Suspicious FileFix Execution Pattern

Severity: high
Author: 0xFustang, Swachchhanda Shrawan Poudel (Nextron Systems)
Source: upstream

Detects suspicious FileFix execution patterns where users are tricked into running malicious commands through browser file upload dialog manipulation. This attack typically begins when users visit malicious websites impersonating legitimate services or news platforms, which may display fake CAPTCHA challenges or direct instructions to open file explorer and paste clipboard content. The clipboard content usually contains commands that download and execute malware, such as information stealing tools.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1204.004` User Execution: Malicious Copy and Paste

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation
Security-Auditing	4688	A new process has been created.

Stages and Predicates

Stage 1: `selection_exec_parent`

or:
ParentImage|endswith: '\brave.exe'
ParentImage|endswith: '\chrome.exe'
ParentImage|endswith: '\firefox.exe'
ParentImage|endswith: '\msedge.exe'
CommandLine|contains: '#'

Stage 2: `1 of selection_cli_lolbin`

or:
CommandLine|contains: '%comspec%'
CommandLine|contains: bitsadmin
CommandLine|contains: certutil
CommandLine|contains: cmd
CommandLine|contains: cscript
CommandLine|contains: curl
CommandLine|contains: finger
CommandLine|contains: mshta
CommandLine|contains: powershell
CommandLine|contains: pwsh
CommandLine|contains: regsvr32
CommandLine|contains: rundll32
CommandLine|contains: schtasks
CommandLine|contains: wget
CommandLine|contains: wscript

Stage 3: `1 of selection_cli_captcha`

or:
CommandLine|contains: account
CommandLine|contains: anti-bot
CommandLine|contains: botcheck
CommandLine|contains: captcha
CommandLine|contains: challenge
CommandLine|contains: confirmation
CommandLine|contains: fraud
CommandLine|contains: human
CommandLine|contains: identification
CommandLine|contains: identificator
CommandLine|contains: identity
CommandLine|contains: robot
CommandLine|contains: validation
CommandLine|contains: verification
CommandLine|contains: verify

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	`#` corpus 3 (sigma 3) `%comspec%` corpus 3 (sigma 3) `account` corpus 2 (sigma 2) `anti-bot` corpus 2 (sigma 2) `bitsadmin` corpus 5 (sigma 5) `botcheck` corpus 2 (sigma 2) `captcha` corpus 2 (sigma 2) `certutil` corpus 5 (sigma 5) `challenge` corpus 2 (sigma 2) `cmd` corpus 5 (sigma 5) `confirmation` corpus 2 (sigma 2) `cscript` corpus 12 (sigma 12) `curl` corpus 3 (sigma 3) `finger` corpus 2 (sigma 2) `fraud` corpus 2 (sigma 2) `human` corpus 2 (sigma 2) `identification` corpus 2 (sigma 2) `identificator` corpus 2 (sigma 2) `identity` corpus 2 (sigma 2) `mshta` corpus 11 (sigma 11) `powershell` corpus 16 (sigma 16) `pwsh` corpus 5 (sigma 5) `regsvr32` corpus 11 (sigma 11) `robot` corpus 2 (sigma 2) `rundll32` corpus 19 (sigma 19) `schtasks` corpus 3 (sigma 3) `validation` corpus 2 (sigma 2) `verification` corpus 2 (sigma 2) `verify` corpus 2 (sigma 2) `wget` corpus 2 (sigma 2) `wscript` corpus 12 (sigma 12)
`ParentImage`	ends_with	`\brave.exe` corpus 2 (sigma 2) `\chrome.exe` corpus 4 (sigma 4) `\firefox.exe` corpus 3 (sigma 3) `\msedge.exe` corpus 3 (sigma 3)