Summary
Go-Guerrilla SMTP Daemon allows the PROXY command to be sent multiple times
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.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2025-31135 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.6.7); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-31135? CVE-2025-31135 is a medium-severity improper input validation vulnerability in github.com/phires/go-guerrilla (go), affecting versions < 1.6.7. It is fixed in 1.6.7. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2025-31135? CVE-2025-31135 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/phires/go-guerrilla are affected by CVE-2025-31135? github.com/phires/go-guerrilla (go) versions < 1.6.7 is affected.
- Is there a fix for CVE-2025-31135? Yes. CVE-2025-31135 is fixed in 1.6.7. Upgrade to this version or later.
- Is CVE-2025-31135 exploitable, and should I be worried? Whether CVE-2025-31135 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-31135 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-31135? Upgrade
github.com/phires/go-guerrillato 1.6.7 or later.