Webserver Indicators of Successful Exchange CVE-2021-28480 Exploitation via OWA Calendar POST

Flags POST requests to OWA calendar endpoint patterns linked to CVE-2021-28480, excluding HTTP 503 responses.

FreeReviewedSigma · Critical · v5

Category: webserver
Author: Florian Roth (Nextron Systems) (SigmaHQ), DRL 1.1
Published: 2021-05-14
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 flags webserver events consistent with exploitation of an Exchange vulnerability tied to CVE-2021-28480, using HTTP POST requests to an OWA calendar endpoint. Attackers may leverage this path to reach Exchange components and achieve unauthorized access, making these requests an important initial signal. The detection relies on webserver request telemetry including the request URI query, HTTP method, and HTTP status while excluding events with specific main-status indicators.

Reporting behind it

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Webserver Indicators of Successful Exchange CVE-2021-28480 Exploitation via OWA Calendar POST

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.

microsoft-exchange-exploitation-attempt-behavior-for-cve-2021-28480-via-owa-cale-a2a9d722

title: Webserver Indicators of Successful Exchange CVE-2021-28480 Exploitation via OWA Calendar POST
id: 166f749c-dea5-4a5b-9ef9-03df848780c2
status: test
description: This rule flags webserver events consistent with exploitation of an Exchange vulnerability tied to CVE-2021-28480, using HTTP POST requests to an OWA calendar endpoint. Attackers may leverage this path to reach Exchange components and achieve unauthorized access, making these requests an important initial signal. The detection relies on webserver request telemetry including the request URI query, HTTP method, and HTTP status while excluding events with specific main-status indicators.
references:
  - https://twitter.com/GossiTheDog/status/1392965209132871683?s=20
  - https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-28480/web_cve_2021_28480_exchange_exploit.yml
author: Florian Roth (Nextron Systems), Huntrule Team
date: 2021-05-14
modified: 2023-01-02
tags:
  - attack.initial-access
  - attack.t1190
  - cve.2021-28480
  - detection.emerging-threats
logsource:
  category: webserver
detection:
  selection:
    cs-uri-query|contains: /owa/calendar/a
    cs-method: POST
  filter_main_status:
    sc-status: 503
  condition: selection and not 1 of filter_*
falsepositives:
  - Unknown
level: critical
license: DRL-1.1
related:
  - id: a2a9d722-0acb-4096-bccc-daaf91a5037b
    type: derived

Convert

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