Sigma rule: Suspicious DNS Z Flag Bit Set

Rule

Title

Suspicious DNS Z Flag Bit Set

Status

test

Level

medium

Author

@neu5ron, SOC Prime Team, Corelight

Date

2021-05-04

Modified

2022-11-29

File

rules/network/zeek/zeek_dns_susp_zbit_flag.yml

Commit

ccbb0369507f796db0b0ae6854c8a2be471b9d84

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'

Log source

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)

False positives (as the rule lists them)

• 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"

Tags

• attack.t1095
• attack.t1571
• attack.command-and-control

MITRE ATT&CK ids this rule names

• T1095
• T1571

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

Read on 2026-11-03T00:58:21.399Z under licence sigma-drl-1.1@d5f4c36a from https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/network/zeek/zeek_dns_susp_zbit_flag.yml