CVE-1999-1129 in IOS
Summary
by MITRE
Cisco Catalyst 2900 Virtual LAN (VLAN) switches allow remote attackers to inject 802.1q frames into another VLAN by forging the VLAN identifier in the trunking tag.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability identified as CVE-1999-1129 represents a critical security flaw in Cisco Catalyst 2900 series Virtual Local Area Network switches that fundamentally undermines network segmentation principles. This vulnerability specifically affects the switch's handling of 802.1q trunking protocols, which are designed to maintain network isolation between different virtual LANs through proper VLAN tagging mechanisms. The flaw enables remote attackers to bypass network security controls by exploiting a weakness in how the switches validate incoming VLAN identifiers, potentially allowing unauthorized access to sensitive network segments.
The technical implementation of this vulnerability stems from insufficient validation of the VLAN identifier field within the 802.1q frame header. When a switch receives a frame with a trunking tag, it should verify that the frame originates from a legitimate source within the specified VLAN. However, the Cisco Catalyst 2900 switches fail to properly authenticate or validate the VLAN identifier, allowing malicious actors to craft frames with arbitrary VLAN tags. This creates a situation where an attacker can inject frames into a VLAN that they should not have access to, effectively breaking the isolation that VLANs are designed to provide. The vulnerability operates at the data link layer and specifically targets the IEEE 802.1q standard implementation, which is a foundational protocol for VLAN operations in enterprise networks.
The operational impact of this vulnerability extends far beyond simple network misconfiguration, as it fundamentally compromises network security boundaries and enables lateral movement within the network infrastructure. An attacker who successfully exploits this vulnerability can gain access to sensitive data, systems, and resources that would normally be isolated within different VLANs. This could result in unauthorized data access, privilege escalation, and potential compromise of critical network resources. The remote nature of the attack means that adversaries do not need physical access to the network infrastructure or direct network connectivity to the switches themselves, making the vulnerability particularly dangerous in environments where network segmentation is critical for security. Organizations relying on VLAN-based security models face significant risk exposure, as this vulnerability essentially nullifies the security benefits of VLAN isolation.
Mitigation strategies for this vulnerability require immediate implementation of network security controls and configuration changes to prevent unauthorized VLAN tagging. Network administrators should implement proper access control lists and port security measures to restrict which devices can participate in trunking operations. The most effective approach involves configuring switches to only accept frames from legitimate trunking partners and implementing strict VLAN access controls that prevent unauthorized frame injection. Additionally, organizations should consider implementing network monitoring solutions that can detect anomalous VLAN tagging patterns and alert administrators to potential exploitation attempts. The vulnerability aligns with CWE-284 which addresses improper access control, and represents a clear violation of the principle of least privilege in network security. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and lateral movement through network infrastructure manipulation, making it a significant concern for organizations implementing defensive security measures.