Detect Cisco FMC/SCC Deserialization RCE Exploitation (CVE-2026-20131) in Splunk
Detects exploitation of CVE-2026-20131, a deserialization of untrusted data vulnerability in Cisco Secure Firewall Management Center (FMC) and Cisco Security Cloud Control (SCC) Firewall Management. Successful exploitation allows unauthenticated or authenticated remote attackers to execute arbitrary commands on the underlying OS. This CVE is listed in CISA's Known Exploited Vulnerabilities catalog.
MITRE ATT&CK
SPL Detection Query
index=network OR index=cisco_fmc OR index=syslog
| eval is_fmc_host=if(match(host, "(?i)(fmc|firesight|sfmc|firepower)"), 1, 0)
| eval is_deser_indicator=if(match(_raw, "(?i)(deserialization|ObjectInputStream|ClassNotFoundException|java\.lang\.Runtime|ProcessBuilder|ysoserial|CommonsCollections)"), 1, 0)
| eval is_suspicious_proc=if(match(process_name, "(?i)(java|sh|bash|python|python3|wget|curl)"), 1, 0)
| where is_fmc_host=1 AND (is_deser_indicator=1 OR is_suspicious_proc=1)
| eval risk_score=case(
is_deser_indicator=1 AND is_suspicious_proc=1, 90,
is_deser_indicator=1, 70,
is_suspicious_proc=1, 40,
true(), 10
)
| stats count, max(risk_score) as max_risk, values(src_ip) as src_ips, values(dest_port) as dest_ports, list(_raw) as raw_events by host, _time
| where max_risk >= 70
| sort - max_risk
| table _time, host, src_ips, dest_ports, max_risk, count, raw_eventsSplunk detection for CVE-2026-20131 exploitation. Identifies deserialization payload indicators in FMC host logs and suspicious process activity, scoring events by risk level.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Routine FMC software updates triggering Java classloading messages
- Authorized red team exercises against FMC infrastructure
- Third-party SIEM integrations using Java-based connectors on the FMC host
Other platforms for CVE-2026-20131
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Simulate Deserialization Payload Delivery to FMC-like Endpoint
Expected signal: Network logs: POST to /j_spring_security_check with binary Content-Type. Process logs: java process spawning 'id' command. File creation event for /tmp/rce_proof.txt.
- Test 2Spawn Reverse Shell from Java Process (Post-Exploitation Simulation)
Expected signal: ProcessRollup2 event showing java parent spawning bash child. NetworkConnectIP4 event for outbound TCP to attacker IP on port 4444.
- Test 3Drop Webshell on FMC Tomcat Webroot (Post-Exploitation Persistence)
Expected signal: File creation event for .jsp file in Tomcat webroot. Syslog entry for file write by java or tomcat process user.
- Test 4Enumerate FMC Management API Without Authentication (Pre-Exploitation Recon)
Expected signal: Web server access logs showing GET/POST to /api/fmc_platform/ and /api/fmc_config/ from an external IP. 401 or 200 responses logged.
Unlock Pro Content
Get the full detection package for CVE-2026-20131 including response playbook, investigation guide, and atomic red team tests.