Detect Microsoft Defender Link Following Privilege Escalation (CVE-2026-41091) in Sumo Logic CSE
Detects exploitation of CVE-2026-41091, a link-following vulnerability (CWE-59) in Microsoft Defender that allows attackers to follow symbolic links or junction points to access or overwrite privileged files. This vulnerability is actively exploited in the wild (CISA KEV) and can lead to privilege escalation or arbitrary file manipulation in the context of the Defender service.
MITRE ATT&CK
Sumo Detection Query
_sourceCategory=Windows/Security OR _sourceCategory=Windows/Sysmon
| where EventID in ("4656", "4663", "1", "4688", "11")
| where (
(toLowerCase(ObjectName) matches "*programdata\\microsoft\\windows defender*"
OR toLowerCase(ObjectName) matches "*program files\\windows defender*"
OR toLowerCase(TargetFilename) matches "*programdata\\microsoft\\windows defender*"
OR toLowerCase(TargetFilename) matches "*program files\\windows defender*")
AND (toLowerCase(ProcessName) matches "*cmd.exe*"
OR toLowerCase(ProcessName) matches "*powershell.exe*"
OR toLowerCase(ProcessName) matches "*wscript.exe*"
OR toLowerCase(ProcessName) matches "*rundll32.exe*")
)
OR (
(toLowerCase(CommandLine) matches "*mklink*" OR toLowerCase(CommandLine) matches "*junction*")
AND (toLowerCase(CommandLine) matches "*defender*" OR toLowerCase(CommandLine) matches "*windefend*")
)
| fields _messageTime, Computer, SubjectUserName, ProcessName, ObjectName, CommandLine, EventID
| count by Computer, SubjectUserName, ProcessName, EventID
| sort by _count descSumo Logic query detecting suspicious file system operations and symlink creation commands targeting Microsoft Defender directories as indicators of CVE-2026-41091 exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Microsoft Defender live protection service writing temporary files
- Authorized red team exercises on endpoints with documented scope
- Windows Update modifying Defender binaries during patching
- Enterprise endpoint management agents adjusting Defender configuration
Other platforms for CVE-2026-41091
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Create Symlink Targeting Defender Definitions Directory
Expected signal: Sysmon Event ID 11 with TargetFilename=C:\Users\Public\DefenderLink and Image=cmd.exe; Windows Security Event 4663 if object auditing is enabled for the Defender directory
- Test 2Junction Point Creation via mklink Referencing Defender Support Logs
Expected signal: Process creation event for cmd.exe with CommandLine containing 'mklink /J' and 'Windows Defender'; Sysmon Event ID 1 capturing the full command line
- Test 3PowerShell-Based Symlink Creation Targeting Defender Quarantine
Expected signal: Sysmon Event ID 1 with Image=powershell.exe and CommandLine referencing mklink and Windows Defender Quarantine path; parent-child chain of powershell.exe → cmd.exe visible in process tree
Unlock Pro Content
Get the full detection package for CVE-2026-41091 including response playbook, investigation guide, and atomic red team tests.