CVE-2026-20048: Cisco Nexus 9000 SNMP DoS Vulnerability

CVE-2026-20048 Overview

A vulnerability in the Simple Network Management Protocol (SNMP) subsystem of Cisco Nexus 9000 Series Fabric Switches in ACI mode could allow an authenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to improper processing when parsing SNMP requests, which can be exploited by continuously sending SNMP queries to a specific MIB of an affected device. A successful exploit could allow the attacker to cause a kernel panic on the device, resulting in a reload and a DoS condition.

Critical Impact

Authenticated attackers can trigger kernel panic and device reload via malformed SNMP requests, causing network infrastructure outages in ACI fabric environments.

Affected Products

• Cisco Nexus 9000 Series Fabric Switches in ACI mode (SNMP v1)
• Cisco Nexus 9000 Series Fabric Switches in ACI mode (SNMP v2c)
• Cisco Nexus 9000 Series Fabric Switches in ACI mode (SNMP v3)

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-20048 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-20048

Vulnerability Analysis

This Denial of Service vulnerability exists in the SNMP subsystem of Cisco Nexus 9000 Series Fabric Switches operating in Application Centric Infrastructure (ACI) mode. The flaw stems from improper input validation when processing SNMP requests targeting specific Management Information Base (MIB) objects on the affected device. When an authenticated attacker sends crafted SNMP queries to certain MIBs, the device fails to properly handle the parsing operation, leading to memory allocation issues that ultimately trigger a kernel panic.

The vulnerability affects all three versions of SNMP (v1, v2c, and v3), though exploitation requires valid credentials for each version. For SNMPv1 and SNMPv2c, the attacker must possess a valid read-only SNMP community string. For SNMPv3, valid SNMP user credentials are required. This authentication requirement somewhat limits the attack surface but does not eliminate the risk, as SNMP credentials are often shared or configured with weak values in enterprise environments.

Root Cause

The root cause of CVE-2026-20048 is CWE-789: Memory Allocation with Excessive Size Value. The SNMP subsystem fails to properly validate the size of memory allocations when processing certain SNMP query requests. When parsing SNMP requests directed at specific MIB objects, the system may attempt to allocate an excessively large amount of memory, which exhausts available resources and triggers a kernel panic. This improper memory allocation handling results in system instability and forces a device reload.

Attack Vector

The attack vector for this vulnerability is network-based, allowing authenticated remote attackers to exploit the flaw without physical access to the device. An attacker with valid SNMP credentials can continuously send SNMP queries targeting specific MIB objects on the affected Nexus 9000 switch. The attack does not require user interaction and can be executed from any network location that has SNMP access to the vulnerable device.

The attack flow proceeds as follows: The attacker first obtains valid SNMP credentials (community string for v1/v2c or user credentials for v3). The attacker then crafts SNMP GET or GETNEXT requests targeting the vulnerable MIB. By continuously sending these requests, the SNMP parsing routine triggers excessive memory allocation, leading to resource exhaustion and a kernel panic. The device subsequently reloads, causing a denial of service that disrupts ACI fabric operations.

Detection Methods for CVE-2026-20048

Indicators of Compromise

• Unexpected device reloads or kernel panic events logged in system event history
• Anomalous SNMP traffic patterns including high volume of requests to specific MIB objects
• Authentication logs showing SNMP access from unexpected IP addresses
• Sudden increase in SNMP GET/GETNEXT requests targeting the affected MIBs

Detection Strategies

• Configure SNMP logging to capture all SNMP requests and monitor for unusual patterns
• Implement network traffic analysis to detect high-frequency SNMP queries from single sources
• Enable syslog monitoring for kernel panic events and unexpected device reload messages
• Deploy intrusion detection signatures to identify potential SNMP-based DoS attack patterns

Monitoring Recommendations

• Monitor Cisco Nexus 9000 devices for unexpected reload events and correlate with SNMP traffic logs
• Implement SNMP access control lists (ACLs) and log denied access attempts
• Set up alerting for kernel panic events and multiple device reloads within short time periods
• Review SNMP community strings and user credentials for any unauthorized access or modifications

How to Mitigate CVE-2026-20048

Immediate Actions Required

• Review the Cisco Security Advisory for the latest patch information
• Restrict SNMP access to trusted management networks using access control lists
• Change default SNMP community strings and implement strong SNMPv3 credentials
• Consider disabling SNMP if not required for network operations

Patch Information

Cisco has released a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory for specific patched software versions and upgrade instructions. Organizations should prioritize patching Nexus 9000 Series switches operating in ACI mode that have SNMP enabled and accessible from untrusted networks.

Workarounds

• Implement SNMP access control lists to restrict SNMP access to trusted management stations only
• Disable SNMP on affected devices if it is not required for network management operations
• Migrate from SNMPv1/v2c to SNMPv3 with strong authentication and encryption to reduce credential exposure risk
• Segment management traffic using dedicated out-of-band management networks

# Example: Configure SNMP ACL on Cisco NX-OS
# Restrict SNMP access to trusted management subnet
configure terminal
ip access-list snmp-acl
  permit ip 10.0.100.0/24 any
  deny ip any any
snmp-server community <community-string> use-acl snmp-acl
end
write memory