Detection: Zeek x509 Certificate with Punycode

EXPERIMENTAL DETECTION

This detection status is set to experimental. The Splunk Threat Research team has not yet fully tested, simulated, or built comprehensive datasets for this detection. As such, this analytic is not officially supported. If you have any questions or concerns, please reach out to us at research@splunk.com.

Updated Date: 2024-05-30 ID: 029d6fe4-a5fe-43af-827e-c78c50e81d81 Author: Michael Haag, Splunk Type: Hunting Product: Splunk Enterprise Security

Description

The following analytic detects the presence of punycode within x509 certificates using Zeek x509 logs. It identifies punycode in the subject alternative name email and other fields by searching for the "xn--" prefix. This activity is significant as punycode can be used in phishing attacks or to bypass domain filters, posing a security risk. If confirmed malicious, attackers could use these certificates to impersonate legitimate domains, potentially leading to unauthorized access or data breaches.

1`zeek_x509` 
2| rex field=san.email{} "\@(?<domain_detected>xn--.*)" 
3| rex field=san.other_fields{} "\@(?<domain_detected>xn--.*)" 
4| stats values(domain_detected) by  basic_constraints.ca source host 
5| `zeek_x509_certificate_with_punycode_filter`

Data Source

Name Platform Sourcetype Source Supported App
N/A N/A N/A N/A N/A

Macros Used

Name Value
zeek_x509 sourcetype="zeek:x509:json"
zeek_x509_certificate_with_punycode_filter search *
zeek_x509_certificate_with_punycode_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK
+ Kill Chain Phases
+ NIST
+ CIS
- Threat Actors
ID Technique Tactic
T1573 Encrypted Channel Command And Control
KillChainPhase.COMMAND_AND_CONTROL
NistCategory.DE_AE
Cis18Value.CIS_13
APT29
BITTER
Magic Hound
Tropic Trooper

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting Value
Disabled true
Cron Schedule 0 * * * *
Earliest Time -70m@m
Latest Time -10m@m
Schedule Window auto
Creates Risk Event False
This configuration file applies to all detections of type hunting.

Implementation

The following analytic requires x509 certificate data to be logged entirely. In particular, for CVE-2022-3602, the punycode will be within the leaf certificate. The analytic may be modified to look for all xn--, or utilize a network IDS/monitoring tool like Zeek or Suricata to drill down into cert captured. Note for Suricata, the certificate is base64 encoded and will need to be decoded to capture the punycode (punycode will need to be decoded after).

Known False Positives

False positives may be present if the organization works with international businesses. Filter as needed.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
A x509 certificate has been identified to have punycode in the subject alternative name on $dest$. 15 50 30
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation Not Applicable N/A N/A N/A
Unit ❌ Failing N/A N/A N/A
Integration ❌ Failing N/A N/A N/A

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 2