Detect Cisco FMC/SCC Deserialization RCE Exploitation (CVE-2026-20131) in Google Chronicle
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
YARA-L Detection Query
rule CVE_2026_20131_Cisco_FMC_Deserialization_RCE {
meta:
author = "df00tech detection engineering"
description = "Detects potential exploitation of CVE-2026-20131 via Java deserialization in Cisco FMC"
severity = "CRITICAL"
priority = "HIGH"
reference = "https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-fmc-rce-NKhnULJh"
events:
(
$e1.metadata.event_type = "NETWORK_CONNECTION"
and (
$e1.target.hostname = /(?i)(fmc|firesight|sfmc|firepower)/
or $e1.target.application = /(?i)(cisco.*fmc|firepower.*management)/
)
and $e1.target.port in (443, 8080, 8443)
and not re.regex($e1.principal.ip, `^(10\.|192\.168\.|172\.(1[6-9]|2[0-9]|3[01])\.).*`)
)
or
(
$e1.metadata.event_type = "PROCESS_LAUNCH"
and re.regex($e1.principal.hostname, `(?i)(fmc|firesight|sfmc)`)
and $e1.target.process.file.full_path = /\/(java|sh|bash|python|python3|wget|curl)$/
and re.regex($e1.principal.process.file.full_path, `(?i)(java|tomcat|httpd|sfmcd)`)
)
or
(
$e1.metadata.event_type = "GENERIC_EVENT"
and re.regex($e1.principal.hostname, `(?i)(fmc|firesight|sfmc)`)
and re.regex($e1.metadata.description, `(?i)(deserialization|ObjectInputStream|ClassNotFoundException|java\.lang\.Runtime|ProcessBuilder)`)
)
condition:
$e1
}Chronicle YARA-L rule for CVE-2026-20131: multi-event detection covering inbound connections to FMC from external IPs, suspicious process spawning from FMC services, and deserialization-related log indicators.
Data Sources
Required Tables
False Positives & Tuning
- External partner access via HTTPS to FMC management UI
- Automated compliance scanners running Java-based checks against FMC
- Authorized remote administration sessions with non-RFC1918 jump hosts
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.
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