Threat Level: green Handler on Duty: Johannes Ullrich

SANS ISC: Windows 0-day SMB mrxsmb.dll vulnerability - SANS Internet Storm Center SANS ISC InfoSec Forums

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Windows 0-day SMB mrxsmb.dll vulnerability

A new vulnerability has been discovered exploiting SMB component of Windows. The attack involves sending of malformed Browser Election requests leading the heap overflow within the mrxsmb.dll driver. The vulnerability is known to be able to cause DoS and fully control of vulnerable machines. Proof of concept code for DoS had been released. There are reports that this exploit only work on local network segment (this hasn't been verified).

The general practice of block port 138, 139 and 445 should be observed especially with this 0-day.

More information on this exploit



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93 Posts
ISC Handler
Feb 16th 2011
I believe that the browser election uses UDP packets. Does this mean that UDP port 137 should be blocked as well?
The advisory doesn't mention it, but if you are blocking NetBIOS traffic, blocking 137/udp and 138/udp should definitely be part of that.

93 Posts
Secunia lists this vulnerability as Moderately Critical but the VUPEN lists it as Critical. Perhaps that is due to it affecting older operating systems? Anyone know if this is wormable?

12 Posts
Straining my language facilities here... "fully control of vulnerable machines" sounds "wormable" to me.

50 Posts
mrxsmb.dll <= i believe that you try to say mrxsmb.sys here if you are referring to BowserWriteErrorLogEntry vulnerability
1 Posts

Analysis from Microsoft suggests that Remote Code Execution is unlikely (Exploitability Index (XI) rating of ‘3’ – Functioning exploit code unlikely.):

3 Posts
So will disabling NetBIOS over TCP/IP or turning off the computer browser service do anything to mitigate against this?
6 Posts
It seems that MS's review is short sighted: if you can combine this and other vulnerabilities the attack would be 2-phased like I LOVE YOU worm. The simplest method - user opens attachment, running code, which kicks of the Master Browser PoC. The attack could even be aimed at the %logonserver%. Am I missing something that would make it any harder?

8 Posts
We have the browser service disabled on our Windows devices; are we insulated from this vulnerability? None of the write-ups I've seen say whether or not this is an effective workaround.

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