Suspicious Word Cab File Write CVE-2021-40444

Status: test
Severity: high
Log source: product windows, category file_event
Author: Florian Roth (Nextron Systems), Sittikorn S
Source: github.com/SigmaHQ/sigma

Detects file creation patterns noticeable during the exploitation of CVE-2021-40444

MITRE ATT&CK coverage

Tactic	Techniques
Resource Development	`T1587` Develop Capabilities

Event coverage

Provider	Event	Title
Sysmon	Event ID 11	FileCreate

Rule body yaml

title: Suspicious Word Cab File Write CVE-2021-40444
id: 60c0a111-787a-4e8a-9262-ee485f3ef9d5
status: test
description: Detects file creation patterns noticeable during the exploitation of CVE-2021-40444
references:
    - https://twitter.com/RonnyTNL/status/1436334640617373699?s=20
    - https://twitter.com/vanitasnk/status/1437329511142420483?s=21
author: Florian Roth (Nextron Systems), Sittikorn S
date: 2021-09-10
modified: 2023-06-22
tags:
    - attack.resource-development
    - attack.t1587
    - detection.emerging-threats
logsource:
    product: windows
    category: file_event
detection:
    selection_cab:
        Image|endswith: '\winword.exe'
        TargetFilename|contains: '\Windows\INetCache'
        TargetFilename|endswith: '.cab'
    selection_inf:
        Image|endswith: '\winword.exe'
        TargetFilename|contains|all:
            - '\AppData\Local\Temp\'
            - '.inf'
    filter_main_legit:
        TargetFilename|startswith: 'C:\Users\'
        TargetFilename|contains: 'AppData\Local\Temp'
        TargetFilename|endswith: '\Content.inf'
    condition: 1 of selection_* and not 1 of filter_main_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

1 of selection_* and not 1 of filter_main_*

Stage 1: `selection_cab`

selection_cab:
    Image|endswith: '\winword.exe'
    TargetFilename|contains: '\Windows\INetCache'
    TargetFilename|endswith: '.cab'

Stage 2: `selection_inf`

selection_inf:
    Image|endswith: '\winword.exe'
    TargetFilename|contains|all:
        - '\AppData\Local\Temp\'
        - '.inf'

Stage 3: `not filter_main_legit`

filter_main_legit:
    TargetFilename|startswith: 'C:\Users\'
    TargetFilename|contains: 'AppData\Local\Temp'
    TargetFilename|endswith: '\Content.inf'

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`TargetFilename`	ends_with	`\Content.inf`
`TargetFilename`	match	`AppData\Local\Temp`
`TargetFilename`	starts_with	`C:\Users\`

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
`Image`	ends_with	`\winword.exe` corpus 20 (sigma 20)
`TargetFilename`	ends_with	`.cab`
`TargetFilename`	match	`.inf` `\AppData\Local\Temp\` corpus 8 (sigma 8) `\Windows\INetCache`

`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 Word Cab File Write CVE-2021-40444

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_cab`

Stage 2: `selection_inf`

Stage 3: `not filter_main_legit`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Suspicious Word Cab File Write CVE-2021-40444

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_cab

Stage 2: selection_inf

Stage 3: not filter_main_legit

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection_cab`

Stage 2: `selection_inf`

Stage 3: `not filter_main_legit`