File Encoded To Base64 Via Certutil.EXE

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

Detects the execution of certutil with the "encode" flag to encode a file to base64. This can be abused by threat actors and attackers for data exfiltration

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`

CommandLine|contains: -encode

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	`-encode` corpus 3 (sigma 3)
`Image`	ends_with	`\certutil.exe` corpus 34 (sigma 34)
`OriginalFileName`	eq	`CertUtil.exe` corpus 13 (sigma 13)