Suspicious Access to Sensitive File Extensions - Zeek

Status: test
Severity: medium
Log source: product zeek, service smb_files
Author: Samir Bousseaden, @neu5ron
Source: github.com/SigmaHQ/sigma

Detects known sensitive file extensions via Zeek

MITRE ATT&CK coverage

Tactic	Techniques
Collection	No specific technique

Rule body yaml

title: Suspicious Access to Sensitive File Extensions - Zeek
id: 286b47ed-f6fe-40b3-b3a8-35129acd43bc
related:
    - id: 91c945bc-2ad1-4799-a591-4d00198a1215
      type: derived
status: test
description: Detects known sensitive file extensions via Zeek
references:
    - Internal Research
author: Samir Bousseaden, @neu5ron
date: 2020-04-02
modified: 2025-10-17
tags:
    - attack.collection
logsource:
    product: zeek
    service: smb_files
detection:
    selection:
        name|endswith:
            - '.pst'
            - '.ost'
            - '.msg'
            - '.nst'
            - '.oab'
            - '.edb'
            - '.nsf'
            - '.bak'
            - '.dmp'
            - '.kirbi'
            # - '\groups.xml'  # Commented out: groups.xml is accessed legitimately by Group Policy processing; high FP rate in enterprise environments
            - '.rdp'
    condition: selection
falsepositives:
    - Help Desk operator doing backup or re-imaging end user machine or backup software
    - Users working with these data types or exchanging message files
level: medium

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    name|endswith:
        - '.pst'
        - '.ost'
        - '.msg'
        - '.nst'
        - '.oab'
        - '.edb'
        - '.nsf'
        - '.bak'
        - '.dmp'
        - '.kirbi'
        - '.rdp'

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
`name`	ends_with	`.bak` `.dmp` `.edb` `.kirbi` `.msg` `.nsf` `.nst` `.oab` `.ost` `.pst` `.rdp`

`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 Access to Sensitive File Extensions - Zeek

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Indicators

Keyboard shortcuts

Search operators

Suspicious Access to Sensitive File Extensions - Zeek

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Indicators

Stage 0: `condition`

Stage 1: `selection`