CVE-2001-0405 in Linux
Summary
by MITRE
ip_conntrack_ftp in the IPTables firewall for Linux 2.4 allows remote attackers to bypass access restrictions for an FTP server via a PORT command that lists an arbitrary IP address and port number, which is added to the RELATED table and allowed by the firewall.
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Analysis
by VulDB Data Team • 11/25/2024
The vulnerability described in CVE-2001-0405 represents a significant flaw in the Linux kernel's netfilter framework, specifically within the ip_conntrack_ftp module that handles connection tracking for ftp protocol traffic. This issue affects Linux 2.4 kernel versions and demonstrates how improper handling of ftp protocol commands can lead to complete bypass of firewall restrictions. The vulnerability stems from the way the kernel processes ftp PORT commands, which are used by ftp clients to specify the IP address and port number where the server should establish data connections. When an ftp client sends a PORT command, the ip_conntrack_ftp module parses this information and creates a connection tracking entry in the RELATED table, allowing subsequent data connections to proceed through the firewall.
The technical exploitation of this vulnerability occurs when a remote attacker crafts a malicious ftp PORT command that specifies an arbitrary IP address and port number different from the actual ftp server. This crafted command gets processed by the ip_conntrack_ftp module, which then adds the specified IP address and port to the connection tracking table with a RELATED state. The firewall rules subsequently allow traffic matching this connection tracking entry, effectively creating a backdoor that bypasses all access restrictions that would normally be enforced by the firewall. This flaw fundamentally undermines the security model of the firewall by allowing unauthorized access to ftp server resources through manipulation of the connection tracking mechanism.
The operational impact of this vulnerability is severe as it provides attackers with a straightforward method to circumvent firewall protections without requiring authentication or sophisticated exploitation techniques. An attacker can use this vulnerability to gain access to ftp servers that are otherwise protected by firewall rules, potentially leading to unauthorized data access, data modification, or complete system compromise. The vulnerability affects any system running Linux 2.4 kernel with iptables firewall configured to handle ftp traffic, making it particularly dangerous in enterprise environments where ftp services are commonly used for file transfers. The flaw also demonstrates how protocol-specific connection tracking modules can introduce security weaknesses that extend beyond the intended scope of firewall protection.
This vulnerability maps to CWE-284 Access Control Bypass and aligns with several ATT&CK techniques including T1071.004 Application Layer Protocol FTP and T1566 Phishing. The issue highlights the importance of proper input validation in protocol handling modules and demonstrates how seemingly benign protocol features can be exploited to undermine security controls. Organizations should implement immediate mitigations including updating to patched kernel versions, disabling ftp connection tracking when not required, or implementing additional firewall rules that specifically block suspicious ftp PORT commands. The vulnerability also underscores the need for comprehensive security testing of connection tracking modules and the importance of maintaining up-to-date kernel versions to protect against known vulnerabilities in network stack components.
The broader implications of this vulnerability extend beyond simple access bypass to highlight fundamental security considerations in kernel-level network processing. It demonstrates that protocol handling modules within the kernel must be carefully designed with security in mind, particularly when dealing with commands that can alter the network state or connection tracking tables. The flaw emphasizes the critical importance of validating all information received from network protocols before incorporating it into connection tracking state tables, as this information directly influences firewall decision-making processes. This vulnerability serves as a reminder that even well-established security mechanisms like connection tracking can introduce new attack surfaces when not properly secured against malicious input manipulation.