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Fencer Vulns
medium 5.3

CVE-2026-23829 json

Mailpit has SMTP Header Injection via Regex Bypass

published 2026-01-18 9 mo ago modified 2026-06-17 4 mo ago also EUVD-2026-3297 , GHSA-54wq-72mp-cq7c , GO-2026-4333

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.4%
EPSS 1.4% chance of exploitation in 30 days, percentile 72%
public exploits
0
none known, plus a Nuclei check

Summary

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.

Patch / fix

  • go github.com/axllent/mailpit 1.28.3
  • axllent mailpit 1.28.3
  • govulncheck-vulndb openSUSE Leap 15.6 0.0.20260205T172317-150000.1.146.1

suse advisories SUSE-SU-2026:0403-1

  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

CISA SSVC

Exploitation
Proof of concept is public
Automatable
yes
Technical impact
partial
Assessed
9 mo ago

Exploit prediction

1.40% chance of exploitation in the next 30 days, higher than 72% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • Nuclei template Mailpit < 1.28.2 - SMTP CRLF Injection nuclei detection template
  • github.com https://github.com/axllent/mailpit/security/advisories/GHSA-54wq-72mp-cq7c referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
GitHub 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected packages

ecosystem package affected fixed in
go github.com/axllent/mailpit
<1.28.3
1.28.3

Affected products

  • axllent mailpit

Classification

Weakness
  • CWE-93 Improper Neutralization of CRLF Sequences
  • CWE-150 Improper Neutralization of Escape, Meta, or Control Sequences

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.