Microsoft Exchange ProxyToken Exploitation via ECP POST and InboxRules NewObject (CVE-2021-33766)

Flags POSTs to Exchange ECP InboxRules endpoints with SecurityToken= that return HTTP 500, indicating ProxyToken exploitation attempts.

FreeReviewedSigma · Critical · v5

Category: webserver
Author: Florian Roth (Nextron Systems), Max Altgelt (Nextron Systems), Christian Burkard (Nextron Systems) (SigmaHQ), DRL 1.1
Published: 2021-08-30
Updated: 2026-07-31

Analysis Detection

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 flags webserver requests that return HTTP 500 while targeting Microsoft Exchange ECP endpoints and creating inbox rules via RulesEditor. It matches POST activity to /ecp/ with /RulesEditor/InboxRules.svc/NewObject, including query strings containing SecurityToken=, which is indicative of attempted ProxyToken abuse. The detection relies on webserver request telemetry including HTTP method, URI query contents, and server response status codes.

Reporting behind it

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Microsoft Exchange ProxyToken Exploitation via ECP POST and InboxRules NewObject (CVE-2021-33766)

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.

webserver-telemetry-detects-exchange-proxytoken-exploitation-attempts-targeting--56973b50

title: Microsoft Exchange ProxyToken Exploitation via ECP POST and InboxRules NewObject (CVE-2021-33766)
id: 2100cd1b-49a1-49cf-8d57-ff5360977781
status: test
description: This rule flags webserver requests that return HTTP 500 while targeting Microsoft Exchange ECP endpoints and creating inbox rules via RulesEditor. It matches POST activity to /ecp/ with /RulesEditor/InboxRules.svc/NewObject, including query strings containing SecurityToken=, which is indicative of attempted ProxyToken abuse. The detection relies on webserver request telemetry including HTTP method, URI query contents, and server response status codes.
references:
  - https://www.zerodayinitiative.com/blog/2021/8/30/proxytoken-an-authentication-bypass-in-microsoft-exchange-server
  - https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-33766/web_cve_2021_33766_msexchange_proxytoken.yml
author: Florian Roth (Nextron Systems), Max Altgelt (Nextron Systems), Christian Burkard (Nextron Systems), Huntrule Team
date: 2021-08-30
modified: 2023-01-02
tags:
  - attack.initial-access
  - attack.t1190
  - cve.2021-33766
  - detection.emerging-threats
logsource:
  category: webserver
detection:
  selection_1:
    cs-method: POST
    cs-uri-query|contains|all:
      - /ecp/
      - /RulesEditor/InboxRules.svc/NewObject
    sc-status: 500
  selection_2:
    cs-uri-query|contains|all:
      - SecurityToken=
      - /ecp/
    sc-status: 500
  condition: 1 of selection_*
falsepositives:
  - Unknown
level: critical
license: DRL-1.1
related:
  - id: 56973b50-3382-4b56-bdf5-f51a3183797a
    type: derived

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