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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-67445: 0.51% estimated exploitation activity in the next 30 days; percentile 43.30 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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: readLine() then buffers until newline without a maximum length: This violates the…

Published by the source

2 Sep 2026, 23:42 UTC

Last modified by the source

10 Sep 2026, 15:15 UTC

Source last checked

26 Oct 2026, 15:29 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-w878-pj84-3j5v

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GO-2026-6363

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-w878-pj84-3j5v

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# Patch Next advisory brief: CVE-2026-67445

Advisory: Mailpit: SMTP command parser buffers unbounded command lines before syntax rejection
IDs: CVE-2026-67445, GHSA-w878-pj84-3j5v
Record: https://patchnext.com/advisories/cve-2026-67445 (revision 3, source checked 26 Oct 2026, 15:29 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/axllent/mailpit 1.30.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/axllent/mailpit: affected <= 1.30.3; fixed in 1.30.4

## Suggested inspection (read-only)
- Go github.com/axllent/mailpit: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/axllent/mailpit.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-w878-pj84-3j5v (OSV record GHSA-w878-pj84-3j5v lists affected ranges: Go github.com/axllent/mailpit >=0 <1.30.4 (fixed))
- Verified: https://osv.dev/vulnerability/GO-2026-6363 (OSV record GO-2026-6363 lists affected ranges: Go github.com/axllent/mailpit >=0 <1.30.4 (fixed))
- Verified: https://github.com/advisories/GHSA-w878-pj84-3j5v (GitHub-reviewed advisory GHSA-w878-pj84-3j5v lists affected ranges: Go github.com/axllent/mailpit >=0 <=1.30.3)

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