CVE-2025-20253: 
Cisco Adaptive Security Appliance (ASA) vulnerability analysis and mitigation

CVE-2025-20253 is a Denial of Service (DoS) vulnerability in the IKEv2 feature of Cisco IOS Software, IOS XE Software, Secure Firewall ASA Software, and Secure Firewall Threat Defense (FTD) Software. The flaw allows an unauthenticated, remote attacker to cause an affected device to reload by sending crafted IKEv2 packets that trigger an infinite loop exhausting system resources. It was discovered internally by Jason Crowder of the Cisco Advanced Security Initiatives Group (ASIG) and publicly disclosed on August 14, 2025, as part of Cisco's August 2025 Semiannual Secure Firewall Security Advisory Bundled Publication. The vulnerability carries a CVSS v3.1 base score of 8.6 (High) (Cisco Advisory).

The root cause is classified as CWE-835 (Loop with Unreachable Exit Condition / 'Infinite Loop'), arising from improper processing of IKEv2 packets in the affected Cisco software stacks. An attacker exploits this by sending specially crafted IKEv2 packets over UDP (ports 500 or 4500) to a device with the IKEv2 VPN feature enabled; no authentication or prior access is required. The malformed packets cause the IKEv2 processing logic to enter an infinite loop, exhausting CPU and memory resources until the device reloads. The vulnerability is tracked under Cisco Bug IDs CSCwn73399 and CSCwn83263, and affects IOS, IOS XE, ASA, and FTD software when IKEv2 is actively configured — the Group Encrypted Transport VPN (GET VPN) feature is explicitly not affected (Cisco Advisory).

Successful exploitation results in a complete availability impact: the targeted device reloads, disrupting all network services it provides, including VPN tunnels, routing, and firewall functions. Because the scope is marked as Changed in the CVSS vector, the impact can extend beyond the directly affected device to dependent network segments and connected systems that rely on the device for connectivity or security enforcement. There is no confidentiality or integrity impact — the vulnerability is purely a DoS condition. Repeated exploitation could render critical network infrastructure persistently unavailable (Cisco Advisory, Feedly).

As of the disclosure date, Cisco PSIRT is not aware of any public announcements or malicious use of this vulnerability in the wild, and no public proof-of-concept exploit code has been identified (Cisco Advisory). The EPSS score is approximately 0.138%, indicating a low probability of exploitation in the near term. The vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. However, the low attack complexity and lack of authentication requirements make it an attractive target for threat actors seeking to disrupt network infrastructure.

1. Reconnaissance: Identify internet-facing Cisco IOS, IOS XE, ASA, or FTD devices with IKEv2 VPN enabled using tools such as Shodan or Censys, filtering for devices with UDP ports 500 or 4500 open.
2. Confirm IKEv2 exposure: Probe the target with standard IKEv2 initiation packets to confirm the device responds to IKEv2 negotiation, indicating the feature is active.
3. Craft malicious IKEv2 packets: Construct specially crafted IKEv2 packets designed to trigger the improper processing condition — specific packet structures that cause the IKEv2 handler to enter an unreachable exit loop.
4. Send crafted packets: Transmit the malformed IKEv2 packets to the target device on UDP port 500 or 4500 from any remote IP address, requiring no credentials or prior session establishment.
5. Trigger infinite loop and DoS: The device's IKEv2 processing enters an infinite loop, exhausting CPU and memory resources, ultimately causing the device to reload and resulting in a DoS condition for all dependent services (Cisco Advisory).

• Network: Unexpected high-volume UDP traffic to ports 500 or 4500 from external or unknown IP addresses; repeated IKEv2 initiation attempts from a single source without completing negotiation.
• Logs: Syslog or device logs showing IKEv2 processing errors or crashes immediately before an unexpected reload; crash dump files referencing IKEv2 or ISAKMP processing modules.
• Device Behavior: Unexpected device reloads or reboots with no corresponding hardware fault; sustained high CPU utilization attributed to IKEv2/ISAKMP processes prior to reload.
• System: Post-reload core dump files on the device filesystem referencing IKEv2 infinite loop conditions; repeated reload events within a short timeframe without administrative action (Cisco Advisory).

Cisco has released free software updates addressing this vulnerability as part of the August 2025 Semiannual Secure Firewall Security Advisory Bundled Publication. Customers should use the Cisco Software Checker to identify the earliest fixed release for their specific platform and version. Cisco has confirmed there are no workarounds that address CVE-2025-20253 specifically. As interim risk reduction measures, organizations should restrict IKEv2 traffic (UDP 500/4500) to known, trusted peer IP addresses using ACLs or upstream firewall rules, and monitor for anomalous IKEv2 traffic patterns. Devices not requiring IKEv2 should have the feature disabled (Cisco Advisory).

The vulnerability was covered by security news outlets including GBHackers and CyberNoz, which highlighted the broader set of Cisco IKEv2 DoS flaws disclosed in the August 2025 bundle (GBHackers, CyberNoz). The Center for Internet Security (CIS) issued an advisory noting that multiple vulnerabilities in Cisco security products could allow for significant impacts (CIS Advisory). CISA included the vulnerability in its weekly vulnerability bulletin for the week of August 11, 2025. Community reaction has been moderate, with security professionals noting the broad scope of affected products and the unauthenticated nature of the attack as key concerns.