SMTP and Extended SMTP Inspection
ESMTP application inspection provides improved protection against SMTP-based attacks by restricting the types of SMTP commands that can pass through the security appliance and by adding monitoring capabilities.
SMTP and Extended SMTP Inspection ■
ESMTP is an enhancement to the SMTP protocol and is similar is most respects to SMTP. For convenience, the term SMTP is used in this document to refer to both SMTP and ESMTP. The application inspection process for extended SMTP is similar to SMTP application inspection and includes support for SMTP sessions. Most commands used in an extended SMTP session are the same as those used in an SMTP session but an ESMTP session is considerably faster and offers more options related to reliability and security, such as delivery status notification.
Extended SMTP application inspection adds support for eight extended SMTP commands, including AUTH, EHLO, ETRN, HELP, SAML, SEND, SOML and VRFY. Along with the support for seven RFC 821 commands (DATA, HELO, MAIL, NOOP, QUIT, RCPT, RSET), the security appliance supports a total of fifteen SMTP commands.
Other extended SMTP commands, such as ATRN, STARTLS, ONEX, VERB, CHUNKING, and private extensions and are not supported. Unsupported commands are translated into Xs, which are rejected by the internal server. This results in a message such as "500 Command unknown: 'XXX'." Incomplete commands are discarded.
The ESMTP inspection engine changes the characters in the server SMTP banner to asterisks except for the "2", "0", "0" characters. Carriage return (CR) and linefeed (LF) characters are ignored.
With SMTP inspection enabled, a Telnet session used for interactive SMTP may hang if the following rules are not observed: SMTP commands must be at least four characters in length; must be terminated with carriage return and line feed; and must wait for a response before issuing the next reply.
An SMTP server responds to client requests with numeric reply codes and optional human-readable strings. SMTP application inspection controls and reduces the commands that the user can use as well as the messages that the server returns. SMTP inspection performs three primary tasks:
• Restricts SMTP requests to seven basic SMTP commands and eight extended commands.
• Monitors the SMTP command-response sequence.
• Generates an audit trail—Audit record 108002 is generated when invalid character embedded in t he mail address is replaced. For more information, see RFC 821.
SMTP inspection monitors the command and response sequence for the following anomalous signatures:
• Truncated commands.
• Incorrect command termination (not terminated with <CR><LR>).
• The MAIL and RCPT commands specify who are the sender and the receiver of the mail. Mail addresses are scanned for strange characters. The pipeline character (I) is deleted (changed to a blank space) and "<" ,">" are only allowed if they are used to define a mail address (">" must be preceded by "<").
• Unexpected transition by the SMTP server.
• For unknown commands, the security appliance changes all the characters in the packet to X. In this case, the server generates an error code to the client. Because of the change in the packed, the TCP checksum has to be recalculated or adjusted.
• TCP stream editing.
• Command pipelining.
Continue reading here: SQLNet Inspection
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