Detect Microsoft Defender Link Following Privilege Escalation (CVE-2026-41091) in IBM QRadar
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
QRadar Detection Query
SELECT
DATEFORMAT(devicetime, 'yyyy-MM-dd HH:mm:ss') AS event_time,
logsourcename(logsourceid) AS log_source,
"username",
"processname",
"filename",
"filepath",
"commandline",
CATEGORYNAME(category) AS event_category
FROM events
WHERE
LOGSOURCETYPENAME(devicetype) IN ('Microsoft Windows Security Event Log', 'Microsoft Sysmon')
AND (
(eventid IN (4656, 4663)
AND LOWER("filepath") MATCHES '.*\\(programdata\\microsoft\\windows defender|program files\\windows defender).*'
AND LOWER("processname") MATCHES '.*(cmd\.exe|powershell\.exe|wscript\.exe|cscript\.exe|mshta\.exe|rundll32\.exe).*')
OR
(eventid IN (1, 4688)
AND LOWER("commandline") MATCHES '.*(mklink|junction|symlink).*'
AND LOWER("commandline") MATCHES '.*(windows defender|windefend).*')
)
AND DATEFORMAT(devicetime, 'yyyy-MM-dd') >= DATEADD('day', -7, NOW())
ORDER BY devicetime DESC
LIMIT 500QRadar AQL query detecting file access events and symlink/junction commands targeting Microsoft Defender paths, consistent with CVE-2026-41091 link-following exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Scheduled Defender definition update tasks
- Microsoft Endpoint Configuration Manager managing Defender settings
- Helpdesk tools accessing Defender logs for troubleshooting
- Vulnerability scanners enumerating installed security software
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.