File In Suspicious Location Encoded To Base64 Via Certutil.EXE

Severity: high
Author: Nasreddine Bencherchali (Nextron Systems)
Source: upstream

Detects the execution of certutil with the "encode" flag to encode a file to base64 where the files are located in potentially suspicious locations

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

Stage 3: `all of selection_extension`

or:
CommandLine|contains: '$Recycle.Bin'
CommandLine|contains: '\AppData\Roaming\'
CommandLine|contains: '\Desktop\'
CommandLine|contains: '\Local\Temp\'
CommandLine|contains: '\PerfLogs\'
CommandLine|contains: '\Users\Public\'
CommandLine|contains: '\Windows\Temp\'

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	`$Recycle.Bin` `-encode` corpus 3 (sigma 3) `\AppData\Roaming\` corpus 10 (sigma 10) `\Desktop\` corpus 11 (sigma 11) `\Local\Temp\` `\PerfLogs\` corpus 2 (sigma 2) `\Users\Public\` corpus 17 (sigma 17) `\Windows\Temp\` corpus 10 (sigma 10)
`Image`	ends_with	`\certutil.exe` corpus 34 (sigma 34)
`OriginalFileName`	eq	`CertUtil.exe` corpus 13 (sigma 13)