zakirkun SMTP Command Injection
Overview
SMTP command injection (distinct from email header injection) occurs when user input is placed directly in raw SMTP protocol commands. By injecting CRLF sequences, attackers can:
- Add additional RCPT TO commands (spam relay)
- Send emails to unintended recipients
- Inject arbitrary SMTP commands (VRFY, EXPN, DATA)
Remediation
- Use email library abstractions that handle SMTP protocol internally
- Strip CRLF characters from all email fields before SMTP transmission
- Validate sender/recipient addresses with strict RFC-compliant regex