CVE-2020-17051: Remote kernel heap overflow in NFSv3 Windows Server

CVE-2020-17051: Remote kernel heap overflow in NFSv3 Windows Server

CVSS Score: 9.8 

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 

Overview 

Microsoft released a patch today for a critical vulnerability (CVE-2020-17051) in the Windows NFSv3 (Network File System) server. NFS is typically used in heterogenous environments of Windows and Unix/Linux for file sharing. The vulnerability can be reproduced to cause an immediate BSOD (Blue Screen of Death) within the nfssvr.sys driver. Interestingly, the November patches from Microsoft also include a remote kernel data read vulnerability in the same nfssvr.sys driver (CVE-2020-17056), which leads to a potential ASLR (address space layout randomizationbypass. The combination of these two vulnerabilities dramatically increases the likelihood of a remote exploit when used on Windows Server to bypass exploit mitigations.  CVE-2020-17051 is the first known vulnerability which has been disclosed within the Windows implementation of the NFSv3 protocol to the best of our knowledge.  

Threat Surface 

The vulnerability is believed to impact all versions of Windows Server when: 

  1. An authenticated user has write access to any NFS share. 
  2. An NFS share has been configured with anonymous write access (no authentication required) 

A Shodan query reported 38,893 servers with port 2049 exposed to the internet; however, it is unknown what percentage of these servers are actually NFS shares and actuallconfigured with anonymous write access. The network share discovery technique is typically used by an adversary within the discovery phase of the MITRE ATT&CK framework with the objective to gain further privileges. CVE-2020-17051 would give adversaries the ability to spread wormlike within heterogenous Windows and Unix/Linux environments using anonymous write access file shares over NFSv3. 

Mitigation 

Patching is always the first and most effective course of action. If it’s not possible to patch, the best mitigation is to limit Windows NFSv3 server share write access internally and block any external access to vulnerable servers. For those McAfee customers who are unable to deploy the Windows patch, the following Network Security Platform (NSP) signatures will provide a virtual patch against attempted exploitation of this vulnerability. 

NSP Attack ID: 0x40c01200 – NFS Microsoft Windows Network File System Remote Code Execution Vulnerability (CVE-2020-17051) 

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