Detect Microsoft Defender Link Following Privilege Escalation (CVE-2026-41091) in Elastic Security
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
Elastic Detection Query
sequence by host.name with maxspan=5m
[
process where event.type == "start"
and process.name in ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe")
and (
process.command_line like~ "*mklink*"
or process.command_line like~ "*junction*"
or process.command_line like~ "*symlink*"
)
]
[
file where event.type in ("creation", "change")
and (
file.path like~ "*\\ProgramData\\Microsoft\\Windows Defender\\*"
or file.path like~ "*\\Program Files\\Windows Defender\\*"
)
]EQL sequence detecting a suspicious process creating a symlink or junction immediately followed by file activity in Microsoft Defender directories, flagging potential CVE-2026-41091 exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Defender update service legitimately modifying its own directories
- Windows Installer processes during Defender component updates
- Security engineers testing Defender bypass techniques in authorized lab environments
- SCCM or Intune deploying Defender policy changes
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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