Mailpit has an SMTP Header Injection via Regex Bypass

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:L/A:N

EPSS

• CVE-2026-23829: 1.40% estimated exploitation activity in the next 30 days; percentile 72.56 (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.

Vulnerability Report: SMTP Header Injection via Regex Bypass Vulnerable Code: mailpit/internal/smtpd/smtpd.go Executive Summary Mailpit's SMTP server is vulnerable to Header Injection due to an insufficient Regular Expression used to validate RCPT TO and MAIL FROM addresses. An attacker can inject arbitrary SMTP headers (or corrupt existing ones) by including carriage return characters (\r) in the email address. This header injection occurs because the regex intended to filter control characters fails to exclude \r and \n when used inside a character class. RFC Compliance & Design Analysis "Is this behavior intentional for a testing tool?" No. While testing tools are often permissive, this specific behavior violates the core SMTP protocol and fails the developer's own intent. 1. RFC 5321 Violation: The SMTP protocol strictly forbids Control Characters (CR, LF, Null) in the envelope address (Mailbox). RFC 5321 Section 4.1.2: A Mailbox consists of an Atom or Quoted-string. An Atom…

Published by the source

20 Jan 2026, 17:54 UTC

Last modified by the source

2 Feb 2026, 14:50 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-54wq-72mp-cq7c

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# Patch Next advisory brief: CVE-2026-23829

Advisory: Mailpit has an SMTP Header Injection via Regex Bypass
IDs: CVE-2026-23829, GHSA-54wq-72mp-cq7c
Record: https://patchnext.com/advisories/cve-2026-23829 (revision 1, source checked 26 Oct 2026, 15:14 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 1.28.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/axllent/mailpit: affected <= 1.28.2; fixed in 1.28.3

## 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://github.com/advisories/GHSA-54wq-72mp-cq7c (GitHub-reviewed advisory GHSA-54wq-72mp-cq7c lists affected ranges: Go github.com/axllent/mailpit >=0 <=1.28.2)

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