CVE-2025-20311 in IOS XE
Summary
by MITRE • 09/24/2025
A vulnerability in the handling of certain Ethernet frames in Cisco IOS XE Software for Catalyst 9000 Series Switches could allow an unauthenticated, adjacent attacker to cause an egress port to become blocked and drop all outbound traffic.
This vulnerability is due to improper handling of crafted Ethernet frames. An attacker could exploit this vulnerability by sending crafted Ethernet frames through an affected switch. A successful exploit could allow the attacker to cause the egress port to which the crafted frame is forwarded to start dropping all frames, resulting in a denial of service (DoS) condition.
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Analysis
by VulDB Data Team • 01/07/2026
This vulnerability resides within Cisco IOS XE Software running on Catalyst 9000 Series Switches, specifically affecting how the system processes certain Ethernet frames at the ingress port level. The flaw manifests when the switch receives malformed or specially crafted Ethernet frames that trigger an unexpected behavior in the frame processing pipeline. According to the Common Weakness Enumeration framework, this vulnerability maps to CWE-129, which describes improper validation of input boundaries, and CWE-20, which covers input validation issues that can lead to unexpected system behavior. The vulnerability is classified as a network-level denial of service that operates at the data link layer, affecting the switch's ability to properly forward traffic through egress ports.
The technical exploitation mechanism involves an adjacent attacker who can directly connect to the switch's network interface and transmit specially crafted Ethernet frames designed to trigger the vulnerable code path. The vulnerability stems from inadequate validation of frame headers and payload structures, particularly when processing frames that contain unexpected or malformed fields that the switch's Ethernet frame parser cannot properly handle. This improper frame handling causes the switch to incorrectly process the frame, leading to a state where the egress port associated with the frame's destination becomes functionally disabled. The switch essentially enters a protective mode where it drops all subsequent frames destined for that port, effectively creating a denial of service condition that impacts network connectivity for all services relying on that specific egress path.
From an operational impact perspective, this vulnerability creates a significant risk for network availability and service continuity, particularly in environments where Catalyst 9000 Series Switches serve as core network infrastructure components. The DoS condition affects only the specific egress port that becomes blocked, but this can cascade into broader network disruption depending on the switch's role and the topology. Network administrators may experience unexpected traffic blackouts on affected ports, potentially impacting critical applications or services that depend on those specific network paths. The vulnerability is particularly concerning because it requires no authentication credentials and can be exploited by attackers who are physically present on the network segment, aligning with the MITRE ATT&CK framework's T1046 technique for network service scanning and T1498 for network denial of service attacks. The impact severity ranges from moderate to high depending on the switch's role in the network infrastructure.
Mitigation strategies should focus on network segmentation and access control measures to prevent unauthorized physical access to network switches. Implementing network access control lists and port security features can help limit the attack surface by restricting which devices can connect to switch ports. Cisco recommends applying the latest software patches and updates to address this vulnerability, as the company has released firmware updates specifically targeting this flaw. Network administrators should also consider implementing monitoring solutions that can detect unusual traffic patterns or port blocking behaviors that might indicate exploitation attempts. Additional defensive measures include disabling unused ports, implementing MAC address filtering, and establishing network segmentation policies that isolate critical infrastructure components from potentially untrusted network segments. The vulnerability's adjacency requirement suggests that physical security controls and network access controls should be strengthened as part of a comprehensive security posture to prevent unauthorized access to network infrastructure devices.