Suspicious File Encoded To Base64 Via Certutil.EXE

Status: test
Severity: high
Log source: product windows, category process_creation
Author: Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects the execution of certutil with the "encode" flag to encode a file to base64 where the extensions of the file is suspicious

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1027` Obfuscated Files or Information

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Suspicious File Encoded To Base64 Via Certutil.EXE
id: ea0cdc3e-2239-4f26-a947-4e8f8224e464
related:
    - id: e62a9f0c-ca1e-46b2-85d5-a6da77f86d1a
      type: derived
status: test
description: Detects the execution of certutil with the "encode" flag to encode a file to base64 where the extensions of the file is suspicious
references:
    - https://www.virustotal.com/gui/file/35c22725a92d5cb1016b09421c0a6cdbfd860fd4778b3313669b057d4a131cb7/behavior
    - https://www.virustotal.com/gui/file/427616528b7dbc4a6057ac89eb174a3a90f7abcf3f34e5a359b7a910d82f7a72/behavior
    - https://www.virustotal.com/gui/file/34de4c8beded481a4084a1fd77855c3e977e8ac643e5c5842d0f15f7f9b9086f/behavior
    - https://www.virustotal.com/gui/file/4abe1395a09fda06d897a9c4eb247278c1b6cddda5d126ce5b3f4f499e3b8fa2/behavior
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-05-15
modified: 2024-03-05
tags:
    - attack.stealth
    - attack.t1027
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith: '\certutil.exe'
        - OriginalFileName: 'CertUtil.exe'
    selection_cli:
        CommandLine|contains|windash: '-encode'
    selection_extension:
        CommandLine|contains:
            - '.acl'
            - '.bat'
            - '.doc'
            - '.gif'
            - '.jpeg'
            - '.jpg'
            - '.mp3'
            - '.pdf'
            - '.png'
            - '.ppt'
            - '.tmp'
            - '.xls'
            - '.xml'
    condition: all of selection_*
falsepositives:
    - Unknown
level: high
regression_tests_path: regression_data/rules/windows/process_creation/proc_creation_win_certutil_encode_susp_extensions/info.yml

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_img`

selection_img:
    - Image|endswith: '\certutil.exe'
    - OriginalFileName: 'CertUtil.exe'

Stage 2: `selection_cli`

selection_cli:
    CommandLine|contains|windash: '-encode'

Stage 3: `selection_extension`

selection_extension:
    CommandLine|contains:
        - '.acl'
        - '.bat'
        - '.doc'
        - '.gif'
        - '.jpeg'
        - '.jpg'
        - '.mp3'
        - '.pdf'
        - '.png'
        - '.ppt'
        - '.tmp'
        - '.xls'
        - '.xml'

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` transforms: `windash` corpus 3 (sigma 3) `.acl` `.bat` corpus 11 (sigma 9, elastic 1, kusto 1) `.doc` corpus 4 (sigma 4) `.gif` corpus 8 (sigma 8) `.jpeg` corpus 6 (sigma 6) `.jpg` corpus 7 (sigma 7) `.mp3` corpus 2 (sigma 2) `.pdf` corpus 3 (sigma 3) `.png` corpus 7 (sigma 7) `.ppt` corpus 4 (sigma 4) `.tmp` corpus 3 (sigma 3) `.xls` corpus 4 (sigma 4) `.xml` corpus 6 (sigma 4, splunk 2)
`Image`	ends_with	`\certutil.exe` corpus 43 (sigma 43)
`OriginalFileName`	eq	`CertUtil.exe` corpus 21 (sigma 14, splunk 6, elastic 1)

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Suspicious File Encoded To Base64 Via Certutil.EXE

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_img`

Stage 2: `selection_cli`

Stage 3: `selection_extension`

Indicators

Keyboard shortcuts

Search operators

Suspicious File Encoded To Base64 Via Certutil.EXE

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_img

Stage 2: selection_cli

Stage 3: selection_extension

Indicators

Stage 0: `condition`

Stage 1: `selection_img`

Stage 2: `selection_cli`

Stage 3: `selection_extension`