From d4bd69dd77bb77a896a9211c8d363928c3e220fd Mon Sep 17 00:00:00 2001
From: Nate Guagenti <neu5ron@users.noreply.github.com>
Date: Tue, 4 May 2021 18:13:08 -0400
Subject: [PATCH] Suspicious DNS Z Flag Set

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.
references:
  - 'https://twitter.com/neu5ron/status/1346245602502443009'
  - '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'
---
 .../zeek/zeek_dns_suspicious_zbit_flag.yml    | 61 +++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 rules/network/zeek/zeek_dns_suspicious_zbit_flag.yml

diff --git a/rules/network/zeek/zeek_dns_suspicious_zbit_flag.yml b/rules/network/zeek/zeek_dns_suspicious_zbit_flag.yml
new file mode 100644
index 000000000..5ea101f56
--- /dev/null
+++ b/rules/network/zeek/zeek_dns_suspicious_zbit_flag.yml
@@ -0,0 +1,61 @@
+title: Suspicious DNS Z Flag Bit Set
+id: ede05abc-2c9e-4624-9944-9ff17fdc0bf5
+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'
+tags:
+  - attack.t1094
+  - attack.t1043
+  - 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'
+      - 'ns'
+  exclude_responses:
+    answers|endswith: '\\x00'
+  condition: NOT z_flag_unset AND most_probable_valid_domain AND NOT (exclude_tlds OR exclude_tlds OR exclude_query_types OR exclude_responses)
+falsepositive:
+  - '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
+fields:
+- ts
+- id.orig_h
+- id.orig_p
+- id.resp_h
+- id.resp_p
+- proto
+- qtype_name
+- qtype
+- query
+- answers
+- rcode
+- rcode_name
+- trans_id
+- qtype
+- ttl
+- AA
+- uid