Sigma rule: CVE-2021-26858 Exchange Exploitation

A detection rule from the SigmaHQ repository, shown as its author published it. Dataset version ref-65aa93b9b1ed, updated 2022-10-09T00:00:00.000Z.

Rule

Title
CVE-2021-26858 Exchange Exploitation
Status
test
Level
high
Author
Bhabesh Raj
Date
2021-03-03
Modified
2022-10-09
File
rules-emerging-threats/2021/Exploits/CVE-2021-26858/file_event_win_cve_2021_26858_msexchange.yml
Commit
ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects possible successful exploitation for vulnerability described in CVE-2021-26858 by looking for
creation of non-standard files on disk by Exchange Server’s Unified Messaging service
which could indicate dropping web shells or other malicious content

Log source

product
windows
category
file_event

Detection

selection:
    Image|endswith: 'UMWorkerProcess.exe'
filter:
    TargetFilename|endswith:
        - 'CacheCleanup.bin'
        - '.txt'
        - '.LOG'
        - '.cfg'
        - 'cleanup.bin'
condition: selection and not filter

False positives (as the rule lists them)

  • Unknown

Tags

  • attack.t1203
  • attack.execution
  • cve.2021-26858
  • detection.emerging-threats

CVEs this rule names

MITRE ATT&CK ids this rule names

References

Read on 2026-11-03T00:58:21.399Z under licence sigma-drl-1.1@d5f4c36a from https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules-emerging-threats/2021/Exploits/CVE-2021-26858/file_event_win_cve_2021_26858_msexchange.yml

Sources and licences

  • SigmaHQ

    "CVE-2021-26858 Exchange Exploitation" by Bhabesh Raj (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules-emerging-threats/2021/Exploits/CVE-2021-26858/file_event_win_cve_2021_26858_msexchange.yml.
    Licensed under the Detection Rule License (DRL) 1.1: https://github.com/SigmaHQ/Detection-Rule-License

    Detection Rule License (DRL) 1.1

Cite this page

Dragons Eye Ransomware Tracker, "Sigma rule: CVE-2021-26858 Exchange Exploitation",
entity page, dataset version ref-65aa93b9b1ed, updated 2022-10-09T00:00:00.000Z,
https://ransomware-tracker.com/detections/sigma/b06335b3-55ac-4b41-937e-16b7f5d57dfd (accessed 2026-11-03).
Underlying source: SigmaHQ rule b06335b3-55ac-4b41-937e-16b7f5d57dfd, commit ccbb0369507f796db0b0ae6854c8a2be471b9d84.

Pages change. The dataset version identifies the data, but no archive of old versions is kept, so this citation cannot be replayed later.