Detect Microsoft Defender Link Following Privilege Escalation (CVE-2026-41091) in CrowdStrike LogScale
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
LogScale Detection Query
#event_simpleName IN (FileWritten, FileCreated, ProcessRollup2, SyntheticProcessRollup2)
| ParentBaseFileName IN ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe")
OR ImageFileName IN ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe")
| TargetDirectoryName IN ("*\\ProgramData\\Microsoft\\Windows Defender*", "*\\Program Files\\Windows Defender*")
OR (CommandLine LIKE "%mklink%" AND (CommandLine LIKE "%defender%" OR CommandLine LIKE "%windefend%"))
OR (CommandLine LIKE "%junction%" AND (CommandLine LIKE "%defender%" OR CommandLine LIKE "%windefend%"))
| eval risk=if(CommandLine LIKE "%mklink%" OR CommandLine LIKE "%junction%", "symlink_creation", "file_write_in_defender_path")
| stats count=count() earliest=min(timestamp) latest=max(timestamp) values(CommandLine) as commands values(TargetDirectoryName) as targets by aid, ComputerName, UserName, ImageFileName, risk
| where count >= 1
| sort - latestCrowdStrike Falcon query detecting file write operations and symlink/junction commands targeting Microsoft Defender directories, surfacing potential CVE-2026-41091 exploitation activity.
Data Sources
Required Tables
False Positives & Tuning
- Falcon sensor itself accessing Defender telemetry for co-existence checks
- Authorized penetration testers with documented CrowdStrike exclusions
- IT ops scripts that manage Defender exclusion lists via PowerShell
- SCCM client writing Defender policy updates to ProgramData
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
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