File Decoded From Base64/Hex Via Certutil.EXE

Severity: high
Author: Florian Roth (Nextron Systems), Jonhnathan Ribeiro, oscd.community
Source: upstream

Detects the execution of certutil with either the "decode" or "decodehex" flags to decode base64 or hex encoded files. This can be abused by attackers to decode an encoded payload before execution

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1027` Obfuscated Files or Information

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `all of selection_img`

or:
Image|endswith: '\certutil.exe'
OriginalFileName: CertUtil.exe

Stage 2: `all of selection_cli`

or:
CommandLine|contains: '-decode '
CommandLine|contains: '-decodehex '

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	`-decode` `-decodehex`
`Image`	ends_with	`\certutil.exe` corpus 34 (sigma 34)
`OriginalFileName`	eq	`CertUtil.exe` corpus 13 (sigma 13)