Rule: 

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Sid: 356

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Summary:
This event is generated when an attempt to retrieve a specific file, in this case the systems user database from an FTP server is made.

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Impact: 
Serious. The attacker may obtain a valid list of user names and/or encrypted passwords from the server.

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Detailed Information:
This event is generated when an attempt to download a copy of the "passwd" file from the server is made.

The UNIX "passwd" file (typically located in "/etc/" directory) is used to hold the authentication information for system logins. This file needs to be readable by all system users. 

Where shadow passwords are used, the actual encrypted passwords are stored in a separate file, only readable by root. It is possible to use various password cracking tools to obtain unencrypted passwords either by trying random character combinations, a predefined word list or a combination of public user information. The attacker may use the information contained in the passwd file to launch a dictionary attack against the victim host or other hosts the same users may have access to.

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Attack Scenarios: 
The attacker downloads a "passwd" file from a machine that does not use shadowed passwords and uses a tool like John-the-Ripper to crack the passwords used for several accounts. He then proceeds to login to the system remotely and possibly gain escalated privileges via a local exploit on the system.

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Ease of Attack: 
Simple. The attack usually requires FTP access to the /etc/ directory either by system misconfiguration or via a directory traversal technique. Also, in the rare circumstances the system administrator may have accidentally left a copy of a "passwd" file in a directory accessible for anonymous or other FTP users, which presents a high security risk and simplifies the attack.

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False Positives: 
If the string "passwd" is contained within an otherwise innocuous filename being retrieved from a server, the rule will generate an event. 

Also, the anonymous FTP account often has a separate password file within the chrooted anonymous FTP directory (e.g. /var/ftp/etc/passwd). This file does not usually contain valid system usernames and passwords. While technically not a false positive, this may be considered a false alarm.

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False Negatives:
None Known

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Corrective Action: 
Identify the downloaded file and confirm that it indeed a valid system password file. Change the user passwords on the system and notify the users.

Ensure that FTP access to sensitive system files is not allowed.

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Contributors: 
Original rule writer Max Vision <vision@whitehats.com>
Snort documentation contributed by Anton Chuvakin <http://www.chuvakin.org>
Sourcefire Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>

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Additional References:

Arachnids:
http://www.whitehats.com/info/IDS319

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