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Vulnerability Note VU#634847XDMCP leaks sensitive information by default configurationOverviewAn information leakage vulnerability exists in the default configuration of the X Display Management Console Protocol (XDMCP) daemon.I. DescriptionOn some operating systems, the X Display Manager Control Protocol (XDMCP) daemon is set to permit remote access to the local machine from any host by default. Upon a request to connect, some XDMCP daemons show a graphical list of users authorized to log in to that machine. The user then selects their username and is prompted for a password. The information leakage occurs when a system displays the username selection screen to any XDMCP client.II. ImpactAn attacker may gain sensitive information about users permitted to login to the system. This may aid in brute-force attacks against the system.III. SolutionIf remote connections to the machine are not required, disable them to mitigate attacks.If disabling is not an option, modify the configuration file to permit remote connections from only authorized addresses. Note that this may not be sufficient to block attacks from hosts that use other methods such as IP address spoofing. In addition, implementing a firewall to permit access to the XDMCP port (177/UDP, may vary based on system) from only authorized sources on the network may also help mitigate the exploitation vulnerability.
* CHOOSER BROADCAST #any indirect host can get a chooser becomes
#* CHOOSER BROADCAST #any indirect host can get a chooser Systems Affected
References
Our thanks to ProCheckUp for the information provided in their security bulletin, and for bringing this vulnerability to our attention. This document was written by Jason Rafail.
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