CVE-2025-31135 is a medium-severity (CVSS 5.3) Improper Input Validation vulnerability in github.com/phires/go-guerrilla. A fix is available for github.com/phires/go-guerrilla — see the affected versions and patch details below.
Go-Guerrilla SMTP Daemon allows the PROXY command to be sent multiple times
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-31135.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/phires/go-guerrillaReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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 enabled (proxyon in the JSON config) is affected.
As far as I'm able to tell, the impact is limited to spoofing the RemoteIP field. This isn't ideal, but it probably has less practical impact on an MTA than, say, a web server.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/phires/go-guerrilla | all versions | 1.6.7go get github.com/phires/go-guerrilla@v1.6.7 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/phires/go-guerrilla, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/phires/go-guerrilla to 1.6.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-31135 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2025-31135 in your dependencies?
Find it across Go, including transitive dependencies.