CVE-2026-20242

Cisco Secure Firewall Management Center (FMC) Software contains a critical insecure deserialization flaw (CWE-502) in its External Database Access feature. An attacker who controls a host listed in the FMC's external database access list can send a crafted serialized Java byte stream to a specific TCP port on the device. Because the deserialization is insecure and the attack is unauthenticated at the protocol level, a successful exploit lets the attacker execute arbitrary commands on the appliance and elevate privileges to root. All deployments of Cisco FMC with the External Database Access feature enabled are potentially affected, though exposure is substantially reduced when the FMC management interface is not reachable from the public internet. Exploitation status: the flaw is not in CISA's KEV catalog and no public proof-of-concept is known.

What to do: Upgrade FMC to a fixed release per Cisco's security advisory as soon as one is published. As interim mitigation, review and restrict the external database access list to fully trusted hosts, limit which hosts can reach the affected TCP port, and ensure the FMC management interface is not exposed to the public internet. Verify configuration via the FMC admin console and monitor Cisco PSIRT for updates.