CVE-2026-20250: Cisco Secure Firewall DTLS DoS Vulnerability

CVE-2026-20250 is a denial of service vulnerability in Cisco Secure Firewall ASA and FTD Software affecting 3100 and 4200 Series devices. Improper DTLS message handling allows attackers to cause device reloads. This article covers technical details, affected versions, impact assessment, and mitigation strategies.

CVE-2026-20250 Overview

Cisco disclosed CVE-2026-20250, a denial of service (DoS) vulnerability in the Datagram Transport Layer Security (DTLS) message handling of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. The flaw affects Cisco Secure Firewall 3100 Series and 4200 Series devices. An unauthenticated remote attacker can send a crafted stream of DTLS traffic to force an affected device to reload. The root cause is improper resource management [CWE-772] during DTLS message processing.

Critical Impact
An unauthenticated remote attacker can reload the firewall by sending crafted DTLS traffic, disrupting perimeter and VPN services.

Affected Products

Cisco Secure Firewall ASA Software on 3100 Series and 4200 Series devices
Cisco Secure Firewall Threat Defense (FTD) Software on 3100 Series and 4200 Series devices
Cisco Secure Firewall 3100 and 4200 Series appliances with DTLS-enabled services

Discovery Timeline

2026-09-16 - CVE-2026-20250 published to the National Vulnerability Database
2026-09-17 - Last updated in NVD database

Technical Details for CVE-2026-20250

Vulnerability Analysis

The vulnerability resides in the DTLS message handling logic of Cisco ASA and FTD Software running on Secure Firewall 3100 and 4200 Series hardware. DTLS is commonly used for remote access VPN data channels, including AnyConnect and Secure Client sessions. Processing certain DTLS messages does not release allocated resources properly, which aligns with the CWE-772 (Missing Release of Resource after Effective Lifetime) classification.

As a stream of crafted DTLS packets arrives, the device consumes internal resources without freeing them. Once these resources are depleted, the device reloads to recover, causing a service outage. The scope change reflected in the CVSS vector indicates the impact extends beyond the DTLS component to the availability of the entire appliance, including traffic inspection and VPN termination.

Root Cause

The defect is improper resource management during DTLS packet processing. The affected code paths allocate memory or session-tracking structures for inbound DTLS messages but fail to release them under specific conditions, leading to resource exhaustion and a forced reload.

Attack Vector

Exploitation requires only network reachability to a DTLS listener on an affected appliance. No authentication or user interaction is needed. The attacker sends a sustained, crafted DTLS traffic stream toward an interface exposing DTLS, such as a remote access VPN endpoint. Successful exploitation causes the device to reload, dropping all firewall and VPN sessions.

No public proof-of-concept, exploit code, or evidence of in-the-wild exploitation was available at publication. Refer to the Cisco Security Advisory for detailed technical guidance.

Detection Methods for CVE-2026-20250

Indicators of Compromise

Unplanned reloads of ASA or FTD devices on Secure Firewall 3100 or 4200 Series hardware
Crash or traceback files referencing DTLS session handling generated near the time of an outage
Sudden termination of active AnyConnect or Secure Client DTLS sessions across multiple users
Spikes in inbound UDP traffic to DTLS ports (typically UDP/443) from unusual sources

Detection Strategies

Monitor syslog for %ASA- reload and traceback messages correlated with DTLS activity
Alert on repeated reboot events on Secure Firewall 3100 and 4200 Series appliances
Baseline DTLS handshake and session volumes and alert on anomalies from single or small source sets
Correlate VPN session drops with device health telemetry to distinguish DoS from routine failover

Monitoring Recommendations

Forward ASA and FTD syslog, SNMP traps, and NetFlow to a central SIEM for cross-referencing
Track CPU, memory, and connection-table utilization on affected appliances at short intervals
Enable and review DTLS-related debug counters using show crypto ssl and related commands during incidents
Ingest firewall telemetry into a data lake to preserve evidence across reload events

How to Mitigate CVE-2026-20250

Immediate Actions Required

Review the Cisco Security Advisory and identify appliances running affected ASA or FTD releases on Secure Firewall 3100 or 4200 Series hardware
Upgrade to the fixed ASA or FTD software versions listed by Cisco as soon as a maintenance window allows
Restrict inbound DTLS traffic to known VPN client source ranges where operationally feasible
Enable enhanced logging on Internet-facing interfaces terminating DTLS to accelerate incident response

Patch Information

Cisco has published fixed software releases in the Cisco Security Advisory cisco-sa-asaftd-dtls-dos-Kp57HkyO. Administrators should consult the advisory for the specific fixed versions applicable to their ASA or FTD deployment and follow standard Cisco upgrade procedures.

Workarounds

Cisco's advisory is authoritative on workarounds; consult it before applying any mitigation
Where DTLS is not required, disable DTLS on the VPN configuration and fall back to TLS for the data channel
Apply upstream access control lists to limit which sources can reach DTLS listeners on the appliance
Use rate limiting or DDoS protection in front of the firewall to blunt sustained crafted DTLS floods

bash

# Example: disable DTLS on an AnyConnect group policy (validate against Cisco guidance before use)
configure terminal
group-policy DfltGrpPolicy attributes
 webvpn
  anyconnect ssl dtls none
end
write memory