CVE-2001-1056 in Linux
Summary
by MITRE
IRC DCC helper in the ip_masq_irc IP masquerading module 2.2 allows remote attackers to bypass intended firewall restrictions by causing the target system to send a "DCC SEND" request to a malicious server which listens on port 6667, which may cause the module to believe that the traffic is a valid request and allow the connection to the port specified in the DCC SEND request.
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Analysis
by VulDB Data Team • 04/13/2019
The vulnerability described in CVE-2001-1056 represents a significant security flaw in the Linux kernel's IP masquerading implementation, specifically within the ip_masq_irc module version 2.2. This issue affects systems running older Linux kernels that utilize the ip_masq_irc module for managing internet relay chat DCC (Direct Client-to-Client) connections. The vulnerability stems from improper validation of DCC SEND requests, which are used by IRC clients to establish direct file transfer connections between users. When a malicious server listens on port 6667 and receives a DCC SEND request from a target system, the module fails to properly authenticate or validate the request source, leading to potential unauthorized network connections.
The technical flaw in this vulnerability resides in the insufficient input validation and trust model implemented within the ip_masq_irc module. According to CWE-284, this represents an improper access control issue where the system grants access based on flawed assumptions about request legitimacy. The module operates under the assumption that any DCC SEND request originating from a valid IRC connection is trustworthy, without adequately verifying whether the request comes from an authorized source or if the destination port specified in the request is legitimate. This trust model creates a pathway for attackers to manipulate the firewall behavior by crafting malicious DCC requests that appear to come from legitimate IRC traffic.
The operational impact of this vulnerability extends beyond simple firewall bypass, creating potential risks for network security and system integrity. Attackers can exploit this flaw to establish unauthorized connections to arbitrary ports on target systems, potentially allowing them to access internal services that would normally be protected by firewall rules. This vulnerability specifically targets the IP masquerading functionality that allows multiple hosts to share a single public IP address, making it particularly dangerous in environments where such masquerading is used for network security. The attack vector involves sending a specially crafted DCC SEND request that tricks the ip_masq_irc module into believing it's a legitimate connection attempt, thereby opening firewall rules for the specified destination port.
From an ATT&CK framework perspective, this vulnerability maps to multiple techniques including T1071.004 for application layer protocol usage and T1562.001 for defense evasion. The exploitation process demonstrates how attackers can leverage protocol-specific weaknesses to circumvent network security controls, representing a classic case of protocol manipulation that bypasses traditional firewall protections. Organizations using affected kernel versions are particularly vulnerable because the flaw exists at the kernel level, making it difficult to patch without system upgrades or kernel modifications. The vulnerability also highlights the importance of proper input validation and the dangers of implementing trust models that rely on superficial protocol characteristics rather than robust authentication mechanisms.
Mitigation strategies for this vulnerability require immediate attention from system administrators and security teams. The most effective approach involves upgrading to newer kernel versions that contain fixed implementations of the IP masquerading modules, as the original vulnerability was addressed in subsequent releases. Organizations should also implement additional network monitoring to detect unusual DCC SEND requests and establish stricter firewall rules that limit outbound connections from IRC clients. The solution aligns with security best practices outlined in NIST SP 800-53 for access control and network protection, emphasizing the need for proper input validation and secure configuration management. Additionally, implementing network segmentation and using more robust firewall solutions that can properly validate connection requests would provide additional layers of protection against similar exploitation techniques.