The pipeline character (|) is replaced with a blank space, and lesser-than and greater-than characters () are allowed only if they are used to define a mail address (> must be preceded by <). Mail addresses are scanned for strange characters. These commands specify the sender and receiver of the mail. Incorrect command termination (not terminated with ).SMTP inspection monitors the command and response sequence for the following anomalous signatures: Audit record 108002 is generated when an invalid character embedded in the mail address is replaced. Monitors the SMTP command-response sequence.Restricts SMTP requests to seven basic SMTP commands and eight extended commands.SMTP inspection performs three primary tasks: SMTP application inspection controls and reduces the commands that the user can use as well as the messages that the server returns. This results in a message such as "500 Command unknown: 'XXX'." Incomplete commands are discarded.Īn SMTP server responds to client requests with numeric reply codes and optional human-readable strings. Unsupported commands are translated into Xs, which are rejected by the internal server. Other extended SMTP commands, such as ATRN, ONEX, VERB, and CHUNKING, and private extensions are not supported. Along with the support for seven RFC 821 commands (DATA, HELO, MAIL, NOOP, QUIT, RCPT, and RSET), the ASA supports a total of 15 SMTP commands.
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It also provides support for application security and protocol conformance, which enforce the sanity of the SMTP messages and also detect several attacks, block senders/receivers, and block mail relay.ĮSMTP application inspection adds support for extended SMTP commands, including AUTH, EHLO, ETRN, HELP, SAML, SEND, SOML, STARTTLS, and VRFY. ESMTP Inspection on the Cisco Adaptive Security ApplianceĮxtended SMTP (ESMTP) application inspection provides improved protection against SMTP-based attacks by restricting the types of SMTP commands that can pass through the Cisco Adaptive Security Appliance (ASA) and by adding monitoring capabilities. This allows SMTP agents to protect some or all of their communications from eavesdroppers and attackers.įor more information on STARTTLS, see RFC 3207: SMTP Service Extension for Secure SMTP over Transport Layer Security. STARTTLS is an extension to the Simple Mail Transfer Protocol (SMTP) service that allows an SMTP server and client to use Transport Layer Security (TLS) to provide private, authenticated communication over the Internet.
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Interaction Between ASA ESMTP Inspection and STARTTLSĭisabling ESMTP Inspection on the Cisco ASA ESMTP Inspection on the Cisco Adaptive Security Appliance