Mailpit is an email testing tool and API for developers. Prior to version 1.30.0, the screenshot/print proxy (/proxy?data=…) maintains a package-level assets map[string]MessageAssets cache, but reads the map without holding assetsMutex while a long-running cleanup goroutine and (re-entrant) CSS-rewriting code path concurrently write to it under the lock. When the unsynchronized read coincides with a synchronized write, Go's runtime raises fatal error: concurrent map read and map write — a runtime.throw that is not recoverable by http.Server's handler-panic recover. The whole Mailpit process exits, taking the SMTP, POP3 and HTTP listeners down with it. Version 1.30.0 contains a patch.

Affected: Go: github.com/axllent/mailpit

### Summary The screenshot/print proxy (/proxy?data=…) maintains a package-level assets map[string]MessageAssets cache, but reads the map without holding assetsMutex while a long-running cleanup goroutine and (re-entrant) CSS-rewriting code path concurrently write to it under the lock. When the unsynchronized read coincides with a synchronized write, Go's runtime raises fatal error: concurrent map read and map write — a runtime.throw that is not recoverable by http.Server's handler-panic recover. The whole Mailpit process exits, taking the SMTP, POP3 and HTTP listeners down with it.

### Details A remote, unauthenticated attacker who can (1) reach /proxy and (2) plant any message with a stylesheet link in the inbox can crash Mailpit by issuing concurrent /proxy?data=… requests against the same message's CSS URL. Mailpit's defaults make both prerequisites trivial: the SMTP listener accepts mail anonymously, the HTTP listener accepts requests anonymously, and the cleanup goroutine fires every minute regardless of whether the map is being read.

Affected code [server/handlers/proxy.go:198-229](https://github.com/axllent/mailpit/blob/develop/server/handlers/proxy.go#L198-L229) [server/handlers/proxy.go:52-66](https://github.com/axllent/mailpit/blob/develop/server/handlers/proxy.go#L52-L66) [server/handlers/proxy.go:244-313](https://github.com/axllent/mailpit/blob/develop/server/handlers/proxy.go#L244-L313)

Go's map runtime sets a hashWriting flag at the start of any write op.…

Ein entfernter, anonymer Angreifer kann mehrere Schwachstellen in MailPit ausnutzen, um Dateien zu manipulieren, vertrauliche Informationen offenzulegen oder einen Denial-of-Service-Zustand zu verursachen.