# CVE-2026-67445: Mailpit SMTP command parser unbounded buffering denial of service

Source: https://junglewise.ai/threats/cve-2026-67445-mailpit-smtp-command-parser-unbounded-buffering-denial-of-service  
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-2026-67445 · Severity: high · CVSS 7.5 · Published 2026-08-20

Mailpit's SMTP server reads incoming command lines into memory without enforcing RFC 5321's 512-octet limit, allowing unauthenticated remote attackers to force excessive heap allocation by sending oversized single commands. By repeating this across multiple connections, an attacker can exhaust server memory and disrupt email processing and availability.

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

## Quick answers

**Is CVE-2026-67445 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-67445 being exploited in the wild.

**How severe is CVE-2026-67445?** High severity, with a CVSS score of 7.5, published 20 August 2026.

**Which products and versions are affected?** Axllent Mailpit <= 1.30.3; github.com/axllent/mailpit (Go) <= 1.30.3, SEMVER:introduced=0,fixed=1.30.4.

**Is there a fix for CVE-2026-67445?** Yes. github.com/axllent/mailpit (Go): upgrade to 1.30.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-67445 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-67445?** Check whether you run Axllent Mailpit or 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 1.30.4 or later

  ```sh
  go get github.com/axllent/mailpit@v1.30.4
  ```


## Technical details

The vulnerability is a resource exhaustion flaw (CWE-400) in Mailpit's SMTP command handling. The `readLine()` function in `internal/smtpd/smtpd.go` uses `bufio.Reader.ReadString('\n')` to read commands without enforcing a maximum length, buffering the entire attacker-supplied line into heap memory before any command validation occurs. This violates RFC 5321 section 4.5.3.1.4, which mandates a 512-octet limit for SMTP command lines. An unauthenticated attacker with network access to the SMTP listener (default `[::]:1025`) can send multi-megabyte command lines repeatedly across concurrent connections to consume process memory and trigger denial of service. The DATA message-size limit (`MaxMessageSize`, default 50 MiB) does not protect against this, as the vulnerability exists in pre-DATA command processing. Patch version 1.30.4 is available; all prior versions including v1.30.3 and develop are affected.

## Affected products

- Axllent Mailpit <= 1.30.3

## Identifiers

- CVE-2026-67445
- GHSA-w878-pj84-3j5v
- GO-2026-6363

## Timeline

- 2026-07-09: disclosed
- 2026-09-02: advisory
- 2026-09-02: patched: Fixed in version 1.30.4

## 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)

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- [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)
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- [Mailpit thumbnail generation unbounded image dimension decode](https://junglewise.ai/threats/cve-2026-67446-mailpit-thumbnail-generation-unbounded-image-dimension-decode)
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