Deserialization in Cisco Secure Firewall Management Center (Fmc)

CVE-2026-20340

A vulnerability in Cisco Secure FMC Software could allow an authenticated, remote attacker to execute arbitrary commands at the root privilege level. This vulnerability is due to unsecured deserialization of web-management user-controlled data. An attacker could exploit this vulnerability by authenticating to the device and sending a crafted HTTP payload. A successful exploit could allow the attacker to save the crafted payload and then execute it on the underlying operating system as root. To exploit this vulnerability, the attacker must have valid credentials for a user account with at least the role of Security Analyst (read-only).

Vulnerability class: Insecure Deserialization

Published · last modified .

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.007 (54.7th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 55th percentile — 54.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-20340: held from 0.007 to 0.007.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-20340?
CVE-2026-20340 is a high-severity vulnerability in Cisco Secure Firewall Management Center (Fmc), classified under Deserialization of Untrusted Data. CVSS score: 8.8/10. Published 2026-09-16.
How severe is CVE-2026-20340?
High severity. CVSS v3 base score is 8.8 out of 10.