GHSA-c2c3-pqw5-5p7c Go-Guerrilla SMTP Daemon allows the PROXY command to be sent multiple times

• Go github.com/phires/go-guerrilla: < 1.6.7 → fixed in 1.6.7

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.