CVE-2025-20311: Cisco IOS XE Catalyst 9000 DoS Vulnerability

CVE-2025-20311 Overview

CVE-2025-20311 is a denial of service vulnerability in Cisco IOS XE Software running on Catalyst 9000 Series Switches. The flaw stems from improper handling of crafted Ethernet frames. An unauthenticated, adjacent attacker can send specially crafted frames through an affected switch to force the egress port to block and drop all outbound traffic.

The vulnerability is tracked under [CWE-19] (Data Processing Errors) and affects network availability without requiring authentication or user interaction. Exploitation requires adjacent network access, limiting attackers to the same broadcast domain or directly connected segments.

Critical Impact

Successful exploitation causes a sustained denial of service on the affected egress port, dropping all forwarded traffic and disrupting connectivity for downstream hosts and services.

Affected Products

• Cisco IOS XE Software
• Cisco Catalyst 9000 Series Switches
• Network segments connected to affected switch egress ports

Discovery Timeline

• 2025-09-24 - CVE-2025-20311 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-20311

Vulnerability Analysis

The vulnerability resides in the Ethernet frame processing logic of Cisco IOS XE Software on Catalyst 9000 Series Switches. When the switch receives certain malformed or crafted Ethernet frames, the forwarding engine fails to handle them correctly. The defect causes the egress interface selected for the offending frame to enter a blocking state.

Once blocked, the egress port drops all outbound traffic, not just the malicious frames. This produces a persistent denial of service condition affecting every host or service that depends on the impacted port. Recovery typically requires administrator intervention to restore the interface.

The issue is categorized under [CWE-19] for data processing errors and impacts availability only. Confidentiality and integrity of switch data are not affected by this flaw.

Root Cause

The root cause is improper validation and handling of specific Ethernet frame structures within the data plane of the Catalyst 9000 platform. The switch's frame processing path does not properly recover from the malformed input. Instead of dropping the individual frame and continuing normal forwarding, the egress port transitions into a state where it discards all subsequent traffic.

Attack Vector

Exploitation requires adjacent network access, meaning the attacker must be on the same Layer 2 segment as an interface of the affected switch. The attacker sends crafted Ethernet frames that traverse the switch and are forwarded toward a target egress port. No credentials and no user interaction are required.

The scope is changed because the impact extends beyond the switch itself to downstream networks and hosts whose connectivity depends on the affected port. Refer to the Cisco Security Advisory for additional technical context.

Detection Methods for CVE-2025-20311

Indicators of Compromise

• Sudden and sustained traffic drops on a Catalyst 9000 egress interface with no corresponding configuration change
• Interface counters showing rising output drops while the link remains operationally up
• Reports of connectivity loss from hosts and services downstream of a specific switch port
• Recent receipt of unusual or malformed Ethernet frames preceding the outage

Detection Strategies

• Monitor show interfaces output for anomalous output drop counts and queue stalls on Catalyst 9000 switches
• Correlate SNMP interface counters and syslog events across switches to identify ports that stop forwarding without administrative action
• Baseline normal Layer 2 frame patterns and alert on unexpected EtherType values, malformed headers, or protocol anomalies

Monitoring Recommendations

• Ingest switch syslog and SNMP telemetry into a centralized logging and analytics platform for correlation
• Configure alerts on interface output drop rate thresholds and unexplained traffic loss on uplink and access ports
• Review port security and storm control logs for adjacent endpoints generating abnormal frame patterns

How to Mitigate CVE-2025-20311

Immediate Actions Required

• Identify all Catalyst 9000 Series Switches running Cisco IOS XE Software in the environment and prioritize patching per the vendor advisory
• Restrict physical and logical access to switch ports so that only trusted endpoints can inject Ethernet frames into the Layer 2 domain
• Establish a recovery procedure to quickly reset affected interfaces using shutdown followed by no shutdown if a port enters the blocked state

Patch Information

Cisco has published fixed software releases in the Cisco Security Advisory cisco-sa-cat9k-PtmD7bgy. Administrators should consult the advisory for the specific fixed IOS XE versions corresponding to their Catalyst 9000 model and upgrade path. No official workarounds replace the patch.

Workarounds

• Apply Layer 2 access controls such as 802.1X authentication, port security, and private VLANs to limit which devices can transmit frames through the switch
• Segment untrusted devices onto isolated VLANs that do not share egress paths with critical infrastructure
• Maintain out-of-band management access so administrators can recover affected interfaces if a denial of service occurs