8/9/2019
01:08 PM
Larry Loeb
Larry Loeb
Larry Loeb

MSFT Walks Back Original Action, Admits RDP Vulnerability Affects Hyper-V

Check Point Research's Eyal Itkin told the world in February of this year that there were multiple critical vulnerabilities in the widely used RDP protocol.



Check Point Research's Eyal Itkin told the world in February of this year that there were multiple critical vulnerabilities in the widely used RDP protocol. RDP is a proprietary protocol developed by Microsoft and is usually used when a user wants to connect to a remote Windows machine.

Check Point started their investigation with open source RDP implementations, and found 11 vulnerabilities with a major security impact (19 vulnerabilities overall in the library) in the "rdesktop" v1.8.3 RDP client.

The FreeRDP 2.0.0-rc3 client performed a bit better, with five vulnerabilities with major security impact, as well as six vulnerabilities overall in the library.

Then then started looking at Mstsc.exe, which is Microsoft's RDP client. As they put it, "Soon enough, we realized that Microsoft's implementation is much better than the implementations we tested previously. Actually, it seems like Microsoft's code is better by several orders of magnitude."

But, what it did with the clipboard was interesting. The client doesn't verify the received blob that comes from the RDP server. And indeed, if the server includes a path traversal path of the form: ..\canary1.txt it was stored "as is" on the client's clipboard. Bingo. Client and server will share a clipboard, if that option is enabled. This means communication in the protocol between server and client is possible in a reverse manner through use of a shared clipboard, which could allow an attack vector for hijacking and remote code execution.

Microsoft's reply at the time to all of this was reported to be, "We determined your finding is valid but does not meet our bar for servicing. For more information, please see the Microsoft Security Servicing Criteria for Windows (https://aka.ms/windowscriteria)." Gee, that seems like a pretty gruff blowoff.

But -- aha -- it seems that the attack vector affects Hyper-V, the MSFT virtual machine controller. In fact, Dana Baril just posted a blog entry about the situation.

Baril says in it that "...because Hyper-V uses RDP, it inherits the security vulnerabilities in RDP. Hyper-V uses RDP behind the scenes for managing the VM, meaning that the vulnerability could be used to escape a Hyper-V VM, resulting in a guest-to-host sandbox escape vulnerability."

That's the sound of crows being eaten.

Microsoft Security Response Center (MSRC) worked with Check Point (they now say) to further investigate and address the vulnerability. The fix for CVE-2019-0887 was released as part of the July 2019 security update.

The icing on the cake is that CheckPoint and Microsoft had a Wednesday buddy session at this week's Black Hat convention in Las Vegas to give the "inside story." There was no word if Entertainment Tonight was going to be covering the story.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.

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