CVE-2026-20131
About
A vulnerability in the web-based management interface of Cisco Secure Firewall Management Center (FMC) Software could allow an unauthenticated, remote attacker to execute arbitrary Java code as root on an affected device.
This vulnerability is due to insecure deserialization of a user-supplied Java byte stream. An attacker could exploit this vulnerability by sending a crafted serialized Java object to the web-based management interface of an affected device. A successful exploit could allow the attacker to execute arbitrary code on the device and elevate privileges to root.
Note: If the FMC management interface does not have public internet access, the attack surface that is associated with this vulnerability is reduced.
Rainforest analyst review
This is an insecure Java deserialization flaw in the web-based management interface of Cisco Secure Firewall Management Center. The interface accepts a serialized Java byte stream from the request and reconstructs it without validation, so an attacker who sends a crafted serialized object triggers a gadget chain that executes arbitrary Java code. The execution runs as root, and no authentication is required, so a single crafted request to the management interface yields full root code execution and privilege escalation on the appliance.
FMC is the centralized management plane for Cisco's Secure Firewall fleet, the console from which firewall policy across the organization is defined and pushed, so root on it means control over the devices guarding the network perimeter. That concentration of authority makes it a top-tier target, and Cisco's own note that reduced internet exposure shrinks the attack surface is the key to prioritization: any FMC with a publicly reachable management interface is at acute risk. Apply Cisco's fixed software immediately, and where patching lags, ensure the FMC management interface is confined to a dedicated, tightly restricted management network. Review administrative access and device configuration state for tampering on any instance that was reachable.
References
- https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-fmc-rce-NKhnULJh
- https://aws.amazon.com/blogs/security/amazon-threat-intelligence-teams-identify-interlock-ransomware-campaign-targeting-enterprise-firewalls/
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-20131
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Frequently asked questions
What is CVE-2026-20131?
A vulnerability in the web-based management interface of Cisco Secure Firewall Management Center (FMC) Software could allow an unauthenticated, remote attacker to execute arbitrary Java code as root on an affected device. This vulnerability is due to insecure deserialization of a user-supplied Java byte stream.
How severe is CVE-2026-20131?
CVE-2026-20131 carries a CVSS 3.1 base score of 10, rated critical. On the Rainforest Risk Index — which weighs the severity alongside how easy the flaw is to reach and how broad its impact is — it scores 100 out of 100, in the critical band.
How is CVE-2026-20131 exploited?
According to the CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H): attack vector Network, attack complexity Low, privileges required None, user interaction None. Impact on confidentiality High, integrity High and availability High.
Which products are affected by CVE-2026-20131?
Public advisories list the following as affected: secure firewall management center. Check the references on this page for the exact versions each vendor confirms.
How do I fix CVE-2026-20131?
Apply the fix the vendor published — the references on this page link to the primary advisories and patches. When patching can't happen right away, reduce the exposure of the affected component and watch it for exploitation attempts. Rainforest customers see this vulnerability correlated to their own assets and prioritized by real exposure, not by score alone.
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