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Upgrade to ProDetect CVE-2024-21413: Microsoft Outlook RCE via Moniker Link (MonikerLink) in Microsoft Sentinel
CVE-2024-21413 is a critical remote code execution vulnerability in Microsoft Outlook caused by improper input validation of hyperlinks using the 'file://' moniker protocol combined with an exclamation mark suffix. When a victim previews or opens a crafted email, Outlook follows the malicious link without the usual Protected View warning, leaking NTLM credentials via an outbound SMB connection and potentially enabling remote code execution. CVSS 9.8. Actively exploited in the wild (CISA KEV).
MITRE ATT&CK
KQL Detection Query
let moniker_pattern = @'(?i)file://[^\s]*![^\s]*';
union
(
EmailUrlInfo
| where Url matches regex moniker_pattern
| project TimeGenerated, NetworkMessageId, Url, UrlDomain, ReportId
| extend DetectionSource = 'EmailUrlInfo'
),
(
DeviceNetworkEvents
| where RemotePort == 445 or RemotePort == 139
| where InitiatingProcessFileName =~ 'OUTLOOK.EXE' or InitiatingProcessFileName =~ 'outlook.exe'
| project TimeGenerated, DeviceName, InitiatingProcessFileName, InitiatingProcessCommandLine, RemoteIP, RemotePort, LocalIP
| extend DetectionSource = 'OutlookSMBEgress'
),
(
DeviceProcessEvents
| where InitiatingProcessFileName =~ 'OUTLOOK.EXE'
| where FileName in~ ('cmd.exe','powershell.exe','wscript.exe','mshta.exe','rundll32.exe','regsvr32.exe')
| project TimeGenerated, DeviceName, InitiatingProcessFileName, FileName, ProcessCommandLine, AccountName
| extend DetectionSource = 'OutlookChildProcess'
)
| where TimeGenerated > ago(7d)
| sort by TimeGenerated descDetects CVE-2024-21413 MonikerLink exploitation patterns: malicious file:// moniker URLs in email, Outlook initiating outbound SMB connections (NTLM credential theft), and Outlook spawning suspicious child processes indicative of RCE.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate internal file:// links in emails referencing corporate UNC shares (common in some enterprise environments)
- Security scanning tools that follow URLs in emails for analysis may trigger the EmailUrlInfo rule
- Outlook accessing internal SMB shares for legitimate business file sharing workflows
- Software deployment or IT tooling that launches processes via Outlook integrations
Other platforms for CVE-2024-21413
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 1MonikerLink NTLM Credential Capture via Crafted Email
Expected signal: Windows Security Event ID 4776 on DC showing NTLM authentication from victim Outlook host to ATTACKER_IP. Sysmon Event ID 3 showing OUTLOOK.EXE network connection to ATTACKER_IP:445.
- Test 2Outlook Child Process Spawn via MonikerLink RCE Simulation
Expected signal: Sysmon Event ID 1 with ParentImage=OUTLOOK.EXE and Image=calc.exe (or substitute payload). DeviceProcessEvents in Defender ATP showing the process chain.
- Test 3MonikerLink Pattern Extraction and Email Header Analysis
Expected signal: Microsoft Defender for Office 365 EmailUrlInfo table entry with Url matching file://[IP]/[share]/[path]![suffix] pattern. Log ingestion into SIEM within 5-15 minutes.
- Test 4NTLM Hash Relay Attempt Post-Credential Capture
Expected signal: Windows Security Event ID 4624 (Type 3 network logon) on lab target using relayed credentials. Sysmon network events showing SMB connection from relay host to target. New process execution on target if relay succeeded.
References (4)
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