Mailpit: SMTP command parser buffers unbounded command lines before syntax rejection

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Mailpit is an email testing tool and API for developers. Prior to 1.30.4, Mailpit reads SMTP commands through internal/smtpd/smtpd.go session.readLine() using bufio.Reader.ReadString before session.parseLine() parses the verb or the RFC 5321 512-octet command-line limit is enforced. An unauthenticated remote SMTP client can send an oversized single command line that is fully allocated before syntax rejection or timeout, and the normal MaxMessageSize and DATA limits do not apply to this pre-DATA path. The same command reader is used by handleAuthLogin(), handleAuthPlain(), and handleAuthCramMD5() continuation lines, so concurrent oversized inputs can create memory pressure and reduce service availability. This issue is fixed in version 1.30.4.

Source published

20 Aug 2026 21:13:27 UTC

Source modified

21 Aug 2026 21:45:36 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Mailpit is an email testing tool and API for developers. Prior to 1.30.4, Mailpit reads SMTP commands through internal/smtpd/smtpd.go session.readLine() using bufio.Reader.ReadString before session.parseLine() parses the verb or the RFC 5321 512-octet command-line limit is enforced. An unauthenticated remote SMTP client can send an oversized single command line that is fully allocated before syntax rejection or timeout, and the normal MaxMessageSize and DATA limits do not apply to this pre-DATA path. The same command reader is used by handleAuthLogin(), handleAuthPlain(), and handleAuthCramMD5() continuation lines, so concurrent oversized inputs can create memory pressure and reduce service availability. This issue is fixed in version 1.30.4.

Source published

Aug 20, 2026, 9:13:27 PM (timezone not stated)

Source modified

Aug 21, 2026, 9:45:36 PM (timezone not stated)

Observed here

7 Nov 2026 14:20:38 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Mailpit is an email testing tool and API for developers. Prior to 1.30.4, Mailpit reads SMTP commands through internal/smtpd/smtpd.go session.readLine() using bufio.Reader.ReadString before session.parseLine() parses the verb or the RFC 5321 512-octet command-line limit is enforced. An unauthenticated remote SMTP client can send an oversized single command line that is fully allocated before syntax rejection or timeout, and the normal MaxMessageSize and DATA limits do not apply to this pre-DATA path. The same command reader is used by handleAuthLogin(), handleAuthPlain(), and handleAuthCramMD5() continuation lines, so concurrent oversized inputs can create memory pressure and reduce service availability. This issue is fixed in version 1.30.4.

