Suspicious DNS Z Flag Bit Set

Status: test
Severity: medium
Log source: product zeek, service dns
Author: @neu5ron, SOC Prime Team, Corelight
Source: github.com/SigmaHQ/sigma

The DNS Z flag is bit within the DNS protocol header that is, per the IETF design, meant to be used reserved (unused). Although recently it has been used in DNSSec, the value being set to anything other than 0 should be rare. Otherwise if it is set to non 0 and DNSSec is being used, then excluding the legitimate domains is low effort and high reward. Determine if multiple of these files were accessed in a short period of time to further enhance the possibility of seeing if this was a one off or the possibility of larger sensitive file gathering. This Sigma query is designed to accompany the Corelight Threat Hunting Guide, which can be found here: https://www3.corelight.com/corelights-introductory-guide-to-threat-hunting-with-zeek-bro-logs'

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1095` Non-Application Layer Protocol, `T1571` Non-Standard Port

Rule body yaml

title: Suspicious DNS Z Flag Bit Set
id: ede05abc-2c9e-4624-9944-9ff17fdc0bf5
status: test
description: |
    The DNS Z flag is bit within the DNS protocol header that is, per the IETF design, meant to be used reserved (unused).
    Although recently it has been used in DNSSec, the value being set to anything other than 0 should be rare.
    Otherwise if it is set to non 0 and DNSSec is being used, then excluding the legitimate domains is low effort and high reward.
    Determine if multiple of these files were accessed in a short period of time to further enhance the possibility of seeing if this was a one off or the possibility of larger sensitive file gathering.
    This Sigma query is designed to accompany the Corelight Threat Hunting Guide, which can be found here: https://www3.corelight.com/corelights-introductory-guide-to-threat-hunting-with-zeek-bro-logs'
references:
    - https://twitter.com/neu5ron/status/1346245602502443009
    - https://tdm.socprime.com/tdm/info/eLbyj4JjI15v#sigma
    - https://tools.ietf.org/html/rfc2929#section-2.1
    - https://www.netresec.com/?page=Blog&month=2021-01&post=Finding-Targeted-SUNBURST-Victims-with-pDNS
author: '@neu5ron, SOC Prime Team, Corelight'
date: 2021-05-04
modified: 2022-11-29
tags:
    - attack.t1095
    - attack.t1571
    - attack.command-and-control
logsource:
    product: zeek
    service: dns
detection:
    z_flag_unset:
        Z: 0
    most_probable_valid_domain:
        query|contains: '.'
    exclude_tlds:
        query|endswith:
            - '.arpa'
            - '.local'
            - '.ultradns.net'
            - '.twtrdns.net'
            - '.azuredns-prd.info'
            - '.azure-dns.com'
            - '.azuredns-ff.info'
            - '.azuredns-ff.org'
            - '.azuregov-dns.org'
    exclude_query_types:
        qtype_name:
            - 'ns'
            - 'mx'
    exclude_responses:
        answers|endswith: '\\x00'
    exclude_netbios:
        id.resp_p:
            - 137
            - 138
            - 139
    condition: not z_flag_unset and most_probable_valid_domain and not (exclude_tlds or exclude_query_types or exclude_responses or exclude_netbios)
falsepositives:
    - 'Internal or legitimate external domains using DNSSec. Verify if these are legitimate DNSSec domains and then exclude them.'
    - 'If you work in a Public Sector then it may be good to exclude things like endswith ".edu", ".gov" and or ".mil"'
level: medium

Stages and Predicates

Stage 0: `condition`

not z_flag_unset and most_probable_valid_domain and not (exclude_tlds or exclude_query_types or exclude_responses or exclude_netbios)

Stage 1: `not z_flag_unset`

z_flag_unset:
    Z: 0

Stage 2: `most_probable_valid_domain`

most_probable_valid_domain:
    query|contains: '.'

Stage 3: `not exclude_tlds`

exclude_tlds:
    query|endswith:
        - '.arpa'
        - '.local'
        - '.ultradns.net'
        - '.twtrdns.net'
        - '.azuredns-prd.info'
        - '.azure-dns.com'
        - '.azuredns-ff.info'
        - '.azuredns-ff.org'
        - '.azuregov-dns.org'

Stage 4: `not exclude_query_types`

exclude_query_types:
    qtype_name:
        - 'ns'
        - 'mx'

Stage 5: `not exclude_responses`

exclude_responses:
    answers|endswith: '\\x00'

Stage 6: `not exclude_netbios`

exclude_netbios:
    id.resp_p:
        - 137
        - 138
        - 139

Exclusions

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

Field	Kind	Excluded values
`Z`	eq	`0`
`query`	ends_with	`.arpa`
`query`	ends_with	`.azure-dns.com`
`query`	ends_with	`.azuredns-ff.info`
`query`	ends_with	`.azuredns-ff.org`
`query`	ends_with	`.azuredns-prd.info`
`query`	ends_with	`.azuregov-dns.org`
`query`	ends_with	`.local`
`query`	ends_with	`.twtrdns.net`
`query`	ends_with	`.ultradns.net`
`qtype_name`	eq	`mx`
`qtype_name`	eq	`ns`
`answers`	ends_with	`\\x00`
`id.resp_p`	eq	`137`
`id.resp_p`	eq	`138`
`id.resp_p`	eq	`139`

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
`query`	match	`.`

`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 DNS Z Flag Bit Set

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `not z_flag_unset`

Stage 2: `most_probable_valid_domain`

Stage 3: `not exclude_tlds`

Stage 4: `not exclude_query_types`

Stage 5: `not exclude_responses`

Stage 6: `not exclude_netbios`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Suspicious DNS Z Flag Bit Set

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: not z_flag_unset

Stage 2: most_probable_valid_domain

Stage 3: not exclude_tlds

Stage 4: not exclude_query_types

Stage 5: not exclude_responses

Stage 6: not exclude_netbios

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `not z_flag_unset`

Stage 2: `most_probable_valid_domain`

Stage 3: `not exclude_tlds`

Stage 4: `not exclude_query_types`

Stage 5: `not exclude_responses`

Stage 6: `not exclude_netbios`