While studying the infamous EternalBlue exploit about 2 months ago, researchers Sean Dillon (zerosum0x0) and Zach Harding (Aleph-Naught-) found a new flaw in the Server Message Block (SMB) protocol that could allow an adversary to interrupt the service by depleting the memory and CPU resources of the targeted machine on a Denial of Service (DoS) attack.

Tweet used to announce the flaw [2]

According to an article posted by ThreatPost [1], the flaw called SMBLoris was privately reported to Microsoft in early June, but the company considered it to be of moderate impact and that it would not be considered a security breach. In addition, it would probably not even be fixed.

As announced, some bug details were presented yesterday during a presentation at DEFCON 25 in Las Vegas. The attack is similar to another called SlowLoris [4] (hence also the similarity of the name) by allowing an attacker with a single machine and low bandwidth to be able to interrupt a service through a DoS attack. The difference is that SlowLoris affected Web servers.

Technically speaking, the problem occurs with the accumulation of a 4-bytes buffer called NBSS used during SMB session establishment which are allocated in the physical RAM and can not be swapped out. Triggering this, an attacker who initiates a large amount of connections to the service will be able to deplete the memory resources and after the CPU on the target.

NBSS buffer details

In the demonstration, an 8 GB memory server became unresponsive in a few seconds - note in the following figure the rapid increase in memory consumption during the attack.

SMBLoris attack demonstration

There is no update from Microsoft to fix the problem - so it has been considered a zero-day. For now, as a mitigation measure, the recommendation is to use a packet filter, like a Firewall, to limit the number of connections from a same source to the Windows servers on port 445 (SMB). 

References

[1] https://threatpost.com/windows-smb-zero-day-to-be-disclosed-during-def-con/126927/?utm_source=kasperskysocialchannel.com&utm_medium=REAL%20Solutions%20Technologies,%20LLC&utm_campaign=kasperskysocialchannel.com
[2] https://twitter.com/zerosum0x0/status/870862422327689216
[3] https://www.defcon.org/html/defcon-25/dc-25-speakers.html#Dillon
[4] https://web.archive.org/web/20090822001255/http://ha.ckers.org/slowloris/

--
Renato Marinho
Morphus Labs | LinkedIn | Twitter