Source published

20 Aug 2026 21:17:07 UTC

Source modified

18 Oct 2026 20:10:00 UTC

Observed here

25 Oct 2026 15:18:51 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

## Summary Mailpit's SMTP server reads each command line with an unbounded `bufio.Reader.ReadString('\n')` before parsing the command or enforcing any protocol length limit. A remote SMTP client can send an oversized single command line and force Mailpit to allocate attacker-controlled memory before the server returns a syntax error or times out, even though RFC 5321 limits SMTP command lines to 512 octets including CRLF. ## Technical Details Mailpit enables SMTP by default. `config/config.go` sets `SMTPListen = "[::]:1025"`, and `cmd/root.go` calls `smtpd.Listen()` during normal startup. The SMTP server configures recipient and message DATA size limits in `internal/smtpd/main.go`, including the default 50 MiB `MaxMessageSize`, but those limits do not apply to command lines. The vulnerable path is in the SMTP command loop. `internal/smtpd/smtpd.go` calls `s.readLine()` for every command before parsing the verb or arguments: ```go line, err := s.readLine() if err != nil { if netErr, ok := err.(net.Error); ok && netErr.Timeout() { s.writef("421 4.4.2 %s %s ESMTP Service closing transmission channel after timeout exceeded", s.srv.Hostname, s.srv.AppName) } break } verb, args := s.parseLine(line) ``` `readLine()` then buffers until newline without a maximum length: ```go func (s *session) readLine() (string, error) { if s.srv.Timeout > 0 { _ = s.conn.SetReadDeadline(time.Now().Add(s.srv.Timeout)) } line, err := s.br.ReadString('\n') if err != nil { return "", err } line = strings.TrimSpace(line) return line, err } ``` This violates the SMTP command-line invariant before later validation can help. Address length validation in `extractAndValidateAddress()` runs only after the entire command line has already been buffered and parsed. The DATA reader has a separate `srv.MaxSize` check, but the issue is pre-DATA command input. ## PoV The following bounded test exercises the same command reader with a normal `NOOP` control and an 8 MiB oversized command line: ```go package smtpd import ( "bufio" "bytes" "strings" "testing" ) func TestUnboundedSMTPCommandLinePoV(t *testing.T) { short := "NOOP\r\n" shortSession := session{srv: &Server{}, br: bufio.NewReader(strings.NewReader(short))} shortLine, err := shortSession.readLine() if err != nil { t.Fatalf("short control readLine failed: %v", err) } t.Logf("short control: accepted len=%d command=%q", len(shortLine), shortLine) oversizedLen := 8 * 1024 * 1024 oversized := strings.Repeat("X", oversizedLen) + "\r\n" oversizedSession := session{srv: &Server{}, br: bufio.NewReader(bytes.NewBufferString(oversized))} oversizedLine, err := oversizedSession.readLine() if err != nil { t.Fatalf("oversized readLine failed: %v", err) } t.Logf("oversized command: accepted len=%d; RFC 5321 command-line limit is 512 octets including CRLF", len(oversizedLine)) if len(oversizedLine) != oversizedLen { t.Fatalf("readLine length = %d, want %d", len(oversizedLine), oversizedLen) } if len(oversizedLine) <= 512 { t.Fatalf("oversized command did not exceed SMTP command-line limit") } } ``` ## PoC From a Mailpit checkout, save the PoV above as `internal/smtpd/smtp_command_line_pov_test.go` and run: ```fish docker run --rm -v "$PWD:/src" -w /src golang:1.25 go test ./internal/smtpd -run TestUnboundedSMTPCommandLinePoV -v ``` On current `develop` commit `cd7661fd5b23cce1e218b583b21e157cfa612051`, the test prints: ```text === RUN TestUnboundedSMTPCommandLinePoV smtp_command_line_pov_test.go:17: short control: accepted len=4 command="NOOP" smtp_command_line_pov_test.go:26: oversized command: accepted len=8388608; RFC 5321 command-line limit is 512 octets including CRLF --- PASS: TestUnboundedSMTPCommandLinePoV (0.01s) PASS ok github.com/axllent/mailpit/internal/smtpd 0.017s ``` The same test against release `v1.30.3` commit `6acf5b8f942ab0e007b1227d31dfb3c3303e8d13` prints: ```text === RUN TestUnboundedSMTPCommandLinePoV smtp_command_line_pov_test.go:17: short control: accepted len=4 command="NOOP" smtp_command_line_pov_test.go:26: oversized command: accepted len=8388608; RFC 5321 command-line limit is 512 octets including CRLF --- PASS: TestUnboundedSMTPCommandLinePoV (0.02s) PASS ok