Cisco Secure Firewall - Blacklisted SSL Certificate Fingerprint

SPLUNK detection rule by Nasreddine Bencherchali, Splunk · Apache-2.0

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.

name: Cisco Secure Firewall - Blacklisted SSL Certificate Fingerprint
id: c43f7b49-2dab-4e76-892e-7f971c2f20f1
version: 7
creation_date: '2025-04-03'
modification_date: '2026-05-13'
author: Nasreddine Bencherchali, Splunk
status: production
type: TTP
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.
data_source:
    - Cisco Secure Firewall Threat Defense Connection Event
search: |
    `cisco_secure_firewall` EventType=* SSL_CertFingerprint=*
    | lookup sslbl_ssl_certificate_blacklist SHA1 as SSL_CertFingerprint OUTPUT Listingdate, Listingreason
    | where isnotnull(Listingreason)
    | stats min(_time) as firstTime max(_time) as lastTime
            values(dest) as dest
            values(dest_port) as dest_port
            values(rule) as rule
            values(url) as url
            values(Listingreason) as Reasons
            values(Listingdate) as "SSL Cert Listing Dates"
            count by SSL_CertFingerprint src transport action
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `cisco_secure_firewall___blacklisted_ssl_certificate_fingerprint_filter`
how_to_implement: |
    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.
references:
    - https://www.cisco.com/c/en/us/td/docs/security/firepower/741/api/FQE/secure_firewall_estreamer_fqe_guide_740.pdf
drilldown_searches:
    - name: View the detection results for - "$src$"
      search: '%original_detection_search% | search  src = "$src$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$src$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$src$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
finding:
    title: Suspicious SSL certificate fingerprint - [$SSL_CertFingerprint$] used in connections [ListingReason - $Reasons$] from $src$
    entity:
        field: src
        type: system
        score: 50
threat_objects:
    - field: SSL_CertFingerprint
      type: tls_hash
    - field: url
      type: url
analytic_story:
    - Cisco Secure Firewall Threat Defense Analytics
asset_type: Network
mitre_attack_id:
    - T1587.002
    - T1588.004
    - T1071.001
    - T1573.002
product:
    - Splunk Enterprise
    - Splunk Cloud
    - Splunk Enterprise Security
category: network
security_domain: network
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/cisco_secure_firewall_threat_defense/connection_event/connection_events.log
          source: not_applicable
          sourcetype: cisco:sfw:estreamer
      test_type: unit

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