CVE-2000-0289 in Linux
Summary
by MITRE
IP masquerading in Linux 2.2.x allows remote attackers to route UDP packets through the internal interface by modifying the external source IP address and port number to match those of an established connection.
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Analysis
by VulDB Data Team • 10/17/2025
The vulnerability described in CVE-2000-0289 represents a critical flaw in the Linux 2.2.x kernel's implementation of IP masquerading functionality, which is a fundamental component of network address translation services. This issue specifically affects the handling of UDP packet routing within the kernel's netfilter framework, where the system fails to properly validate source IP addresses and port numbers when processing packets that should be routed through internal interfaces. The vulnerability stems from an insufficient validation mechanism that allows malicious actors to exploit the masquerading system by crafting packets with modified external source addresses that match existing connection tuples.
The technical flaw manifests when the Linux kernel processes UDP packets destined for internal network segments through the masquerading subsystem. Normally, the kernel maintains connection tracking tables that store information about established connections including source and destination IP addresses and port numbers. However, in this vulnerability, attackers can manipulate UDP packets by altering the external source IP address and port number to match those of an already established legitimate connection. This allows the kernel to incorrectly route the malicious packet through the internal interface, effectively bypassing normal network security controls. The flaw operates at the network layer and exploits weaknesses in the connection tracking mechanism, specifically in how the kernel validates packet headers against existing connection state information.
The operational impact of this vulnerability is severe and can lead to significant security breaches within networks that rely on Linux-based masquerading systems for network protection. Attackers can leverage this weakness to gain unauthorized access to internal network resources, potentially allowing them to perform man-in-the-middle attacks, redirect traffic, or establish covert communication channels. The vulnerability essentially allows remote attackers to circumvent network segmentation controls, as packets that should be rejected or properly routed through the external interface are instead forwarded through internal network paths. This creates a pathway for attackers to access sensitive internal systems and data without proper authentication or authorization, particularly affecting organizations using Linux firewalls or routers that implement IP masquerading for network security.
Mitigation strategies for CVE-2000-0289 should focus on immediate kernel updates to versions that address the connection tracking validation issues, as this represents a fundamental flaw in the network stack implementation. Network administrators should implement additional firewall rules to restrict UDP packet forwarding and establish more stringent validation mechanisms at the packet filtering layer. The solution aligns with security best practices outlined in the CWE-119 category for buffer overflow vulnerabilities and follows the ATT&CK framework's network ingress and evasion techniques, specifically targeting the network traffic manipulation and tunneling methods that attackers use to bypass security controls. Organizations should also consider implementing network segmentation strategies that reduce the attack surface and deploy intrusion detection systems that can monitor for anomalous packet routing patterns that may indicate exploitation attempts. This vulnerability highlights the critical importance of proper connection state validation in network security implementations and demonstrates how flaws in core kernel functionality can undermine entire network protection architectures.