GAC DLL Loaded Via Office Applications

Status: test
Severity: high
Log source: category image_load, product windows
Author: Antonlovesdnb
Source: github.com/SigmaHQ/sigma

Detects any GAC DLL being loaded by an Office Product

Known false positives

Legitimate macro usage. Add the appropriate filter according to your environment

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1204.002` User Execution: Malicious File

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 7: Image loaded

Rule body

title: GAC DLL Loaded Via Office Applications
id: 90217a70-13fc-48e4-b3db-0d836c5824ac
status: test
description: Detects any GAC DLL being loaded by an Office Product
references:
    - https://medium.com/threatpunter/detecting-adversary-tradecraft-with-image-load-event-logging-and-eql-8de93338c16
author: Antonlovesdnb
date: 2020-02-19
modified: 2023-02-10
tags:
    - attack.execution
    - attack.t1204.002
logsource:
    category: image_load
    product: windows
detection:
    selection:
        Image|endswith:
            - '\excel.exe'
            - '\mspub.exe'
            - '\onenote.exe'
            - '\onenoteim.exe' # Just in case
            - '\outlook.exe'
            - '\powerpnt.exe'
            - '\winword.exe'
        ImageLoaded|startswith: 'C:\Windows\Microsoft.NET\assembly\GAC_MSIL'
    condition: selection
falsepositives:
    - Legitimate macro usage. Add the appropriate filter according to your environment
level: high

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    Image|endswith:
        - '\excel.exe'
        - '\mspub.exe'
        - '\onenote.exe'
        - '\onenoteim.exe'
        - '\outlook.exe'
        - '\powerpnt.exe'
        - '\winword.exe'
    ImageLoaded|startswith: 'C:\Windows\Microsoft.NET\assembly\GAC_MSIL'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\excel.exe` corpus 16 (sigma 16) `\mspub.exe` corpus 10 (sigma 10) `\onenote.exe` corpus 6 (sigma 6) `\onenoteim.exe` corpus 6 (sigma 6) `\outlook.exe` corpus 13 (sigma 13) `\powerpnt.exe` corpus 14 (sigma 14) `\winword.exe` corpus 17 (sigma 17)	`field:"Image" kind:ends_with`
`ImageLoaded`	starts_with	`C:\Windows\Microsoft.NET\assembly\GAC_MSIL`	`field:"ImageLoaded" kind:starts_with value:"C:\Windows\Microsoft.NET\assembly\GAC_MSIL"`

GAC DLL Loaded Via Office Applications

Known false positives

MITRE ATT&CK coverage

Telemetry coverage

Rule body

Stages and Predicates

Stage 0: condition

Stage 1: selection

Indicators

Stage 0: `condition`

Stage 1: `selection`