Detect Microsoft Defender Link Following Privilege Escalation (CVE-2026-41091) in Google Chronicle
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
YARA-L Detection Query
rule cve_2026_41091_defender_link_following {
meta:
author = "df00tech Detection Engineering"
description = "Detects exploitation of CVE-2026-41091 Microsoft Defender link-following vulnerability"
severity = "HIGH"
priority = "HIGH"
reference = "https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2026-41091"
events:
(
$e1.metadata.event_type = "PROCESS_LAUNCH"
AND (
re.regex($e1.principal.process.command_line, `(?i)(mklink|junction|symlink)`)
AND re.regex($e1.principal.process.command_line, `(?i)(windows.defender|windefend|msmpeng)`)
)
AND re.regex($e1.target.process.file.full_path, `(?i)(cmd\.exe|powershell\.exe|wscript\.exe|rundll32\.exe)`)
)
OR
(
$e1.metadata.event_type = "FILE_CREATION"
AND re.regex($e1.target.file.full_path, `(?i)(ProgramData\\Microsoft\\Windows Defender|Program Files\\Windows Defender)`)
AND re.regex($e1.principal.process.file.full_path, `(?i)(cmd\.exe|powershell\.exe|wscript\.exe|mshta\.exe)`)
)
condition:
$e1
}Chronicle YARA-L rule detecting symlink/junction creation targeting Microsoft Defender paths or suspicious process file creation in Defender directories, indicating CVE-2026-41091 link-following exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Defender exclusion management by IT administrators using command-line tools
- Legitimate junction points created by storage tiering software
- Windows servicing stack operations during major OS updates
- Security product integrations that inspect Defender telemetry directories
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
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