Which tool enumerates OS-level user accounts on Solaris via the SMTP service?

• A. RCPT TO
• B. VRFY
• C. EXPN
• D. smtp-user-enum

Answer: (D)

Enabling enumeration of OS-level user accounts over the SMTP service hinges on using a tool that automates probing the SMTP server with commands that reveal valid usernames. The smtp-user-enum tool is designed for this purpose: it connects to the target’s SMTP service and tests potential username strings using commands such as VRFY, EXPN, and RCPT TO, collecting any positive responses into a list of existing accounts. This automation is what makes it effective for discovering OS accounts on systems like Solaris, where local user accounts may be exposed through misconfigured or permissive SMTP responses.

The other items are standard SMTP commands themselves. While VRFY, EXPN, and RCPT TO can be used to probe for valid users, they are individual commands and don’t provide the automated, comprehensive enumeration capability and result aggregation that smtp-user-enum offers.