Microsoft yesterday warned of a critical flaw affecting all versions of its operating systems bar Windows 98 and ME.
The critical vulnerability opens the way for crackers to run malicious code and take over vulnerable machines. The flaw affects Windows NT 4, NT 4 Terminal Edition, Win 2000, XP and Win 2003 (irrespective of any service pack applied). Redmond has issued a fix, which users are strongly urged to review.
The flaw, uncovered by Polish white hats Last Stage of Delirium (LSD), arises from a stack buffer overflow vulnerability within an integral component of Windows - an RPC interface implementing Distributed Component Object Model services (DCOM).
RPC provides an inter-process communication mechanism that allows a program running on one computer to seamlessly execute code on a remote system
Because of an implementation error in a function responsible for instantiation of DCOM objects, remote attackers can obtain remote access to vulnerable systems.
By sending specially crafted message to the TCP port 135 of vulnerable Windows system, an attacker can exploit the vulnerability and execute any code with system privileges. On intranets access to this port can easily be blocked by a properly configured firewall, but that still leaves enormous scope for mischief.
In an advisory LSD warns: "The impact of the vulnerability can be hardly overestimated. It affects every installation of the Windows NT/2K/XP/2003 operating system not protected by additional security mechanisms for access control, such as firewall systems.
"The vulnerability may also cause enormous harm if its exploitation would be conducted with the usage of even primitive worm technologies," it adds.
Members of the LSD Research Group were able to develop two fully functional proof of concept codes, respectively for Windows 2000/XP and Windows 2003 Server.
These exploits - which LSD has decided not to make public - work irrespective of the service pack a user might apply to the system. The vulnerability is also exploitable in the case of Windows 2003 Server, regardless of increased buffer overflow prevention mechanisms built into the latest version of Microsoft's OS. A noteworthy point, particularly when we remember how Microsoft went out its way to say the last Win 2003 patch that went out was really to fix an IE bug.
LSD warns: "Although exploitation of this vulnerability should not be considered as trivial, due to its potential impact, exploits codes from various sources may be expected in the wild very soon."
Microsoft advisory on the problem can be found here.
Wednesday night is, of course, patch issuing night for Microsoft. As well as a fix for the critical flaw detailed above, Redmond also posted fixes for two less serious vulnerabilities. These cover fixes for a flaw in ISA Server's error pages that could allow cross-site scripting attacks (advisory here) and an unchecked buffer in Windows Shell could enable system compromise of Win XP machines (advisory here).
Microsoft designates both fixes as important - and not critical - because although they might sound bad, the problems covered are in practice difficult to exploit. ®