Re: iDEFENSE Security Advisory 02.25.05: WU-FTPD File Globbing Denial of Service Vulnerability

[Stan Bubrouski <stan () ccs neu edu>]

Okay this is getting really old.

This is a problem with almost any service that allows multiple
wild-card characters.  UW-IMAPD is another example where this
type of exploit works and has been discussed in great detail
in the past.  The only difference here is that the ftpd doesn't
allow a compile time limit to the number of wildcards.

The attack specifically requires that the ftproot has a rather
large directory structure or else the wildcards are parsed
quickly (I.E. the ftp server must be huge).  If this is the case
than you shouldn't be allowing wildcards AT ALL, regardless of
what kind of service it is.  Aside from that who the hell uses
wu-ftpd for large sites anymore?  It's performance without
wildcards is pretty dismal to start with.

-sb


iDEFENSE Labs wrote:
> WU-FTPD File Globbing Denial of Service Vulnerability
>
> iDEFENSE Security Advisory 02.25.05
> www.idefense.com/application/poi/display?id=207&type=vulnerabilities
> February 25, 2005
>
> I. BACKGROUND
>
> WU-FTPD is an ftp daemon for Unix systems developed at Washington 
> University. More information is available at:
>
>     http://www.wu-ftpd.org/
>
> II. DESCRIPTION
>
> Remote exploitation of an input validation vulnerability in version
> 2.6.2 of WU-FPTD could allow for a denial of service of the system by 
> resource exhaustion.
>
> The vulnerability specifically exists in the wu_fnmatch() function in
> wu_fnmatch.c. When a pattern containing a '*' character is supplied as 
> input, the function calls itself recursively on a smaller substring. By 
> supplying a string which contains a large number of '*' characters, the 
> system will take a long time to return the results, during which time it
>
> will be using a large amount of CPU time.
>
> III. ANALYSIS
>
> After a user logs into the ftpd, an attacker can send a simple command 
> which will cause high CPU utilization.
>
> To exploit this vulnerability, a simple ftp client is sufficient. Once
> logged  in, either anonymously or as an authenticated user, issuing the
> following command will cause the machine to become less responsive.
>
> ftp> dir ***************************************************************
>          ***************************************************************
>          ***************************************************************
>          **.*
>
> By re-connecting and issuing the command multiple times, the system can
> be made completely unresponsive. This may prevent legitimate access to 
> services provided by the system for the period of the attack.
>
> IV. DETECTION
>
> iDEFENSE has confirmed the existence of this vulnerability in version 
> 2.6.2 and 2.6.1 of WU-FTPD. It is suspected that previous versions are 
> also affected by this vulnerability.
>
> V. WORKAROUND
> Consider disabling the ftpd. If this is not viable as an option, 
> consider disabling anonymous access. Disabling anonymous access will not
>
> prevent local users from exploiting this vulnerability.
>
> VI. VENDOR RESPONSE
>
> No vendor response received.
>
> VII. CVE INFORMATION
>
> The Common Vulnerabilities and Exposures (CVE) project has assigned the
> names CAN-2005-0256 to these issues. This is a candidate for inclusion
> in the CVE list (http://cve.mitre.org), which standardizes names for
> security problems.
>
> VIII. DISCLOSURE TIMELINE
>
> 02/09/2005  Initial vendor notification - No response
> 02/18/2005  Initial vendor notification - No response
> 02/25/2005  Public disclosure
>
> IX. CREDIT
>
> Adam Zabrocki (pi3 / pi3ki31ny) is credited with this discovery.
>
> Get paid for vulnerability research
> http://www.idefense.com/poi/teams/vcp.jsp
>
> Free tools, research and upcoming events
> http://labs.idefense.com
>
> X. LEGAL NOTICES
>
> Copyright (c) 2005 iDEFENSE, Inc.
>
> Permission is granted for the redistribution of this alert
> electronically. It may not be edited in any way without the express
> written consent of iDEFENSE. If you wish to reprint the whole or any
> part of this alert in any other medium other than electronically, please
> email customerservice () idefense com for permission.
>
> Disclaimer: The information in the advisory is believed to be accurate
> at the time of publishing based on currently available information. Use
> of the information constitutes acceptance for use in an AS IS condition.
> There are no warranties with regard to this information. Neither the
> author nor the publisher accepts any liability for any direct, indirect,
> or consequential loss or damage arising from use of, or reliance on,
> this information.