CVE-2025-31135: Go-Guerrilla SMTP Daemon allows the PROXY command to be sent multiple times
First published: Tue Apr 01 2025(Updated: )
### 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](https://github.com/phires/go-guerrilla/blob/fca3b2d8957a746997c7e71fca39004f5c96e91f/server.go#L495), 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](https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt). 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.
Credit: security-advisories@github.com
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Go-Guerrilla SMTP Daemon | <1.6.7 | |
| go/github.com/phires/go-guerrilla | <1.6.7 | 1.6.7 |
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Reference Links
Frequently Asked Questions
What is the severity of CVE-2025-31135?
CVE-2025-31135 is considered a medium-severity vulnerability due to its impact on SMTP communication integrity.
How do I fix CVE-2025-31135?
To mitigate CVE-2025-31135, upgrade Go-Guerrilla SMTP Daemon to version 1.6.7 or later.
What does CVE-2025-31135 affect?
CVE-2025-31135 affects versions of Go-Guerrilla SMTP Daemon prior to 1.6.7 with the ProxyOn feature enabled.
What is the main issue with CVE-2025-31135?
The main issue with CVE-2025-31135 is that multiple PROXY command invocations can lead to unintentional overwrites of previous settings.
Can CVE-2025-31135 lead to security risks?
Yes, CVE-2025-31135 can lead to security risks by allowing potentially malicious proxy settings to override legitimate configurations.
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