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Manipulating FTP Clients Using the PASV Command
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SUMMARY
This paper discusses a common implementation flaw in the File Transfer
Protocol (FTP). Several popular FTP clients are affected including web
browsers. Some proof of concept code is presented to demonstrate how the
vulnerability can be used to extend existing JavaScript-based port scans.
Finally, some consideration is given to other ways in which this flaw
could present a security risk to other FTP clients.
DETAILS
Vulnerable FTP Clients:
The following web browsers have been found to respond to malformed PASV
responses in the way described above:
* Firefox 1.5.0.9
* Firefox 2.0.0.2
* Opera 9.10
* Konqueror 3.5.5
Several command line FTP clients have also been found to be vulnerable.
However as the vendors have not been notified (and the author cannot think
of an interesting way of exploiting command line clients), they have been
omitted from this paper.
Immune FTP Clients
The following web browsers seem to ignore the IP address returned in PASV
responses. They simply connect to the IP address to which the original
control connection (21/TCP) was made:
* Microsoft Internet Explorer 7.0.5730.11
* Microsoft Internet Explorer 6.0.3790.0
FTP Client Implementation Flaw
It is possible for malicious FTP servers to cause some popular FTP clients
to connect to TCP ports on other hosts. This allows us to extend existing
JavaScript-based port scan techniques [spi] in the follow ways:
* Scan ports which modern browsers would not normally connect to
[portban]
* Fingerprint services which do not send a banner by timing how long the
server takes to terminate the connection
* Perform simple banner grabbing to identify services running on other
hosts
Vendor Responses:
No response was provided by either Mozilla or Opera.
KDE responded and discussed both issues. However, they have yet to be
convinced of the severity of the FTP PASV Vulnerability. Unfortunately,
providing POC to demonstrate banner grabbing was made harder (impossible?)
by the crash during the reading of child FTP iframes. KDE have reproduced
the crash and produced a patch [konqcrash].
ADDITIONAL INFORMATION
The information has been provided by <mailto:mark@bindshell.net> mark.
The original article can be found at:
<http://bindshell.net/papers/ftppasv/ftp-client-pasv-manipulation.pdf>
http://bindshell.net/papers/ftppasv/ftp-client-pasv-manipulation.pdf
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