CVE-2021-43217: Windows Encrypting File System (EFS) Remote Code Execution Vulnerability

Overview

Severity
High (CVSS 8.1)
CVSS Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
Category
Remote Code Execution
Exploit Status
Not Exploited
Exploitation Likelihood
Less Likely
Patch Tuesday
2021-Dec
Released
2021-12-14
Last Updated
2021-12-15
EPSS Score
23.66% (percentile: 96.0%)

FAQ

What is the attack vector for this vulnerability? An attacker could cause a buffer overflow write leading to unauthenticated non-sandboxed code execution. Does EFS need to be in use for this to be exploited? No. EFS interfaces trigger a start to the EFS service if it isn’t already running. How does Microsoft plan to address this vulnerability? Microsoft is addressing the vulnerability in a phased two-part rollout. These updates address the vulnerability by modifying how EFS makes connections from client to server. For guidelines on how to manage the changes required for this vulnerability and more information on the phased rollout, see KB5009763: EFS security hardening changes in CVE-2021-43217. When the second phase of Windows updates become available in Q1 2022, customers will be notified via a revision to this security vulnerability. If you wish to be notified when these updates are released, we recommend that you register for the security notifications mailer to be alerted of content changes to this advisory. See Microsoft Technical Security Notifications. How do the two deployment phases address the vulnerability? The initial deployment phase starts with the Windows updates released on December 14, 2021. The updates will enable packet-level privacy for EFS when the client initiates a connection, and the server will only allow connections with packet-level privacy. The second phase, planned for a Q1 2022 release, marks the transition into the enforcement phase. Support for the AllowAllCliAuth registry key will be removed and servers will require packet-level privacy regardless of the registry key setting. Is there a recommended order in which I should install these updates on client and server machines? Yes. We recommend that you install the security updates to address this vulnerability on client machines first and then on servers.

Detection & Weaponization (1 sources)

Maturity: Exploit

  • GitHub PoC: 1 repositories

Affected Products (46)

Windows

  • Windows 10 Version 1809 for 32-bit Systems
  • Windows 10 Version 1809 for x64-based Systems
  • Windows 10 Version 1809 for ARM64-based Systems
  • Windows Server 2019
  • Windows Server 2019 (Server Core installation)
  • Windows 10 Version 1909 for 32-bit Systems
  • Windows 10 Version 1909 for x64-based Systems
  • Windows 10 Version 1909 for ARM64-based Systems
  • Windows 10 Version 21H1 for x64-based Systems
  • Windows 10 Version 21H1 for ARM64-based Systems
  • Windows 10 Version 21H1 for 32-bit Systems
  • Windows Server 2022
  • Windows Server 2022 (Server Core installation)
  • Windows 10 Version 2004 for 32-bit Systems
  • Windows 10 Version 2004 for ARM64-based Systems
  • Windows 10 Version 2004 for x64-based Systems
  • Windows Server, version 2004 (Server Core installation)
  • Windows 10 Version 20H2 for 32-bit Systems
  • Windows 10 Version 20H2 for ARM64-based Systems
  • Windows Server, version 20H2 (Server Core Installation)
  • Windows 11 version 21H2 for x64-based Systems
  • Windows 11 version 21H2 for ARM64-based Systems
  • Windows 10 Version 21H2 for 32-bit Systems
  • Windows 10 Version 21H2 for ARM64-based Systems
  • Windows 10 Version 21H2 for x64-based Systems
  • Windows 10 for 32-bit Systems
  • Windows 10 for x64-based Systems
  • Windows 10 Version 1607 for 32-bit Systems
  • Windows 10 Version 1607 for x64-based Systems
  • Windows Server 2016
  • Windows Server 2016 (Server Core installation)
  • Windows 8.1 for 32-bit systems
  • Windows 8.1 for x64-based systems
  • Windows RT 8.1

ESU

  • Windows 7 for 32-bit Systems Service Pack 1
  • Windows 7 for x64-based Systems Service Pack 1
  • Windows Server 2008 for 32-bit Systems Service Pack 2
  • Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation)
  • Windows Server 2008 for x64-based Systems Service Pack 2
  • Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation)
  • Windows Server 2008 R2 for x64-based Systems Service Pack 1
  • Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation)
  • Windows Server 2012
  • Windows Server 2012 (Server Core installation)
  • Windows Server 2012 R2
  • Windows Server 2012 R2 (Server Core installation)

Security Updates (15)

Acknowledgments

<a href="https://www.cyberkl.com/">Asuka</a> with <a href="https://www.cyberkl.com/">https://www.cyberkl.com/</a>

Revision History

  • 2021-12-14: Information published.
  • 2021-12-15: Added an FAQ to indicate the order in which the security updates to address this vulnerability should be installed on client and server machines. This is an informational change only.