Potentially Suspicious NTFS Symlink Behavior Modification

Severity: medium
Author: frack113, The DFIR Report
Source: upstream

Detects the modification of NTFS symbolic link behavior using fsutil, which could be used to enable remote to local or remote to remote symlinks for potential attacks.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059` Command and Scripting Interpreter
Defense Evasion	`T1222.001` File and Directory Permissions Modification: Windows File and Directory Permissions Modification

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `all of selection_img_proxy`

or:
Image|endswith: '\cmd.exe'
Image|endswith: '\powershell.exe'
Image|endswith: '\pwsh.exe'
OriginalFileName: Cmd.Exe
OriginalFileName: PowerShell.EXE
OriginalFileName: pwsh.dll

Stage 2: `all of selection_fsutil_cli`

CommandLine|contains: SymlinkEvaluation
CommandLine|contains: behavior
CommandLine|contains: fsutil
CommandLine|contains: set

Stage 3: `all of selection_symlink_params`

or:
CommandLine|contains: 'L2L:1'
CommandLine|contains: 'R2L:1'
CommandLine|contains: 'R2R:1'

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
`CommandLine`	match	`L2L:1` `R2L:1` `R2R:1` `SymlinkEvaluation` `behavior` `fsutil` `set` corpus 7 (sigma 7)
`Image`	ends_with	`\cmd.exe` corpus 92 (sigma 92) `\powershell.exe` corpus 143 (sigma 143) `\pwsh.exe` corpus 140 (sigma 140)
`OriginalFileName`	eq	`Cmd.Exe` corpus 32 (sigma 30, splunk 2) `PowerShell.EXE` corpus 64 (sigma 60, splunk 4) `pwsh.dll` corpus 72 (sigma 68, splunk 4)