Files With System DLL Name In Unsuspected Locations

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

Detects the creation of a file with the ".dll" extension that has the name of a System DLL in uncommon or unsuspected locations. (Outisde of "System32", "SysWOW64", etc.). It is highly recommended to perform an initial baseline before using this rule in production.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1036.005` Masquerading: Match Legitimate Resource Name or Location

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `selection`

or:
TargetFilename|endswith: '\secur32.dll'
TargetFilename|endswith: '\tdh.dll'

Stage 2: `not 1 of filter_main_generic`

or:
TargetFilename|contains: 'C:\$WINDOWS.~BT\'
TargetFilename|contains: 'C:\$WinREAgent\'
TargetFilename|contains: 'C:\Windows\SoftwareDistribution\'
TargetFilename|contains: 'C:\Windows\SysWOW64\'
TargetFilename|contains: 'C:\Windows\System32\'
TargetFilename|contains: 'C:\Windows\WinSxS\'
TargetFilename|contains: 'C:\Windows\uus\'

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
`TargetFilename`	ends_with	`\secur32.dll` `\tdh.dll`
`TargetFilename`	match	`C:\$WINDOWS.~BT\` corpus 2 (sigma 2) `C:\$WinREAgent\` corpus 2 (sigma 2) `C:\Windows\SoftwareDistribution\` corpus 2 (sigma 2) `C:\Windows\SysWOW64\` corpus 2 (sigma 2) `C:\Windows\System32\` corpus 2 (sigma 2) `C:\Windows\WinSxS\` corpus 2 (sigma 2) `C:\Windows\uus\` corpus 2 (sigma 2)