Suspicious Svchost Process Access

Status: test
Severity: high
Log source: category process_access, product windows
Author: Tim Burrell
Source: github.com/SigmaHQ/sigma

Detects suspicious access to the "svchost" process such as that used by Invoke-Phantom to kill the thread of the Windows event logging service.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Impairment	`T1685.001` Disable or Modify Tools: Disable or Modify Windows Event Log

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 10: ProcessAccess

Rule body

title: Suspicious Svchost Process Access
id: 166e9c50-8cd9-44af-815d-d1f0c0e90dde
status: test
description: Detects suspicious access to the "svchost" process such as that used by Invoke-Phantom to kill the thread of the Windows event logging service.
references:
    - https://github.com/hlldz/Invoke-Phant0m
    - https://twitter.com/timbmsft/status/900724491076214784
author: Tim Burrell
date: 2020-01-02
modified: 2023-01-30
tags:
    - attack.defense-impairment
    - attack.t1685.001
logsource:
    category: process_access
    product: windows
detection:
    selection:
        TargetImage|endswith: ':\Windows\System32\svchost.exe'
        GrantedAccess: '0x1F3FFF'
        CallTrace|contains: 'UNKNOWN'
    filter_main_msbuild:
        SourceImage|contains: ':\Program Files\Microsoft Visual Studio\'
        SourceImage|endswith: '\MSBuild\Current\Bin\MSBuild.exe'
        # Just to make sure it's "really" .NET :)
        CallTrace|contains:
            - 'Microsoft.Build.ni.dll'
            - 'System.ni.dll'
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_*

Stage 1: `selection`

selection:
    TargetImage|endswith: ':\Windows\System32\svchost.exe'
    GrantedAccess: '0x1F3FFF'
    CallTrace|contains: 'UNKNOWN'

Stage 2: `not filter_main_msbuild`

filter_main_msbuild:
    SourceImage|contains: ':\Program Files\Microsoft Visual Studio\'
    SourceImage|endswith: '\MSBuild\Current\Bin\MSBuild.exe'
    CallTrace|contains:
        - 'Microsoft.Build.ni.dll'
        - 'System.ni.dll'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`CallTrace`	match	`Microsoft.Build.ni.dll`	`excludes:CallTrace` `field:"CallTrace" value:"Microsoft.Build.ni.dll"`
`CallTrace`	match	`System.ni.dll`	`excludes:CallTrace` `field:"CallTrace" value:"System.ni.dll"`
`SourceImage`	ends_with	`\MSBuild\Current\Bin\MSBuild.exe`	`excludes:SourceImage` `field:"SourceImage" value:"\MSBuild\Current\Bin\MSBuild.exe"`
`SourceImage`	match	`:\Program Files\Microsoft Visual Studio\`	`excludes:SourceImage` `field:"SourceImage" value:":\Program Files\Microsoft Visual Studio\"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CallTrace`	match	`UNKNOWN` corpus 5 (sigma 3, elastic 2)	`field:"CallTrace" kind:match value:"UNKNOWN"`
`GrantedAccess`	eq	`0x1F3FFF` corpus 3 (sigma 2, splunk 1)	`field:"GrantedAccess" kind:eq value:"0x1F3FFF"`
`TargetImage`	ends_with	`:\Windows\System32\svchost.exe`	`field:"TargetImage" kind:ends_with value:":\Windows\System32\svchost.exe"`