Assembly DLL Creation Via AspNetCompiler

Status: test
Severity: medium
Log source: category file_event, product windows
Author: Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects the creation of new DLL assembly files by "aspnet_compiler.exe", which could be a sign of "aspnet_compiler" abuse to proxy execution through a build provider.

Known false positives

Legitimate assembly compilation using a build provider

MITRE ATT&CK coverage

Tactic	Techniques
Execution	No specific technique

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 11: FileCreate

Rule body

title: Assembly DLL Creation Via AspNetCompiler
id: 4c7f49ee-2638-43bb-b85b-ce676c30b260 # TMP File
related:
    - id: 9ccba514-7cb6-4c5c-b377-700758f2f120 # SuspChild
      type: similar
    - id: 9f50fe98-fe5c-4a2d-86c7-fad7f63ed622 # Susp Paths
      type: similar
    - id: a01b8329-5953-4f73-ae2d-aa01e1f35f00 # Exec
      type: similar
status: test
description: |
    Detects the creation of new DLL assembly files by "aspnet_compiler.exe", which could be a sign of "aspnet_compiler" abuse to proxy execution through a build provider.
references:
    - Internal Research
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-08-14
tags:
    - attack.execution
logsource:
    product: windows
    category: file_event
detection:
    selection:
        Image|endswith: '\aspnet_compiler.exe'
        TargetFilename|contains|all:
            - '\Temporary ASP.NET Files\'
            - '\assembly\tmp\'
            - '.dll'
    condition: selection
falsepositives:
    - Legitimate assembly compilation using a build provider
level: medium

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    Image|endswith: '\aspnet_compiler.exe'
    TargetFilename|contains|all:
        - '\Temporary ASP.NET Files\'
        - '\assembly\tmp\'
        - '.dll'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\aspnet_compiler.exe` corpus 3 (sigma 3)	`field:"Image" kind:ends_with value:"\aspnet_compiler.exe"`
`TargetFilename`	match	`.dll` `\Temporary ASP.NET Files\` corpus 2 (sigma 2) `\assembly\tmp\`	`field:"TargetFilename" kind:match`