CVE-2026-45712: Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write) ### Summary The…

Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write)

### 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. Concurrent map reads check the flag and call throw("concurrent map read and map write") — throw is not caught by defer recover and is not caught by http.Server's handler-panic guard. The process exits with a stack trace.

### PoC
1. Deposit any message with a in the store (SMTP or /api/v1/send, both unauthenticated by default).
2. Make a few hundred concurrent requests

Affected