Windows Binaries Write Suspicious Extensions

Severity: high
Author: Nasreddine Bencherchali (Nextron Systems)
Source: upstream

Detects Windows executables that write files with suspicious extensions

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1036` Masquerading

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `1 of selection_generic`

or:
Image|endswith: '\RuntimeBroker.exe'
Image|endswith: '\csrss.exe'
Image|endswith: '\lsass.exe'
Image|endswith: '\sihost.exe'
Image|endswith: '\smss.exe'
Image|endswith: '\wininit.exe'
Image|endswith: '\winlogon.exe'
or:
TargetFilename|endswith: .bat
TargetFilename|endswith: .dll
TargetFilename|endswith: .exe
TargetFilename|endswith: .hta
TargetFilename|endswith: .iso
TargetFilename|endswith: .ps1
TargetFilename|endswith: .txt
TargetFilename|endswith: .vbe
TargetFilename|endswith: .vbs

Stage 2: `1 of selection_special`

or:
Image|endswith: '\dllhost.exe'
Image|endswith: '\rundll32.exe'
Image|endswith: '\svchost.exe'
or:
TargetFilename|endswith: .bat
TargetFilename|endswith: .hta
TargetFilename|endswith: .iso
TargetFilename|endswith: .ps1
TargetFilename|endswith: .vbe
TargetFilename|endswith: .vbs

Stage 3: `not 1 of filter_main_*`

or:
Image: ['C:\Windows\SysWOW64\svchost.exe', 'C:\Windows\system32\svchost.exe']
or:
TargetFilename|startswith: 'C:\Program Files (x86)\WindowsApps\Microsoft.PowerShellPreview'
TargetFilename|startswith: 'C:\Program Files\WindowsApps\Microsoft.PowerShellPreview'
TargetFilename|endswith: .ps1
or:
TargetFilename|endswith: .bat
TargetFilename|endswith: .ps1
Image: 'C:\Windows\system32\svchost.exe'
TargetFilename|contains: 'C:\Windows\System32\GroupPolicy\DataStore\'
TargetFilename|contains: '\Machine\Scripts\Startup\'
TargetFilename|contains: '\Policies\'
TargetFilename|contains: '\sysvol\'
Image: 'C:\Windows\System32\dllhost.exe'
TargetFilename|endswith: .ps1
TargetFilename|contains: ':\Users\'
TargetFilename|contains: '\AppData\Local\Temp\__PSScriptPolicyTest_'
Image: 'C:\Windows\system32\svchost.exe'
TargetFilename|contains: .ps1
TargetFilename|contains: 'C:\Program Files\WindowsApps\Clipchamp'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`Image`	ends_with	`\RuntimeBroker.exe` corpus 4 (sigma 4) `\csrss.exe` corpus 3 (sigma 3) `\dllhost.exe` corpus 8 (sigma 8) `\lsass.exe` corpus 5 (sigma 5) `\rundll32.exe` corpus 76 (sigma 76) `\sihost.exe` corpus 2 (sigma 2) `\smss.exe` corpus 2 (sigma 2) `\svchost.exe` corpus 20 (sigma 20) `\wininit.exe` corpus 3 (sigma 3) `\winlogon.exe` corpus 5 (sigma 5)
`Image`	eq	`C:\Windows\SysWOW64\svchost.exe` corpus 3 (sigma 3) `C:\Windows\System32\dllhost.exe` `C:\Windows\system32\svchost.exe` corpus 5 (sigma 5)
`TargetFilename`	ends_with	`.bat` corpus 15 (sigma 15) `.dll` corpus 21 (sigma 21) `.exe` corpus 18 (sigma 18) `.hta` corpus 12 (sigma 12) `.iso` corpus 5 (sigma 5) `.ps1` corpus 15 (sigma 15) `.txt` corpus 2 (sigma 2) `.vbe` corpus 13 (sigma 13) `.vbs` corpus 16 (sigma 16)
`TargetFilename`	match	`.ps1` `:\Users\` corpus 3 (sigma 3) `C:\Program Files\WindowsApps\Clipchamp` `C:\Windows\System32\GroupPolicy\DataStore\` `\AppData\Local\Temp\__PSScriptPolicyTest_` `\Machine\Scripts\Startup\` `\Policies\` `\sysvol\`
`TargetFilename`	starts_with	`C:\Program Files (x86)\WindowsApps\Microsoft.PowerShellPreview` `C:\Program Files\WindowsApps\Microsoft.PowerShellPreview`