# CVE-2026-67447: Mailpit SMTP DATA reader unbounded line buffering denial of service

Source: https://junglewise.ai/threats/cve-2026-67447-mailpit-smtp-data-reader-unbounded-line-buffering-denial-of  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-20. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-67447 · Severity: medium · CVSS 5.3 · Published 2026-08-20

Mailpit is an email testing tool with an SMTP server enabled by default. The SMTP server improperly validates message size limits by buffering entire lines into memory before checking the size cap. An attacker can send oversized SMTP DATA lines without authentication to force excessive memory allocation, potentially exhausting server resources and causing denial of service to legitimate mail operations.

Technologies: [github.com/axllent/mailpit (Go)](https://junglewise.ai/threats/technologies/github-com-axllent-mailpit). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-67447 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-67447 being exploited in the wild.

**How severe is CVE-2026-67447?** Medium severity, with a CVSS score of 5.3, published 20 August 2026.

**Which products and versions are affected?** github.com/axllent/mailpit (Go) >= 1.30.0, < 1.30.5, SEMVER:introduced=1.30.0,fixed=1.30.5.

**Is there a fix for CVE-2026-67447?** Yes. github.com/axllent/mailpit (Go): upgrade to 1.30.5 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-67447 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-67447?** Check whether you run github.com/axllent/mailpit (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- github.com/axllent/mailpit (Go): upgrade to 1.30.5 or later

  ```sh
  go get github.com/axllent/mailpit@v1.30.5
  ```


## Technical details

The vulnerability exists in the SMTP DATA reader's size-enforcement logic. The code calls bufio.Reader.ReadBytes('\n') to read a complete line before checking if len(data)+len(line) exceeds srv.MaxSize. This ordering allows an attacker to cause the full oversized line to be buffered into memory before the size limit is enforced. An unauthenticated remote client can reach the SMTP listener and exploit this with a 64 MiB single DATA line on a default 50 MiB size limit, allocating ~130 MiB of memory before receiving rejection. The issue was introduced after an earlier patch that assigned srv.MaxSize; multi-line messages are bounded, but individual oversized lines bypass the check. No patch has been released as of the advisory date.

## Affected products

- Mailpit Mailpit >=1.30.0, <1.30.5

## Identifiers

- CVE-2026-67447
- GHSA-r553-m4fv-5v97
- GO-2026-6272

## Timeline

- 2026-07-20: disclosed: Vulnerability details published via GHSA-r553-m4fv-5v97
- 2026-07-09: other: Vulnerability confirmed in v1.30.4 release and develop branch
- 2026-09-02: patched: Fix included in v1.30.5 release

## References

- [https://api.github.com/users/rexpository](https://api.github.com/users/rexpository)
- [https://github.com/rexpository](https://github.com/rexpository)
- [https://api.github.com/users/rexpository/gists%7B/gist_id%7D](https://api.github.com/users/rexpository/gists%7B/gist_id%7D)
- [https://api.github.com/users/rexpository/repos](https://api.github.com/users/rexpository/repos)
- [https://avatars.githubusercontent.com/u/30176934?v=4](https://avatars.githubusercontent.com/u/30176934?v=4)
- [https://api.github.com/users/rexpository/events%7B/privacy%7D](https://api.github.com/users/rexpository/events%7B/privacy%7D)

## Related threats

- [Axllent Mailpit WebSocket origin check bypass via percent-encoded path](https://junglewise.ai/threats/cve-2026-67448-axllent-mailpit-websocket-origin-check-bypass-via-percent-encoded)
- [Mailpit thumbnail generation unbounded image dimension decode](https://junglewise.ai/threats/cve-2026-67446-mailpit-thumbnail-generation-unbounded-image-dimension-decode)
- [Mailpit SMTP command parser unbounded buffering denial of service](https://junglewise.ai/threats/cve-2026-67445-mailpit-smtp-command-parser-unbounded-buffering-denial-of-service)
- [axllent Mailpit memory exhaustion DoS in multiple API endpoints](https://junglewise.ai/threats/cve-2026-48824-axllent-mailpit-memory-exhaustion-dos-in-multiple-api-endpoints)
- [axllent Mailpit denial of service via unbounded message size](https://junglewise.ai/threats/cve-2026-45713-axllent-mailpit-denial-of-service-via-unbounded-message-size)