Cisco Nexus 3000 & 9000 Vulnerability Enables DoS Attacks

Cisco has issued a high-severity security advisory warning of a dangerous vulnerability in its Nexus 3000 and 9000 Series switches that could allow attackers to trigger denial of service (DoS) attacks through crafted network packets.

The vulnerability, tracked as CVE-2025-20241 and assigned a CVSS score of 7.4, affects the Intermediate System-to-Intermediate System (IS-IS) feature in Cisco NX-OS Software running on Nexus 3000 Series Switches and Nexus 9000 Series Switches in standalone NX-OS mode.

The flaw was disclosed in Cisco’s security advisory cisco-sa-n39k-isis-dos-JhJA8Rfx on August 27, 2025.

The vulnerability stems from insufficient input validation when parsing incoming IS-IS packets.

An unauthenticated attacker who is adjacent to the network (Layer 2-adjacent) can exploit this weakness by sending specially crafted IS-IS packets to vulnerable devices, potentially causing the IS-IS process to restart unexpectedly and triggering a complete device reload.

Technical Details and Attack Vector

The IS-IS protocol is a routing protocol commonly used in enterprise networks for dynamic routing.

To successfully exploit this vulnerability, attackers must be positioned on the same network segment as the target device, making it particularly concerning for insider threats or attackers who have already gained initial network access.

The attack mechanism is straightforward but effective: malformed IS-IS packets bypass proper input validation, causing the routing process to crash and forcing the entire switch to reload.

This creates a significant denial of service condition that can disrupt network operations and potentially impact business continuity.

Cisco has confirmed that the vulnerability can only be exploited by an adjacent IS-IS peer in the UP state.

If IS-IS authentication is enabled on the network, attackers would need valid authentication keys to successfully trigger the exploit, providing some level of protection.

Affected Products and Detection

The vulnerability specifically impacts:

• Cisco Nexus 3000 Series Switches
• Cisco Nexus 9000 Series Switches (in standalone NX-OS mode only)

Network administrators can determine if their systems are vulnerable by checking if the IS-IS protocol is enabled using the command show running-config | include isis.

Vulnerable configurations will show features including feature isis, router isis name, and at least one instance of ip router isis name.

To identify IS-IS peers that could potentially exploit this vulnerability, administrators should use the show isis adjacency command to view the current adjacency database.

Cisco has released software updates that completely address this vulnerability, and the company strongly recommends immediate patching as the primary defense measure. No workarounds are available that can fully mitigate the risk.

However, Cisco suggests implementing IS-IS area authentication as a best-practice mitigation strategy.

The relatively high CVSS score and potential for complete device failure make this a priority issue for organizations running affected Cisco equipment.

The timing of this disclosure, as part of Cisco’s August 2025 semiannual security advisory bundle, demonstrates the vendor’s commitment to coordinated vulnerability disclosure while emphasizing the critical nature of maintaining updated network infrastructure in today’s threat landscape.

Organizations should immediately assess their exposure and develop remediation plans to address this vulnerability before it can be exploited by malicious actors.

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