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CVE-2021-26887: Microsoft Windows Folder Redirection Elevation of Privilege Vulnerability

• Severity: medium (CVSS 4.6 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-11 · updated 2026-08-19
• Weakness: CWE-59

Description

An elevation of privilege vulnerability exists in Microsoft Windows when Folder redirection has been enabled via Group Policy. When folder redirection file server is co-located with Terminal server, an attacker who successfully exploited the vulnerability would be able to begin redirecting another user's personal data to a created folder. To exploit the vulnerability, an attacker can create a new folder under the Folder Redirection root path and create a junction on a newly created User folder. When the new user logs in, Folder Redirection would start redirecting to the folder and copying personal data. This elevation of privilege vulnerability can only be addressed by reconfiguring Folder Redirection with Offline files and restricting permissions, and NOT via a security update for affected Windows Servers. See the FAQ section of this CVE for configuration guidance.

Affected products

• Microsoft Windows 10 Version 1607: 20h2, 1607, 1803, 1809, 1909, 2004
• Microsoft Windows 7: all versions
• Microsoft Windows 8.1: all versions
• Microsoft Windows RT 8.1: all versions
• Microsoft Windows Server 2008: r2-sp1
• Microsoft Windows Server 2012: r2
• Microsoft Windows Server 2016: 20h2, 1909, 2004
• Microsoft Windows Server 2019: all versions
• Microsoft Windows 10 Version 1507: all versions
• Microsoft Windows 10 1607: all versions
• Microsoft Windows 10 1803: all versions
• Microsoft Windows 10: all versions
• Microsoft Windows 10 1909: all versions
• Microsoft Windows 10 2004: all versions
• Microsoft Windows 10 20H2: all versions
• Microsoft Windows 7: all versions
• Microsoft Windows 7 Service Pack 1: all versions
• Microsoft Windows 8.1: all versions
• Microsoft Windows Server 2008 R2 Service Pack 1: all versions
• Microsoft Windows Server 2008 Service Pack 2: all versions
• Microsoft Windows Server 2012: from 6.2.9200.0 before 6.2.9200.23298 — fixed in 6.2.9200.23298
• Microsoft Windows Server 2012 R2: all versions
• Microsoft Windows Server 2016: all versions
• Microsoft Windows Server 2019: all versions
• Microsoft Windows Server, version 1909 (Server Core installation): all versions
• Microsoft Windows Server version 2004: all versions
• Microsoft Windows Server version 20H2: all versions

How to fix

Upgrade to a fixed version: Microsoft Windows Server 2012 6.2.9200.23298. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26887
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26887

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