CVE-2025-20316 in IOS XE
Summary
by MITRE • 09/24/2025
A vulnerability in the access control list (ACL) programming of Cisco IOS XE Software for Cisco Catalyst 9500X and 9600X Series Switches could allow an unauthenticated, remote attacker to bypass a configured ACL on an affected device.
This vulnerability is due to the flooding of traffic from an unlearned MAC address on a switch virtual interface (SVI) that has an egress ACL applied. An attacker could exploit this vulnerability by causing the VLAN to flush its MAC address table. This condition can also occur if the MAC address table is full. A successful exploit could allow the attacker to bypass an egress ACL on an affected device.
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Analysis
by VulDB Data Team • 10/10/2025
This vulnerability resides within the access control list implementation of Cisco IOS XE Software running on Cisco Catalyst 9500X and 9600X Series Switches, representing a critical weakness in network device security architecture. The flaw manifests when traffic is flooded from an unlearned MAC address on a switch virtual interface where egress ACLs have been configured, creating an unintended pathway for unauthorized network access. The vulnerability stems from the improper handling of MAC address table flush conditions that occur under specific operational circumstances, allowing malicious actors to circumvent security controls that should otherwise restrict traffic flow.
The technical exploitation mechanism involves leveraging the switch's MAC address learning behavior to trigger a table flush event, either through deliberate flooding of traffic from unlearned MAC addresses or when the MAC address table reaches its capacity limits. This condition effectively resets the switch's understanding of legitimate network endpoints, causing the egress ACL enforcement to be bypassed temporarily. The vulnerability is particularly concerning because it operates without requiring authentication credentials, making it accessible to remote attackers who can potentially manipulate network traffic flow and access restricted network segments that should be protected by the configured ACL policies.
From an operational impact perspective, this vulnerability undermines fundamental network security controls by allowing attackers to bypass configured access restrictions that are critical for network segmentation and traffic management. The attack scenario enables unauthorized access to network resources that should be protected by egress ACLs, potentially leading to data exfiltration, network reconnaissance, or lateral movement within the network infrastructure. The vulnerability affects the core functionality of network access control mechanisms and could result in significant security breaches when exploited in environments where network segmentation relies heavily on ACL enforcement.
Security professionals should implement immediate mitigations including network segmentation strategies to limit the impact of potential exploitation, monitoring for unusual MAC address table flush events, and applying Cisco's recommended software updates to address the underlying implementation flaw. The vulnerability aligns with CWE-284 Access Control Bypass, which specifically addresses weaknesses in access control enforcement mechanisms, and corresponds to ATT&CK technique T1046 Network Service Scanning and T1566 Phishing, as attackers may use this vulnerability to establish persistent access to network segments. Organizations should also consider implementing additional network monitoring controls to detect anomalous MAC address table behavior and ensure proper access control enforcement across all network devices.