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Cisco Systems released a security advisory about a critical denial-of-service vulnerability in the Nexus 3000 and 9000 Series Switches using NX-OS software. The flaw, identified as CVE-2025-20241 and rated 7.4 on the CVSS scale, can let an unauthenticated nearby attacker interrupt essential network services.
Cisco explains that “a vulnerability in the Intermediate System-to-Intermediate System (IS-IS) feature of Cisco NX-OS Software… could allow an unauthenticated, adjacent attacker to cause the IS-IS process to unexpectedly restart, which could cause an affected device to reload.”
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Insufficient input validation when parsing IS-IS packets can lead to device crashes. Attackers can exploit this by sending a malicious packet, causing the IS-IS process to crash and resulting in a denial-of-service.
Exploitation is limited to attackers with Layer 2 adjacency to the targeted device. The advisory clarifies: “This vulnerability can only be exploited by an adjacent IS-IS peer in the UP state. If IS-IS authentication is enabled, the IS-IS peer would need to use a valid key to exploit this vulnerability.”
Administrators can check whether IS-IS is enabled by running:
show running-config | include isis
If configuration commands such as feature isis, router isis <name>, and ip router isis <name> appear, IS-IS is active. Similarly, the show isis adjacency command reveals active IS-IS peers.
The vulnerability affects:
Cisco Nexus 3000 Series Switches
Cisco Nexus 9000 Series Switches in standalone NX-OS mode
Only devices with the IS-IS routing protocol enabled are vulnerable.
Cisco has released free software updates to fix the flaw. Customers with service contracts can access these fixes through their usual update channels.
For organizations those who can’t upgrade right away, using strong keys for IS-IS authentication can lower the risk of attacks.
