Detect CVE-2026-21510: Microsoft Windows Shell Protection Mechanism Failure in Google Chronicle
Detects exploitation of CVE-2026-21510, a Microsoft Windows Shell protection mechanism failure (CWE-693) that allows attackers to bypass security controls enforced by the Windows Shell. This vulnerability is actively exploited in the wild (CISA KEV). Attackers may abuse this flaw to execute unauthorized code, bypass security prompts, or escalate privileges via crafted shell interactions.
MITRE ATT&CK
YARA-L Detection Query
rule cve_2026_21510_windows_shell_bypass {
meta:
author = "df00tech"
description = "Detects CVE-2026-21510 Windows Shell protection mechanism failure exploitation"
severity = "HIGH"
priority = "HIGH"
reference = "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21510"
yara_version = "YL2.0"
rule_version = "1.0"
events:
$proc.metadata.event_type = "PROCESS_LAUNCH"
$proc.principal.hostname = $hostname
$proc.target.process.file.full_path = /(?i)(explorer\.exe|cmd\.exe|powershell\.exe|pwsh\.exe|wscript\.exe|cscript\.exe|mshta\.exe)$/
$proc.principal.process.file.full_path = /(?i)(winlogon\.exe|services\.exe|svchost\.exe|lsass\.exe|csrss\.exe)$/
condition:
$proc
}Chronicle YARA-L 2.0 rule detecting Windows Shell process launches from anomalous system-level parent processes, a key indicator of CVE-2026-21510 protection mechanism failure exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate system processes that occasionally spawn shell children during Windows maintenance tasks
- Remote desktop session initialization that may trigger winlogon.exe spawning explorer.exe
- Automated recovery mechanisms in enterprise environments restarting explorer.exe from service context
- Security scanning tools that enumerate processes by briefly spawning shell instances
Other platforms for CVE-2026-21510
Testing Methodology
Validate this detection against 4 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 1Simulate Shell Spawn from Winlogon Context
Expected signal: Sysmon Event ID 1: cmd.exe process with ParentImage path containing winlogon.exe or SYSTEM session identifier; Windows Security Event 4688 showing cmd.exe process creation under SYSTEM account.
- Test 2Explorer.exe Factory Flag Instantiation
Expected signal: Sysmon Event ID 1: explorer.exe process with CommandLine containing /factory and ParentImage of powershell.exe; corresponding network or registry activity from the new explorer instance.
- Test 3PowerShell Encoded Command via Shell Bypass Chain
Expected signal: Sysmon Event ID 1: powershell.exe with -EncodedCommand in CommandLine, parent cmd.exe; Sysmon Event ID 3: any outbound connection if payload includes network activity.
- Test 4Shell Bypass Followed by Discovery Commands
Expected signal: Sysmon Event IDs 1 for cmd.exe (parent: psexec/SYSTEM), then whoami.exe and ipconfig.exe as children within 30 seconds; all events share the same host identifier.
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