CVE-2026-20342: Cisco Secure FMC Information Disclosure

CVE-2026-20342 Overview

CVE-2026-20342 is an arbitrary file download vulnerability in a specific file download API of Cisco Secure Firewall Management Center (FMC) Software. The flaw stems from missing input sanitization on user-supplied parameters. An authenticated, remote attacker can send a crafted HTTPS request to download arbitrary files from the affected system. Exploitation requires valid credentials for an account with at least the Security Analyst (read-only) role. The weakness is classified under [CWE-639] (Authorization Bypass Through User-Controlled Key).

Critical Impact
A low-privileged authenticated user can retrieve arbitrary files from the FMC host, exposing configuration data, credentials, and sensitive management artifacts across a scope-changed boundary.

Affected Products

Cisco Secure Firewall Management Center (FMC) Software
Cisco Secure FMC file download API endpoint
Refer to the Cisco Security Advisory for the full list of fixed releases

Discovery Timeline

2026-09-16 - CVE-2026-20342 published to NVD
2026-09-17 - Last updated in NVD database

Technical Details for CVE-2026-20342

Vulnerability Analysis

The vulnerability resides in a file download API exposed by Cisco Secure FMC Software. The API accepts user-controlled input that identifies the file to be returned but does not validate or sanitize that input against an authorized resource set. As a result, an attacker with a valid Security Analyst account can request files outside the intended scope. The issue crosses a security boundary because a read-only analyst role gains access to system files that role should not reach. Successful exploitation is limited to confidentiality impact; the API does not permit modification or service disruption through this flaw.

Root Cause

The root cause is improper authorization on a user-controlled key, mapped to [CWE-639]. The download handler trusts the file identifier supplied by the client and returns the referenced content without confirming the caller is authorized for that specific object. Missing sanitization further allows the identifier to reference files outside the intended download directory.

Attack Vector

Exploitation occurs over the network via HTTPS to the FMC management interface. The attacker authenticates with valid credentials at the Security Analyst role or higher, then sends a crafted request to the vulnerable file download API. The scope change reflects that data outside the FMC application's authorization scope becomes readable. No user interaction is required beyond the attacker's authenticated session.

No verified public proof-of-concept code is available. See the Cisco Security Advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-20342

Indicators of Compromise

Unexpected HTTPS requests from Security Analyst accounts to the FMC file download API endpoints.
Download of files whose names or paths fall outside standard analyst workflows, such as configuration, credential, or system files.
Elevated request volume to the file download API from a single authenticated session.

Detection Strategies

Review FMC audit logs for file download API calls issued by low-privileged accounts.
Correlate download activity with user role assignments to flag reads that exceed the Security Analyst scope.
Alert on repeated download requests referencing unusual file identifiers or path-like values in API parameters.

Monitoring Recommendations

Forward FMC audit and web-server logs to a centralized log platform for retention and correlation.
Baseline normal analyst API usage and alert on deviations in endpoint, parameter values, or response sizes.
Monitor for authentication anomalies on Security Analyst accounts, including new source IPs, off-hours use, and impossible-travel patterns.

How to Mitigate CVE-2026-20342

Immediate Actions Required

Apply the fixed Cisco Secure FMC Software release listed in the vendor advisory as soon as a maintenance window permits.
Audit and reduce the number of accounts assigned the Security Analyst role or higher; remove stale users.
Rotate credentials for FMC user accounts and enforce multi-factor authentication on the management interface.
Restrict network access to the FMC management interface to trusted administrative networks only.

Patch Information

Cisco has published fixed software addressing CVE-2026-20342. Consult the Cisco Security Advisory cisco-sa-fmc-mulivulns-4PsnFwvx for the specific fixed releases, upgrade guidance, and any dependency requirements.

Workarounds

Cisco has not published a workaround for this vulnerability; upgrading to a fixed release is the required remediation.
Until patching is complete, limit management-plane exposure by placing FMC behind a bastion or dedicated management VLAN.
Review role assignments and remove unnecessary Security Analyst accounts to shrink the pool of users capable of exploitation.

bash

# Restrict FMC management access to a trusted admin subnet (example ACL concept)
access-list FMC_MGMT permit tcp 10.10.0.0 0.0.0.255 host <FMC_IP> eq 443
access-list FMC_MGMT deny   tcp any host <FMC_IP> eq 443