github.com/axllent/mailpit/internal/smtpd 0.020s ``` The `NOOP` control shows the normal reader path. The oversized case shows the parser accepting an 8 MiB command line into memory instead of rejecting at the SMTP command-line limit. ## Impact An unauthenticated remote client that can reach Mailpit's SMTP listener can force heap allocation proportional to a single command line before any SMTP command is parsed. Repeating the input across concurrent connections can consume process memory and degrade or deny Mailpit service availability. This is distinct from message DATA size handling: the default `MaxMessageSize` cap is enforced after `DATA`, while the command-line reader is reached before DATA and before `MAIL FROM SIZE=` handling. Suggested advisory metadata: CWE-400 (Uncontrolled Resource Consumption). Conservative CVSS 3.1 vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`, base score 7.5 (High). If a deployment explicitly binds SMTP to trusted loopback-only clients, environmental severity may be lower; the project default is a network SMTP listener on `[::]:1025`. ## Suggested Fix Bound SMTP command-line reads before buffering the full line. For ordinary SMTP commands, reject input exceeding RFC 5321's 512-octet command-line limit including CRLF, returning a `500 5.5.2`-style command-line-too-long response before allocating the entire attacker-controlled line. Apply the same pre-parse bound to AUTH continuation lines because `handleAuthLogin()`, `handleAuthPlain()`, and `handleAuthCramMD5()` also call `readLine()`. Consider adding a similar POP3 command-line cap in `internal/pop3/server.go`, where the optional POP3 server also uses `ReadString('\n')` for commands. Regression tests should cover a normal short command, an over-limit command line, and over-limit AUTH continuation input. They should assert that the over-limit cases fail without returning the oversized string to command parsing. ## Affected Package/Versions Confirmed affected: - Current `develop`: `cd7661fd5b23cce1e218b583b21e157cfa612051` - Latest release: `v1.30.3`, tag commit `6acf5b8f942ab0e007b1227d31dfb3c3303e8d13`, published 2026-06-27 No fixed version was identified during this review. ## Advisory History The closest published Mailpit advisory is `GHSA-fpxj-m5q8-fphw`, which covers unauthenticated memory exhaustion through unlimited SMTP DATA and `/api/v1/send` body sizes. This report is different: it targets the pre-DATA SMTP command-line reader before `srv.MaxSize`, `MAIL FROM SIZE=`, or DATA handling applies. Other published Mailpit advisories checked were `GHSA-28pq-6qxg-wg5r` for HTTP JSON body limits, `GHSA-54wq-72mp-cq7c` for SMTP header injection, `GHSA-w4vj-r5pg-3722` for proxy CSS map concurrency, `GHSA-qx5x-85p8-vg4j` for dump path traversal, the SSRF/link-check/proxy advisories `GHSA-8v65-47jx-7mfr`, `GHSA-mpf7-p9x7-96r3`, `GHSA-6jxm-fv7w-rw5j`, `GHSA-j3fj-qppj-fmmc`, `GHSA-w4mc-hhc6-xp28`, and `GHSA-524m-q5m7-79mm` for CSWSH. None of these describe unbounded SMTP command-line buffering. Public Mailpit issue searches for `SMTP command line too long ReadString`, `SMTP 512 octets command line`, `SMTP memory DoS command line`, and `ReadString smtpd` found no matching issue. Public commit searches for `ReadString smtpd`, `command line too long`, and `MaxMessageSize smtp` found no matching fix. No prior submitted, ready-for-review, or completed-but-unsubmitted Mailpit report available in the review materials matched this root cause. ## References - RFC 5321 section 4.5.3.1.4, command-line length limit: https://www.rfc-editor.org/rfc/rfc5321#section-4.5.3.1.4 - Mailpit security policy: https://github.com/axllent/mailpit/security/policy - `GHSA-fpxj-m5q8-fphw`: https://github.com/axllent/mailpit/security/advisories/GHSA-fpxj-m5q8-fphw - `GHSA-28pq-6qxg-wg5r`: https://github.com/axllent/mailpit/security/advisories/GHSA-28pq-6qxg-wg5r - `GHSA-54wq-72mp-cq7c`: https://github.com/axllent/mailpit/security/advisories/GHSA-54wq-72mp-cq7c

Source published

2 Sep 2026 23:42:25 UTC

Source modified

10 Sep 2026 15:25:27 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH

Mailpit: SMTP command parser buffers unbounded command lines before syntax rejection in github.com/axllent/mailpit

Source published

10 Sep 2026 14:48:42 UTC

Source modified

10 Sep 2026 15:15:14 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.