Suspicious MSExchangeMailboxReplication ASPX Write

Status: test
Severity: high
Log source: category file_event, product windows
Author: Florian Roth (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects suspicious activity in which the MSExchangeMailboxReplication process writes .asp and .apsx files to disk, which could be a sign of ProxyShell exploitation

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1190` Exploit Public-Facing Application
Persistence	`T1505.003` Server Software Component: Web Shell

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 11: FileCreate

Rule body

title: Suspicious MSExchangeMailboxReplication ASPX Write
id: 7280c9f3-a5af-45d0-916a-bc01cb4151c9
status: test
description: Detects suspicious activity in which the MSExchangeMailboxReplication process writes .asp and .apsx files to disk, which could be a sign of ProxyShell exploitation
references:
    - https://redcanary.com/blog/blackbyte-ransomware/
author: Florian Roth (Nextron Systems)
date: 2022-02-25
tags:
    - attack.initial-access
    - attack.t1190
    - attack.persistence
    - attack.t1505.003
logsource:
    product: windows
    category: file_event
detection:
    selection:
        Image|endswith: '\MSExchangeMailboxReplication.exe'
        TargetFilename|endswith:
            - '.aspx'
            - '.asp'
    condition: selection
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    Image|endswith: '\MSExchangeMailboxReplication.exe'
    TargetFilename|endswith:
        - '.aspx'
        - '.asp'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\MSExchangeMailboxReplication.exe`	`field:"Image" kind:ends_with value:"\MSExchangeMailboxReplication.exe"`
`TargetFilename`	ends_with	`.asp` corpus 4 (sigma 4) `.aspx` corpus 5 (sigma 5)	`field:"TargetFilename" kind:ends_with`