CVE-2026-23829 PoC — Mailpit has SMTP Header Injection via Regex Bypass

Source

https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-23829.yaml

Associated Vulnerability

Title: Mailpit has SMTP Header Injection via Regex Bypass (CVE-2026-23829)
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.

Description

Mailpit < 1.28 contains a header injection caused by insufficient regex validation of `RCPT TO` and `MAIL FROM` addresses in the SMTP server, letting attackers inject arbitrary SMTP headers, exploit requires crafted email addresses

File Snapshot

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