CVE-2026-23829 Mailpit has SMTP Header Injection via Regex Bypass
Attackers can inject arbitrary SMTP headers or corrupt existing ones in Mailpit by exploiting a regex bypass in address validation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23829?
CVE-2026-23829 is a medium-severity vulnerability in axllent mailpit, published on 18 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-23829?
3 open source packages ship an affected version, including github.com/axllent/mailpit. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/axllent/mailpit Go | All versions | 1.28.3 |
github.com/axllent/mailpit Go | All versions | 1.28.3 |
github.com/axllent/mailpit Go | All versions | 1.28.3 |
Products include Axllent Mailpit.
How do I fix CVE-2026-23829?
Upgrade github.com/axllent/mailpit to 1.28.3 or later.
Is CVE-2026-23829 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-23829 exists.
Public exploit code: 1 Nuclei template and 2 proofs of concept on GitHub.
Other vulnerabilities in axllent mailpit
- CVE-2026-45713 Mailpit: Unauthenticated remote memory-exhaustion DoS via unlimited SMTP DATA and /api/v1/send body sizes
- CVE-2026-22689 Mailpit is vulnerable to Cross-Site WebSocket Hijacking (CSWSH) allowing unauthenticated access to emails
- CVE-2026-45711 Mailpit: Path traversal & arbitrary file write in mailpit dump --http via attacker-controlled message IDs
- CVE-2026-45712 Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write)
- CVE-2026-45709 Mailpit has an incomplete fix for GHSA-6jxm: HTML check still permits SSRF to private/loopback/IMDS via missing IP-filter dialer
Sources
Also known as GHSA-54wq-72mp-cq7c, GO-2026-4333.