huntrule

Windows Exchange UMWorkerProcess File Drops Indicating CVE-2021-26858 Exploitation

Alerts on Exchange Unified Messaging (UMWorkerProcess.exe) creating unusual files, excluding common benign names consistent with CVE-2021-26858 activity.

FreeReviewedSigma · High · v5
Product
windows
Category
file_event
Author
Bhabesh Raj (SigmaHQ), DRL 1.1
Published
2021-03-03
Updated
2026-07-31
AnalysisDetection

ATT&CK techniques

Execution

  1. Recon

  2. Resource Dev

  3. Initial Access

  4. Execution

  5. Persistence

  6. Priv Esc

  7. Defense Evasion

  8. Cred Access

  9. Discovery

  10. Lateral Movement

  11. Collection

  12. C2

  13. Exfiltration

  14. Impact

What it detects

This rule flags file creation events where the process name ends with UMWorkerProcess.exe, which is associated with Exchange Unified Messaging. It filters out specific benign-looking target filenames and alerts on other non-standard files that could represent dropped web shells or other attacker content following exploitation. The detection relies on Windows file event telemetry that captures the writing process and the target filename.

Reporting behind it

Related detections9 linkedT1203 — drag to rearrange
Malicious Script Execution from WinRAR Extraction Directory via CVE-2023-38831
Suspicious Execution From WinRAR Temporary Extraction Path via Command Line (via process_creation)
Malicious Shell Execution via Foomatic-rip Print Filter through CUPS Exploit
Suspicious Network Connection From CUPS Foomatic-Rip Child Process
Antivirus signatures matching APT malware naming patterns
Antivirus Remote Access Tool Signature Matches Known RAT Names
Malicious Microsoft Word Spawning Anomalous Child Process via CVE-2023-36884 (via process_creation)
Possible ExifTool CVE-2026-3102 Command Injection via Child Process Spawn (via process_creation)
Malicious Equation Editor Child Process Execution via process_creation
Windows Exchange UMWorkerProcess File Drops Indicating CVE-2021-26858 Exploitation
Pivot detection · T1203 · 9 related

Changelog

v5
  1. v5
    Candidate ingested via manual entry.2026-07-31
  2. v4
    Candidate ingested via manual entry.2026-07-31
  3. v3
    Candidate ingested via manual entry.2026-07-31
  4. v2
    Candidate ingested via manual entry.2026-07-31
  5. v1
    No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.