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A vulnerability involving an obscure UDP protocol might permits crackers to obtain remote control of Unix workstations, security experts have warned.

Security firm ProCheckUp has issued an advisory warning that anonymous XDMCP connections allow remote attackers to obtain a remote console identical to a local X-Windows session, using a command enabled by default on most Unix boxes.

Richard Brain, ProCheckUp's technical director, described the flaw as a "Back Orifice
for Unix" because it might permit anonymous attackers to shut down or gain remote control of a victim's machine. This seems slightly overstated because there are a number of mitigating factors.

To exploit the vulnerability an attacker would still need to guess the correct X-Windows password, but given the lame passwords many users pick this is hardly a high enough barrier.

A properly configured firewall will block the UDP session but that too won't protect everyone - particularly when you consider a tool that scans for XDMCP traffic has become available on the Internet.

Worrying.

Because of the existence of this utility ProCheckUp has released a notice on the vulnerability ahead of security patches from vendors or complete testing on all platforms the flaw might affect. It seems the vulnerability has existed for some time but was dismissed because of assumptions about hardware, software and the possibilities of hacking them, that have now been called into question.

This flaw appears to affect all versions of Sun Solaris and versions of Linux Mandrake up to 8.1, though Red Hat Linux is believed to be immune to that attack. The jury is out on IBM's AIX and HP/UX.

Admins can guard against the problem by implementing workarounds such as disabling remote connection and blocking traffic on ports used by the XDMCP protocol. This is explained in more detail in ProCheckUp's advisory here. ®

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