Detection: Cisco Secure Firewall - Blacklisted SSL Certificate Fingerprint

Description

The following analytic detects the use of known suspicious SSL certificates in any observed event where the SSL_CertFingerprint field is present. It leverages Cisco Secure Firewall logs and compares the SSL certificate SHA1 fingerprint against a blacklist of certificates associated with malware distribution, command and control (C2) infrastructure, or phishing campaigns. This activity is significant as adversaries often reuse or self-sign certificates across malicious infrastructure, allowing defenders to track and detect encrypted sessions even when domains or IPs change. If confirmed malicious, this may indicate beaconing, malware download, or data exfiltration over TLS/SSL.

Search

 1`cisco_secure_firewall` EventType=* SSL_CertFingerprint=*
 2
 3| lookup sslbl_ssl_certificate_blacklist SHA1 as SSL_CertFingerprint OUTPUT Listingdate, Listingreason
 4
 5| where isnotnull(Listingreason)
 6
 7| stats min(_time) as firstTime max(_time) as lastTime
 8        values(dest) as dest
 9        values(dest_port) as dest_port
10        values(rule) as rule
11        values(url) as url
12        values(Listingreason) as Reasons
13        values(Listingdate) as "SSL Cert Listing Dates"
14        count by SSL_CertFingerprint src_ip transport action
15
16| `security_content_ctime(firstTime)`
17
18| `security_content_ctime(lastTime)`
19
20| `cisco_secure_firewall___blacklisted_ssl_certificate_fingerprint_filter`

Data Source

Macros Used

Annotations

Default Configuration

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

Implementation

This search requires Cisco Secure Firewall Threat Defense Logs, which includes the ConnectionEvent EventType. This search uses an input macro named cisco_secure_firewall. We strongly recommend that you specify your environment-specific configurations (index, source, sourcetype, etc.) for Cisco Secure Firewall Threat Defense logs. Replace the macro definition with configurations for your Splunk environment. The search also uses a post-filter macro designed to filter out known false positives. The logs are to be ingested using the Splunk Add-on for Cisco Security Cloud (https://splunkbase.splunk.com/app/7404). The access policy must also enable logging. This search also make use of lookup based on https://sslbl.abuse.ch/blacklist/sslblacklist.csv. Make sure its available

Known False Positives

Certain SSL certificates may be flagged in threat intelligence feeds due to historical misuse, yet still be used by legitimate services, particularly in content delivery or shared hosting environments. Internal or self-signed certificates used in testing or development environments may inadvertently match known blacklisted fingerprints. It is recommended to validate the connection context (destination IP, domain, ClientApplication) and correlate with other indicators before taking action.

Associated Analytic Story

• Cisco Secure Firewall Threat Defense Analytics

Risk Based Analytics (RBA)

Risk Message:

Suspicious SSL certificate fingerprint - [$SSL_CertFingerprint$] used in connections [ListingReason - $Reasons$] from $src_ip$

References

• https://www.cisco.com/c/en/us/td/docs/security/firepower/741/api/FQE/secure_firewall_estreamer_fqe_guide_740.pdf

Detection Testing

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