CVE-2026-23829

Mailpit has SMTP Header Injection via Regex Bypass

Mailpit is an email testing tool and API for developers. Prior to version 1.28.3, 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. Version 1.28.3 fixes this issue.

Affected

axllent · mailpit

CVSS v3.1 GITHUB

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

EPSS

1.40% chance of exploitation in the next 30 days, 72nd percentile

Nuclei

medium · CWE-93

Published

2026-01-18

Updated

2026-01-20

Proof-of-concept exploits (3)

• advisories/GHSA-54wq-72mp-cq7c
• SimoesCTT/-CVE-2026-23829-CTT-Mailpit-phase-reconstruction-0★ · 2026-02-19
• SimoesCTT/CTT-Mailpit-RCE-v1.0---Temporal-Resonance-Mail-Server-Takeover0★ · 2026-01-28

Nuclei templates (1)

• projectdiscovery/nuclei-templates/blob/main/network/cves/2026/CVE-2026-23829.yaml

Security advisories

• axllent/mailpit/security/advisories/GHSA-54wq-72mp-cq7c
• axllent/mailpit/commit/36cc06c125954dec6673219dafa084e13cc14534

References

• CVE Record
• NVD entry

Related

• CVE-2014-0160
• CVE-2021-26855
• CVE-2022-0847
• CVE-2022-21820
• CVE-2022-22965
• CVE-2023-6246