FTP bounce attack is anexploit of theFTP protocol whereby an attacker is able to use thePORT command to request access toports indirectly through the use of the victim machine, which serves as a proxy for the request, similar to anOpen mail relay usingSMTP.^[1]

This technique can be used toport scan hosts discreetly, and to potentially bypass a network'saccess-control list to access specific ports that the attacker cannot access through a direct connection, for example with thenmapport scanner.^[2]

Nearly all modern FTP server programs are configured by default to refusePORT commands that would connect to any host but the originating host, thwarting FTP bounce attacks.

• Confused deputy problem

• CERT Advisory on FTP Bounce Attack
• CERT Article on FTP Bounce Attack
• Original posting describing the attack

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