Exchange ProxyLogon activity: IIS POST SetObject Reset VirtualDirectory requests

Alerts on successful POSTs to ECP DDIService SetObject resetting a VirtualDirectory with a '$' username suffix.

FreeReviewedSigma · Critical · v5

Category: webserver
Author: frack113 (SigmaHQ), DRL 1.1
Published: 2021-08-10
Updated: 2026-07-31

ATT&CK techniques

Initial Access

Recon
Resource Dev
Initial Access
- T1190Exploit Public-Facing Application
Execution
Persistence
Priv Esc
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule matches HTTP POST requests returning status 200 that target the /ecp/DDI/DDIService.svc/SetObject endpoint with query parameters indicating schema=Reset and VirtualDirectory. It also looks for usernames ending with '$', which can be associated with certain automated behaviors during exploitation attempts. Telemetry relies on webserver HTTP fields including request method, status code, URI stem, URI query contents, and authenticated username where available.

Reporting behind it

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Exchange ProxyLogon activity: IIS POST SetObject Reset VirtualDirectory requests

Pivot detection · T1190 · 9 related

Changelog

v5
Candidate ingested via manual entry.2026-07-31
v4
Candidate ingested via manual entry.2026-07-31
v3
Candidate ingested via manual entry.2026-07-31
v2
Candidate ingested via manual entry.2026-07-31
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.

iis-webserver-ssrf-proxylogon-reset-virtual-directory-via-ecp-setobject-post-effee1f6

title: "Exchange ProxyLogon activity: IIS POST SetObject Reset VirtualDirectory requests"
id: d0308e86-9088-4fd2-a7e6-ad04312d79a7
status: test
description: This rule matches HTTP POST requests returning status 200 that target the /ecp/DDI/DDIService.svc/SetObject endpoint with query parameters indicating schema=Reset and VirtualDirectory. It also looks for usernames ending with '$', which can be associated with certain automated behaviors during exploitation attempts. Telemetry relies on webserver HTTP fields including request method, status code, URI stem, URI query contents, and authenticated username where available.
references:
  - https://bi-zone.medium.com/hunting-down-ms-exchange-attacks-part-1-proxylogon-cve-2021-26855-26858-27065-26857-6e885c5f197c
  - https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-26858/web_cve_2021_26858_iis_rce.yml
author: frack113, Huntrule Team
date: 2021-08-10
modified: 2023-05-08
tags:
  - cve.2021-26858
  - detection.emerging-threats
  - attack.initial-access
  - attack.t1190
logsource:
  category: webserver
  definition: "Requirements: The POST request body data must be collected in order to make use of this detection"
detection:
  selection:
    cs-method: POST
    sc-status: 200
    cs-uri-stem: /ecp/DDI/DDIService.svc/SetObject
    cs-uri-query|contains|all:
      - schema=Reset
      - VirtualDirectory
    cs-username|endswith: $
  keywords:
    "|all":
      - POST
      - 200
      - /ecp/DDI/DDIService.svc/SetObject
      - schema=Reset
      - VirtualDirectory
      - $
  condition: selection or keywords
falsepositives:
  - Unlikely
level: critical
license: DRL-1.1
related:
  - id: effee1f6-a932-4297-a81f-acb44064fa3a
    type: derived

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