Introduction
A single malformed packet can take critical network infrastructure offline. The recently disclosed CVE-2025-20253 is a flaw in Cisco's IKEv2 implementation that lets unauthenticated remote attackers force device reloads, causing denial of service on enterprise and service provider networks.
Cisco is a global leader in networking and security appliances, and its IOS, IOS XE, ASA, and FTD software run a large share of the world's business and service provider networks. Global connectivity and business operations depend on these products being secure and reliable.
Technical information
CVE-2025-20253 comes from improper processing of IKEv2 packets in Cisco IOS Software, IOS XE Software, Secure Firewall ASA Software, and Secure FTD Software. Specifically, these products do not validate input well enough when they parse IKEv2 packets. An unauthenticated remote attacker can send specially crafted IKEv2 packets to a vulnerable device. Processing those packets sends the IKEv2 process into an infinite loop, which exhausts system resources and forces the device to reload. The result is a denial of service.
The vulnerability is classified as CWE-835 (Loop with Unreachable Exit Condition). Attackers need no authentication and can exploit it remotely by targeting UDP ports 500 and 4500, the standard ports for IKEv2 traffic. No public code snippets or detailed protocol structures have been disclosed for this vulnerability.
Affected systems and versions
The public advisory does not specify exact affected versions for any of these products:
- Cisco IOS Software
- Cisco IOS XE Software
- Cisco Secure Firewall ASA Software
- Cisco Secure Firewall Threat Defense (FTD) Software
Any configuration with IKEv2 enabled and exposed to untrusted networks is potentially vulnerable. The flaw affects devices that process IKEv2 packets on UDP ports 500 and 4500.
Vendor security history
Cisco has had IKEv2 and protocol parsing vulnerabilities before. CVE-2025-20182, disclosed in May 2025, affected similar product lines and let unauthenticated remote attackers cause a denial of service with crafted IKEv2 messages. CVE-2023-20109 was another IKEv2 flaw in Cisco products. Cisco typically responds promptly, with bundled advisories and coordinated patch releases, but protocol parsing flaws keep recurring, which shows how hard complex network software remains to secure.
