-2025-31135 (revision 1, source checked 26 Oct 2026, 15:23 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/phires/go-guerrilla 1.6.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/phires/go-guerrilla: affected < 1.6.7; fixed in 1.6.7

## Suggested inspection (read-only)
- Go github.com/phires/go-guerrilla: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/phires/go-guerrilla.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-c2c3-pqw5-5p7c (GitHub-reviewed advisory GHSA-c2c3-pqw5-5p7c lists affected ranges: Go github.com/phires/go-guerrilla >=0 <1.6.7)

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