CVE-2026-23829
MediumPublic exploitMailpit has an SMTP Header Injection via Regex Bypass
Published Jan 20, 2026 · Updated Aug 12, 2026 · Also known as GHSA-54wq-72mp-cq7c, GO-2026-4333
Key Facts
Routine patching- CVE-2026-23829 is a medium-severity vulnerability (CVSS 5.3 v3.1) affecting Mailpit (Axllent), github.com/axllent/mailpit (go).
- It is not listed in any known-exploited vulnerabilities catalog. Public exploit code is available.
- EPSS estimates a 1.4% probability of exploitation in the next 30 days.
- Fixed versions are available, including Mailpit 1.28.3, github.com/axllent/mailpit 1.28.3.
- Brandefense priority: routine patching, as no exploitation is known and its severity is moderate or low.
CVSS v3.1
5.3
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
EPSS (30-day exploit likelihood)
1.4%
Higher than 72.0% of all scored CVEs
Exploited in the wild
No known
Not in a known-exploited catalog
Public exploit code
Available
Proof-of-concept or weaponized module
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Description
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 Products
3 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| MailpitAxllent | >= < 1.28.3 | 1.28.3 |
| github.com/axllent/mailpitgo | See advisory | 1.28.3 |
Patched Releases
Mailpit
1.28.3
github.com/axllent/mailpit
1.28.3
Timeline
Publicly disclosed
Jan 20, 2026
Last updated
Aug 12, 2026
Weakness Type
- CWE-93 Improper Neutralization of CRLF Sequences ('CRLF Injection')
- CWE-150 Improper Neutralization of Escape, Meta, or Control Sequences
References
- advisoryGHSA advisory
- advisoryhttps://github.com/CVEProject/cvelistV5/tree/main/cves/2026/23xxx/CVE-2026-23829.json
- advisoryhttps://nvd.nist.gov/vuln/detail/CVE-2026-23829
- articlehttps://github.com/axllent/mailpit/commit/36cc06c125954dec6673219dafa084e13cc14534
- articlehttps://github.com/axllent/mailpit/releases/tag/v1.28.3
FAQ
Common Questions.
Mailpit is an email testing tool and API for developers.
Affected: Mailpit (Axllent) versions >= < 1.28.3; github.com/axllent/mailpit (go). The full list is in the CVE brief.
It is not listed in any known-exploited vulnerabilities catalog.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
Public exploit code is available.
Fixed versions are available, including Mailpit 1.28.3, github.com/axllent/mailpit 1.28.3.
Brandefense priority: routine patching, as no exploitation is known and its severity is moderate or low.
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