Potential CVE-2021-26857 Exploitation Attempt

Status: stable
Severity: high
Log source: category process_creation, product windows
Author: Bhabesh Raj
Source: github.com/SigmaHQ/sigma

Detects possible successful exploitation for vulnerability described in CVE-2021-26857 by looking for | abnormal subprocesses spawning by Exchange Server's Unified Messaging service

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1203` Exploitation for Client Execution

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation
Security-Auditing	Event ID 4688: A new process has been created.

Rule body

title: Potential CVE-2021-26857 Exploitation Attempt
id: cd479ccc-d8f0-4c66-ba7d-e06286f3f887
status: stable
description: Detects possible successful exploitation for vulnerability described in CVE-2021-26857 by looking for | abnormal subprocesses spawning by Exchange Server's Unified Messaging service
references:
    - https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers/
author: Bhabesh Raj
date: 2021-03-03
modified: 2023-02-07
tags:
    - attack.t1203
    - attack.execution
    - cve.2021-26857
    - detection.emerging-threats
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        ParentImage|endswith: '\UMWorkerProcess.exe'
    filter:
        Image|endswith:
            - 'wermgr.exe'
            - 'WerFault.exe'
    condition: selection and not filter
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection and not filter

Stage 1: `selection`

selection:
    ParentImage|endswith: '\UMWorkerProcess.exe'

Stage 2: `not filter`

filter:
    Image|endswith:
        - 'wermgr.exe'
        - 'WerFault.exe'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`Image`	ends_with	`WerFault.exe`	`excludes:Image` `field:"Image" value:"WerFault.exe"`
`Image`	ends_with	`wermgr.exe`	`excludes:Image` `field:"Image" value:"wermgr.exe"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`ParentImage`	ends_with	`\UMWorkerProcess.exe` corpus 2 (sigma 2)	`field:"ParentImage" kind:ends_with value:"\UMWorkerProcess.exe"`