Invoke-Obfuscation Obfuscated IEX Invocation - Security

Severity: high
Author: Daniel Bohannon (@Mandiant/@FireEye), oscd.community
Source: upstream

Detects all variations of obfuscated powershell IEX invocation code generated by Invoke-Obfuscation framework from the code block linked in the references

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1027` Obfuscated Files or Information

Event coverage

Provider	Event ID	Title
Security-Auditing	4697	A service was installed in the system.

Stages and Predicates

Stage 1: `all of selection_eid`

Stage 2: `all of selection_servicefilename`

or:
ServiceFileName|re: '\$PSHome\[\s*\d{1,3}\s*\]\s*\+\s*\$PSHome\['
ServiceFileName|re: '\$ShellId\[\s*\d{1,3}\s*\]\s*\+\s*\$ShellId\['
ServiceFileName|re: '\$VerbosePreference\.ToString\('
ServiceFileName|re: '\$env:ComSpec\[(\s*\d{1,3}\s*,){2}'
ServiceFileName|re: '\$env:Public\[\s*\d{1,3}\s*\]\s*\+\s*\$env:Public\['
ServiceFileName|re: '\String\]\s*\$VerbosePreference'
ServiceFileName|re: '\\*mdr\*\W\s*\)\.Name'

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
`ServiceFileName`	regex_match	`\$PSHome\[\s\d{1,3}\s\]\s\+\s\$PSHome\[` `\$ShellId\[\s\d{1,3}\s\]\s\+\s\$ShellId\[` `\$VerbosePreference\.ToString$` `\$env:ComSpec\[(\s\d{1,3}\s,){2}` `\$env:Public\[\s\d{1,3}\s\]\s\+\s\$env:Public\[` `\String\]\s\$VerbosePreference` `\\mdr\\W\s$\.Name`

Neighbors

Stricter alternatives (narrower than this rule)

The rules below may be useful if you find the current rule is too noisy / lacks specificity.

Windows Service Installed via an Unusual Client [elastic] (cross-vendor) (adds 5 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation Via Use Rundll32 - Security [sigma] (adds 5 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation VAR++ LAUNCHER OBFUSCATION - Security [sigma] (adds 5 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation VAR+ Launcher - Security [sigma] (adds 4 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation COMPRESS OBFUSCATION - Security [sigma] (adds 4 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation RUNDLL LAUNCHER - Security [sigma] (adds 4 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation Via Use MSHTA - Security [sigma] (adds 4 filters) (first-stage match only; downstream stages may differ)
Metasploit Or Impacket Service Installation Via SMB PsExec [sigma] (adds 4 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation CLIP+ Launcher - Security [sigma] (adds 3 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation Via Stdin - Security [sigma] (adds 3 filters) (first-stage match only; downstream stages may differ)
HybridConnectionManager Service Installation [sigma] (adds 2 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation STDIN+ Launcher - Security [sigma] (adds 2 filters) (first-stage match only; downstream stages may differ)
Invoke-Obfuscation Via Use Clip - Security [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)
Credential Dumping Tools Service Execution - Security [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)
Windows Pcap Drivers [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)
PowerShell Scripts Installed as Services - Security [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)
Remote Access Tool Services Have Been Installed - Security [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)
Tap Driver Installation - Security [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)