A penetration tester has compiled a list of potential employee usernames from a public code repository. The tester now needs to confirm which of these usernames correspond to valid email accounts on the target's mail server. Which of the following techniques provides the most direct method for validating these accounts against the mail server itself?
Enumerating Server Message Block (SMB) shares to find user home directories.
Searching public data breach dumps for passwords associated with company email addresses.
Scraping the organization's public staff directory for contact information.
Querying the Simple Mail Transfer Protocol (SMTP) service with commands such as VRFY or RCPT TO.
The most direct method for validating email accounts is to interact with the SMTP service. Specialized SMTP commands such as VRFY (Verify), EXPN (Expand), and RCPT TO (Recipient To) are designed to check for the existence of an email address on the server. Although many modern servers disable VRFY and EXPN for security purposes, attempting to use these commands is a direct enumeration technique. Scraping public directories is an OSINT method for gathering potential addresses, not for validating them against the server in real-time. Searching data breaches is a different reconnaissance technique that does not confirm if an account is currently active on the live server. Enumerating SMB shares may reveal system-level user accounts but does not directly confirm the existence of email mailboxes.
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