CVE-2002-1108 in VPN Client
Summary
by MITRE
Cisco Virtual Private Network (VPN) Client software 2.x.x, and 3.x before 3.6(Rel), when configured with all tunnel mode, can be forced into acknowledging a TCP packet from outside the tunnel.
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Analysis
by VulDB Data Team • 06/16/2019
The vulnerability described in CVE-2002-1108 represents a significant security flaw in Cisco's VPN client software versions 2.x.x and 3.x prior to 3.6 release. This issue specifically affects configurations using all tunnel mode, which is a critical deployment scenario for many enterprise networks relying on secure remote access solutions. The vulnerability stems from improper handling of TCP packet acknowledgment mechanisms within the VPN client's network stack implementation, creating a pathway for external adversaries to manipulate the client's network behavior through crafted packet sequences.
The technical flaw manifests when the VPN client software receives TCP packets from external sources that are not part of the established secure tunnel. Under normal operating conditions, the client should only acknowledge and process packets that originate from within the trusted tunnel environment. However, this vulnerability allows attackers to force the client into acknowledging external TCP packets, effectively bypassing the normal packet validation and filtering mechanisms. This occurs due to insufficient input validation and improper state management within the TCP stack implementation of the affected Cisco VPN client versions, creating a condition where external network traffic can influence the client's TCP acknowledgment behavior.
The operational impact of this vulnerability extends beyond simple network disruption to potentially enable more sophisticated attacks within the compromised environment. An attacker exploiting this vulnerability could manipulate the TCP window scaling, sequence number handling, or connection state information within the VPN client, potentially leading to connection hijacking, data injection, or denial of service conditions. The vulnerability is particularly concerning for enterprise environments where VPN clients are used for remote access to sensitive corporate resources, as it could allow unauthorized network access or compromise the integrity of the secure communication channel. This flaw essentially undermines the fundamental security assumptions of the VPN client's network isolation capabilities, making it easier for attackers to establish persistent access or conduct man-in-the-middle attacks against the client's network communications.
Organizations should immediately upgrade their Cisco VPN client installations to version 3.6 or later, which includes patches addressing this specific TCP acknowledgment vulnerability. The fix typically involves implementing proper input validation for incoming TCP packets and strengthening the state management mechanisms within the client's TCP stack. Security administrators should also consider implementing network segmentation and additional firewall rules to limit external access to VPN client systems, particularly in environments where the vulnerable software is still in use. This vulnerability aligns with CWE-119, which addresses improper restriction of operations within a limited context, and relates to ATT&CK technique T1046, Network Service Scanning, as attackers could leverage this weakness to gain deeper network access. Regular security assessments and network monitoring should be implemented to detect potential exploitation attempts, particularly in environments where legacy VPN client versions may still be deployed due to compatibility requirements or operational constraints.