CVE-2025-20251: Cisco Firewall File Manipulation Vulnerability

CVE-2025-20251 Overview

CVE-2025-20251 is a high-severity vulnerability in the Remote Access SSL VPN service of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. An authenticated VPN user can send crafted HTTP requests to create or delete arbitrary files on the underlying operating system. Manipulation of critical system files denies new SSL VPN sessions, drops existing sessions, and requires a manual reboot to recover. The flaw is categorized under [CWE-1287] Improper Validation of Specified Type of Input.

Critical Impact
An authenticated remote attacker can trigger a denial of service (DoS) on the Remote Access SSL VPN service, requiring a manual device reboot to restore functionality.

Affected Products

Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
Cisco Secure Firewall Threat Defense (FTD) Software
Devices with Remote Access SSL VPN service enabled

Discovery Timeline

2025-08-14 - CVE-2025-20251 published to NVD
2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-20251

Vulnerability Analysis

The vulnerability resides in the HTTP request processing logic of the Remote Access SSL VPN service on Cisco ASA and FTD Software. The service does not sufficiently validate input contained in HTTP requests submitted by authenticated VPN users. An attacker who holds valid VPN credentials can issue crafted HTTP requests that cause the service to create or delete files on the underlying operating system. When files required for SSL VPN operation are removed or overwritten, the service stops accepting new sessions and terminates active sessions. Recovery requires a manual reboot of the device, extending the duration of the outage.

Root Cause

The root cause is improper validation of input parameters processed by the SSL VPN HTTP handler. Specifically, the service does not constrain file path or file operation parameters supplied through HTTP requests. This permits authenticated callers to influence file system operations outside the bounds intended by the application logic. The weakness maps to [CWE-1287], Improper Validation of Specified Type of Input.

Attack Vector

Exploitation requires network access to the SSL VPN interface and authentication as a valid VPN user. After establishing an authenticated session, the attacker sends crafted HTTP requests containing parameters that direct the service to create or delete files on the underlying OS. Because the attack uses the standard SSL VPN HTTPS channel, no additional network position or privilege escalation step is required. The scope is changed because impact extends beyond the SSL VPN service to the underlying operating system file state.

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

Detection Methods for CVE-2025-20251

Indicators of Compromise

Unexpected creation or deletion of files within ASA/FTD system directories following SSL VPN sessions.
Sudden inability to establish new Remote Access SSL VPN sessions and unexplained drops of active sessions.
Device requires manual reboot to restore SSL VPN service after a period of authenticated VPN activity.

Detection Strategies

Inspect HTTP request logs from the SSL VPN service for unusual parameters referencing file paths or non-standard request patterns from authenticated users.
Correlate authenticated VPN session events with subsequent SSL VPN service failures or session-drop bursts.
Monitor Cisco ASA/FTD syslog facilities for errors related to the SSL VPN process, file access failures, or service restart events.

Monitoring Recommendations

Enable verbose syslog forwarding from ASA/FTD devices to a centralized log platform and alert on SSL VPN service degradation.
Track per-user VPN session activity and flag accounts that generate atypical request volumes or session anomalies.
Establish a baseline of normal SSL VPN HTTP request patterns to detect deviations indicative of crafted requests.

How to Mitigate CVE-2025-20251

Immediate Actions Required

Apply the fixed software releases identified in the Cisco Security Advisory as the primary remediation.
Audit VPN user accounts and disable any unused or stale credentials to reduce the authenticated attack surface.
Enforce strong authentication, including multi-factor authentication, on all Remote Access SSL VPN accounts.

Patch Information

Cisco has published a security advisory tracking this issue under the identifier cisco-sa-asaftd-http-file-hUyX2jL4. Administrators should consult the advisory for the list of affected versions and corresponding fixed releases for ASA and FTD Software, then schedule upgrades according to vendor guidance.

Workarounds

No vendor workaround is documented; upgrading to a fixed release is required to address the vulnerability.
If immediate patching is not possible, restrict SSL VPN access to known source networks and reduce the user population permitted to authenticate to the service.
Increase monitoring of SSL VPN service health so that exploitation attempts that lead to DoS conditions are detected and remediated quickly.

bash

# Configuration example
# Refer to the Cisco Security Advisory for the fixed version applicable to your platform
# https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-http-file-hUyX2jL4