CVE-2026-20131
Published: 04 March 2026
Summary
CVE-2026-20131 is a critical-severity Deserialization of Untrusted Data (CWE-502) vulnerability in Cisco Secure Firewall Management Center. Its CVSS base score is 10.0 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 2.2% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2026-20131 is a critical vulnerability in the web-based management interface of Cisco Secure Firewall Management Center (FMC) Software, stemming from insecure deserialization of a user-supplied Java byte stream. It enables an unauthenticated, remote attacker to execute arbitrary Java code with root privileges on affected devices. The issue carries a CVSS v3.1 base score of 10.0 (AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H) and is classified under CWE-502 (Deserialization of Untrusted Data). The vulnerability was published on 2026-03-04.
An unauthenticated, remote attacker can exploit this vulnerability by sending a crafted serialized Java object to the web-based management interface. Successful exploitation allows the attacker to execute arbitrary code on the device and elevate privileges to root. The attack surface is reduced if the FMC management interface lacks public internet access.
The Cisco Security Advisory (https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-fmc-rce-NKhnULJh) details affected versions and available patches. CISA has added CVE-2026-20131 to its Known Exploited Vulnerabilities catalog (https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-20131), indicating active exploitation.
Amazon Threat Intelligence has identified the Interlock ransomware campaign targeting enterprise firewalls, as noted in their blog (https://aws.amazon.com/blogs/security/amazon-threat-intelligence-teams-identify-interlock-ransomware-campaign-targeting-enterprise-firewalls/), highlighting real-world exploitation risks.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-9444
Vulnerability details
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…
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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.
- CWE(s)
- KEV Date Added
- 19 March 2026
Related Threats
Threat-Actor AttributionAI
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Unauthenticated remote code execution via insecure deserialization in the web-based management interface of a network device directly enables T1190: Exploit Public-Facing Application.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mitigates the CVE by identifying, prioritizing, and applying vendor patches to remediate the insecure deserialization flaw.
Validates user-supplied Java byte streams at the web interface to block crafted serialized objects that enable arbitrary code execution.
Enforces network boundaries to restrict remote access to the web management interface, reducing the unauthenticated attack surface especially from the public internet.