Microsoft Flaw Demonstrates Dangers Of Remote Desktop Access

Microsoft's Patch Tuesday warning of possible attacks emerging quickly for a vulnerability in its Remote Desktop Protocol (RDP) is a chilling reminder of the potential dangers of desktop remote-access tools commonly used by IT departments to handle from afar help-desk issues and by administrators to manage virtualized machines.

The sole "critical" vulnerability among the updates in Microsoft's batch of patches yesterday was for a flaw in Microsoft's implementation of RDP, which gives one user remote access to another's desktop. Microsoft strongly urged users to patch this one (CVE-2012-002 in MS12-020) as soon as possible, or to apply the FixIt temporary fix that was also issued among the updates. Other security organizations, such as SANS, and vendors echoed Microsoft's warning that this particular patch should be a priority this month.

"Although Remote Desktop Protocol, RDP, is not enabled on the default configuration for any version of Windows, Microsoft strongly recommends that all customers prioritize the MS12-020 security update this month, in order to ensure that systems are protected," a Microsoft spokesperson said. Microsoft says an attacker could reverse-engineer its new patch for the RDP bug in relatively short order, which raises the potential for exploits to be written for a targeted attack or for automatic-propagation worms that would let attacks quickly take over systems within corporate networks for botnets or other purposes.

That's a scary scenario, security experts say. "We saw this two months ago with Symantec's pcAnywhere" remote-desktop tool code leak," says Wolfgang Kandek, CTO at Qualys. "All of these remote-access tools are great for productivity, but when you set them up and look at who connects and how you can restrict that," it's just username and password, which is weak, Kandek says.

Remote-desktop access tools are popular among organizations with branch offices and users who work at home: Rather than the old days where the IT guy flew to the branch office to physically work on the user's machine, the tools allow IT and security teams to work on users' machines from their own desktops. And this feature is used for managing virtualized servers and machines, including Amazon's EC2.

This particular vulnerability with Microsoft's RDP is a preauthentication one, so an attacker could get into the machine without even using credentials. "It seems rather easy to exploit, which is why they are urging [customers] to patch this as quickly as possible," Kandek says.

SANS Internet Storm Center handler Lenny Zeltser recommended that users move their RDP "listeners" to non-standard ports, rather than TCP port 3389, where it traditionally sits. "Until you install the patch, consider moving your RDP listeners to non-standard ports," Zeltser said in a SANS ISC Diary post.

Microsoft yesterday also offered up the FixIt tool for the vulnerability that lets users enable the Network Level Authentication (NLA) feature in RDP. NLA would require the attacker to authenticate to the server before trying to run an exploit, and ISC's Seltzer also recommends checking out NLA as an option.

[Remote VPN connections are not necessarily as secure as you'd think -- how enterprises can get infected by far-flung users via their SSL VPNs. See VPN An Oft-Forgotten Attack Vector. ]

Leaving RDP open basically increases your attack surface, says Rainer Enders, CTO Americas at NCP Engineering. "If you have it running, you have an active connection that can be attacked," he says. "And the way it's used, an app can be used from any machine."

Jason Miller, manager of research and development for VMware, says RDP is commonly used to connect to virtualized machines. "Although Microsoft is stating that most machines do not have RDP enabled by default, I know of many organizations that use RDP to troubleshoot machines," Miller said in a blog post. "This Windows component comes even more into play with machines that are not physically located next to users, such as virtualized machines."

He worries that the vulnerability could be exploited as a worm: "This bulletin simply scares me when it comes to protecting an environment from future attacks. This vulnerability has the real potential to become victim to a worm outbreak if this vulnerability is not patched," he said. "Although this vulnerability may be difficult to exploit, I can assure you attackers will be working hard to create a valid attack against the vulnerability."

So how can you use remote desktop access safely? Qualys' Kandek recommends adding another security control to the picture, including firewalls or first requiring a VPN log-in.

"The security battlefield today is the access device," NCP's Enders adds. "Administrators need to pay more attention [to these devices]."

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