Mailpit: SMTP DATA line reader buffers over-limit input before size enforcement

Exploitation

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

CVSS

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

EPSS

• CVE-2026-67447: 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 DATA reader enforces the configured MaxMessageSize only after bufio.Reader.ReadBytes('\n') has already buffered a complete DATA line. A remote unauthenticated SMTP client can send one line larger than the configured message-size cap and force memory allocation before Mailpit returns the expected 552 5.3.4 rejection, leaving patched versions still exposed to a single-line incomplete-fix variant of the earlier SMTP DATA body-size issue. Technical Details Mailpit enables SMTP by default. The SMTP server now wires config.MaxMessageSize into srv.MaxSize: The DATA reader then checks that cap, but only after reading a full newline-terminated line into memory: This ordering violates the size-limit invariant. The configured cap can reject the message only after the attacker has supplied the line terminator and ReadBytes('\n') has allocated the over-limit line. With the default 50 MiB cap, a 64 MiB single DATA line is still buffered before Mailpit returns 552 5.3.4…

Published by the source

20 Aug 2026, 21:34 UTC

Last modified by the source

2 Sep 2026, 23:40 UTC

Source last checked

26 Oct 2026, 15:28 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-r553-m4fv-5v97

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GO-2026-6272

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-r553-m4fv-5v97

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# Patch Next advisory brief: CVE-2026-67447

Advisory: Mailpit: SMTP DATA line reader buffers over-limit input before size enforcement
IDs: CVE-2026-67447, GHSA-r553-m4fv-5v97
Record: https://patchnext.com/advisories/cve-2026-67447 (revision 3, source checked 26 Oct 2026, 15:28 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
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 1.30.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/axllent/mailpit: affected >= 1.30.0, < 1.30.5; fixed in 1.30.5

## 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-r553-m4fv-5v97 (OSV record GHSA-r553-m4fv-5v97 lists affected ranges: Go github.com/axllent/mailpit >=1.30.0 <1.30.5 (fixed))
- Verified: https://osv.dev/vulnerability/GO-2026-6272 (OSV record GO-2026-6272 lists affected ranges: Go github.com/axllent/mailpit >=1.30.0 <1.30.5 (fixed))
- Verified: https://github.com/advisories/GHSA-r553-m4fv-5v97 (GitHub-reviewed advisory GHSA-r553-m4fv-5v97 lists affected ranges: Go github.com/axllent/mailpit >=1.30.0 <1.30.5)

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