Potential File Extension Spoofing Using Right-to-Left Override

Severity: high
Author: Jonathan Peters (Nextron Systems), Florian Roth (Nextron Systems), Swachchhanda Shrawan Poudel (Nextron Systems)
Source: upstream

Detects suspicious filenames that contain a right-to-left override character and a potentially spoofed file extensions.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1036.002` Masquerading: Right-to-Left Override

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `all of selection_rtlo_unicode`

or:
TargetFilename|contains: '[U+202E]'
TargetFilename|contains: '\u202e'
TargetFilename|contains: '‮'

Stage 2: `all of selection_extensions`

or:
TargetFilename|contains: 3pm.
TargetFilename|contains: 4pm.
TargetFilename|contains: cod.
TargetFilename|contains: fdp.
TargetFilename|contains: ftr.
TargetFilename|contains: gepj.
TargetFilename|contains: gnp.
TargetFilename|contains: gpj.
TargetFilename|contains: ism.
TargetFilename|contains: lmth.
TargetFilename|contains: nls.
TargetFilename|contains: piz.
TargetFilename|contains: slx.
TargetFilename|contains: tdo.
TargetFilename|contains: vsc.
TargetFilename|contains: vwm.
TargetFilename|contains: xcod.
TargetFilename|contains: xslx.
TargetFilename|contains: xtpp.

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`	match	`3pm.` `4pm.` `[U+202E]` `\u202e` `cod.` `fdp.` `ftr.` `gepj.` `gnp.` `gpj.` `ism.` `lmth.` `nls.` `piz.` `slx.` `tdo.` `vsc.` `vwm.` `xcod.` `xslx.` `xtpp.` `‮`