Uncommon File Created In Office Startup Folder

Status: test
Severity: high
Log source: product windows, category file_event
Author: frack113, Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects the creation of a file with an uncommon extension in an Office application startup folder

MITRE ATT&CK coverage

Tactic	Techniques
Resource Development	`T1587.001` Develop Capabilities: Malware

Event coverage

Provider	Event	Title
Sysmon	Event ID 11	FileCreate

Rule body yaml

title: Uncommon File Created In Office Startup Folder
id: a10a2c40-2c4d-49f8-b557-1a946bc55d9d
status: test
description: Detects the creation of a file with an uncommon extension in an Office application startup folder
references:
    - https://app.any.run/tasks/d6fe6624-6ef8-485d-aa75-3d1bdda2a08c/
    - http://addbalance.com/word/startup.htm
    - https://answers.microsoft.com/en-us/msoffice/forum/all/document-in-word-startup-folder-doesnt-open-when/44ab0932-2917-4150-8cdc-2f2cf39e86f3
    - https://en.wikipedia.org/wiki/List_of_Microsoft_Office_filename_extensions
author: frack113, Nasreddine Bencherchali (Nextron Systems)
date: 2022-06-05
modified: 2023-12-13
tags:
    - attack.resource-development
    - attack.t1587.001
logsource:
    product: windows
    category: file_event
detection:
    selection_word_paths:
        - TargetFilename|contains: '\Microsoft\Word\STARTUP'
        - TargetFilename|contains|all:
              - '\Office'
              - '\Program Files'
              - '\STARTUP'
    filter_exclude_word_ext:
        TargetFilename|endswith:
            - '.docb' # Word binary document introduced in Microsoft Office 2007
            - '.docm' # Word macro-enabled document; same as docx, but may contain macros and scripts
            - '.docx' # Word document
            - '.dotm' # Word macro-enabled template; same as dotx, but may contain macros and scripts
            - '.mdb' # MS Access DB
            - '.mdw' # MS Access DB
            - '.pdf' # PDF documents
            - '.wll' # Word add-in
            - '.wwl' # Word add-in
    selection_excel_paths:
        - TargetFilename|contains: '\Microsoft\Excel\XLSTART'
        - TargetFilename|contains|all:
              - '\Office'
              - '\Program Files'
              - '\XLSTART'
    filter_exclude_excel_ext:
        TargetFilename|endswith:
            - '.xll'
            - '.xls'
            - '.xlsm'
            - '.xlsx'
            - '.xlt'
            - '.xltm'
            - '.xlw'
    filter_main_office_click_to_run:
        Image|contains: ':\Program Files\Common Files\Microsoft Shared\ClickToRun\'
        Image|endswith: '\OfficeClickToRun.exe'
    filter_main_office_apps:
        Image|contains:
            - ':\Program Files\Microsoft Office\'
            - ':\Program Files (x86)\Microsoft Office\'
        Image|endswith:
            - '\winword.exe'
            - '\excel.exe'
    condition: ((selection_word_paths and not filter_exclude_word_ext) or (selection_excel_paths and not filter_exclude_excel_ext)) and not 1 of filter_main_*
falsepositives:
    - False positive might stem from rare extensions used by other Office utilities.
level: high

Stages and Predicates

Stage 0: `condition`

((selection_word_paths and not filter_exclude_word_ext) or (selection_excel_paths and not filter_exclude_excel_ext)) and not 1 of filter_main_*

Stage 1: `selection_word_paths`

selection_word_paths:
    - TargetFilename|contains: '\Microsoft\Word\STARTUP'
    - TargetFilename|contains|all:
          - '\Office'
          - '\Program Files'
          - '\STARTUP'

Stage 2: `not filter_exclude_word_ext`

filter_exclude_word_ext:
    TargetFilename|endswith:
        - '.docb'
        - '.docm'
        - '.docx'
        - '.dotm'
        - '.mdb'
        - '.mdw'
        - '.pdf'
        - '.wll'
        - '.wwl'

Stage 3: `selection_excel_paths`

selection_excel_paths:
    - TargetFilename|contains: '\Microsoft\Excel\XLSTART'
    - TargetFilename|contains|all:
          - '\Office'
          - '\Program Files'
          - '\XLSTART'

Stage 4: `not filter_exclude_excel_ext`

filter_exclude_excel_ext:
    TargetFilename|endswith:
        - '.xll'
        - '.xls'
        - '.xlsm'
        - '.xlsx'
        - '.xlt'
        - '.xltm'
        - '.xlw'

Stage 5: `not filter_main_*`

filter_main_office_click_to_run:
    Image|contains: ':\Program Files\Common Files\Microsoft Shared\ClickToRun\'
    Image|endswith: '\OfficeClickToRun.exe'
filter_main_office_apps:
    Image|contains:
        - ':\Program Files\Microsoft Office\'
        - ':\Program Files (x86)\Microsoft Office\'
    Image|endswith:
        - '\winword.exe'
        - '\excel.exe'

Exclusions

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

Field	Kind	Excluded values
`TargetFilename`	ends_with	`.docb`
`TargetFilename`	ends_with	`.docm`
`TargetFilename`	ends_with	`.docx`
`TargetFilename`	ends_with	`.dotm`
`TargetFilename`	ends_with	`.mdb`
`TargetFilename`	ends_with	`.mdw`
`TargetFilename`	ends_with	`.pdf`
`TargetFilename`	ends_with	`.wll`
`TargetFilename`	ends_with	`.wwl`
`TargetFilename`	ends_with	`.xll`
`TargetFilename`	ends_with	`.xls`
`TargetFilename`	ends_with	`.xlsm`
`TargetFilename`	ends_with	`.xlsx`
`TargetFilename`	ends_with	`.xlt`
`TargetFilename`	ends_with	`.xltm`
`TargetFilename`	ends_with	`.xlw`
`Image`	ends_with	`\excel.exe`
`Image`	ends_with	`\winword.exe`
`Image`	match	`:\Program Files (x86)\Microsoft Office\`
`Image`	match	`:\Program Files\Microsoft Office\`
`Image`	ends_with	`\OfficeClickToRun.exe`
`Image`	match	`:\Program Files\Common Files\Microsoft Shared\ClickToRun\`

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
`TargetFilename`	match	`\Microsoft\Excel\XLSTART` corpus 2 (sigma 2) `\Microsoft\Word\STARTUP` corpus 2 (sigma 2) `\Office` corpus 2 (sigma 2) `\Program Files` corpus 2 (sigma 2) `\STARTUP` corpus 2 (sigma 2) `\XLSTART` corpus 2 (sigma 2)

`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)

Uncommon File Created In Office Startup Folder

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_word_paths`

Stage 2: `not filter_exclude_word_ext`

Stage 3: `selection_excel_paths`

Stage 4: `not filter_exclude_excel_ext`

Stage 5: `not filter_main_*`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Uncommon File Created In Office Startup Folder

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_word_paths

Stage 2: not filter_exclude_word_ext

Stage 3: selection_excel_paths

Stage 4: not filter_exclude_excel_ext

Stage 5: not filter_main_*

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection_word_paths`

Stage 2: `not filter_exclude_word_ext`

Stage 3: `selection_excel_paths`

Stage 4: `not filter_exclude_excel_ext`

Stage 5: `not filter_main_*`