Attacker Value
Moderate
(1 user assessed)
Exploitability
Very High
(1 user assessed)
User Interaction
None
Privileges Required
Low
Attack Vector
Local
1

CVE-2021-26431

Disclosure Date: August 12, 2021
Add MITRE ATT&CK tactics and techniques that apply to this CVE.

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1
Ratings
Technical Analysis

Contrary to what the title says this is not a elevation of privilege vulnerability. As noted at https://halove23.blogspot.com/2021/09/zdi-21-1053-bypassing-windows-lock.html and at https://www.zerodayinitiative.com/advisories/ZDI-21-1053/ this is actually an authentication bypass vulnerability that allows one to bypass the Windows lockscreen by taking advantage of a URL link within the page.

This link appears when one tries to log into an Microsoft connected account and hits the “I Forgot My PIN” link, attempts to sign in to a Microsoft account with an invalid password, hits the back link that appears at the top left of the page, then at the sign on page, hits the question mark icon that appears next to “Sign in with a security key”,

This will display a dialog with the title “Sign in with security key” that has a URL link called “Learn how to set this up”. Clicking on this link after enabling Narrator shows that this link actually opens an “How do you want to open this?” prompt. However this prompt is hidden which is why Narrator is used to tell us which elements we are hovered over.

As mentioned in https://halove23.blogspot.com/2021/09/zdi-21-1053-bypassing-windows-lock.html, this can then be used to open Edge, and then via Edge open the Settings window, at which point we can then open Explorer, and finally open a command window where we can execute arbitrary commands.

The risk of this vulnerability overall is still somewhat low. Due to the number of steps which must be taken using soely Narrator, unless someone automates the key presses (something that is entirely possible) you will still have to do a lot of listening and keypressing to get the vulnerability to work, and any one listening in to you attacking the PC will likely find it rather odd what you are doing.

Additionally you only gain privileges as a local user. This vulnerability does not grant you permissions as a administrative user or any privileged user on the system, and whilst you can use other EOP vulnerabilities that take advantage of the NT AUTHORITY\Authenticated Users the user will be granted, you are still reliant on other EoP vulnerabilities not being patched on the target system for you to get SYSTEM level access.

Overall this is a medium-low severity bug with a high degree of exploitability.

CVSS V3 Severity and Metrics
Base Score:
7.8 High
Impact Score:
5.9
Exploitability Score:
1.8
Vector:
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Attack Vector (AV):
Local
Attack Complexity (AC):
Low
Privileges Required (PR):
Low
User Interaction (UI):
None
Scope (S):
Unchanged
Confidentiality (C):
High
Integrity (I):
High
Availability (A):
High

General Information

Vendors

  • Microsoft

Products

  • 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 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 x64-based Systems,
  • Windows 10 Version 20H2 for 32-bit Systems,
  • Windows 10 Version 20H2 for ARM64-based Systems,
  • Windows Server, version 20H2 (Server Core Installation)

Additional Info

Technical Analysis