CVE-2001-1155 in FreeBSD
Summary
by MITRE
TCP Wrappers (tcp_wrappers) in FreeBSD 4.1.1 through 4.3 with the PARANOID ACL option enabled does not properly check the result of a reverse DNS lookup, which could allow remote attackers to bypass intended access restrictions via DNS spoofing.
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Analysis
by VulDB Data Team • 02/11/2017
The vulnerability described in CVE-2001-1155 represents a critical flaw in the TCP Wrappers implementation within FreeBSD operating systems version 4.1.1 through 4.3 when configured with the PARANOID access control list option. This issue stems from insufficient validation of reverse DNS lookup results, creating a security gap that adversaries can exploit to circumvent access controls. The TCP Wrappers mechanism serves as a fundamental network access control tool that provides a layer of security by allowing or denying access based on client hostnames, IP addresses, or other criteria through the use of access control lists. When the PARANOID option is enabled, the system performs additional checks including reverse DNS lookups to verify client hostnames against configured access restrictions, but this process contains a critical implementation flaw.
The technical flaw manifests in how the system handles reverse DNS resolution results when the PARANOID ACL option is active. During normal operation, TCP Wrappers performs a reverse DNS lookup to resolve client IP addresses to hostnames, then compares these hostnames against access control rules. However, the implementation fails to properly validate the DNS lookup results, particularly when DNS spoofing attacks occur. This validation gap allows attackers to manipulate DNS responses in a way that bypasses the intended access restrictions. The vulnerability specifically affects systems where the PARANOID option is enabled, which is designed to provide enhanced security by performing additional hostname verification steps. When an attacker can successfully spoof DNS responses, they can present false hostname information that passes the flawed validation checks, thereby gaining unauthorized access that would otherwise be blocked by the access control mechanisms.
The operational impact of this vulnerability is significant as it undermines the fundamental security model of TCP Wrappers and exposes systems to unauthorized access. Attackers exploiting this vulnerability can bypass network access restrictions that are typically enforced by services using TCP Wrappers, potentially gaining access to sensitive network services and resources. This flaw is particularly dangerous because it operates at the network layer, affecting multiple services that rely on TCP Wrappers for access control. The vulnerability can be exploited from remote locations without requiring local system access, making it a serious concern for network administrators. Systems configured with PARANOID ACL settings become particularly vulnerable, as these configurations are often chosen specifically to provide enhanced security through additional hostname verification. The implications extend beyond simple access bypass to potentially allow for more sophisticated attacks, as compromised services may provide attackers with additional attack vectors and potential system compromise opportunities.
Mitigation strategies for this vulnerability involve multiple approaches that address both the immediate flaw and broader security considerations. The most direct solution is to disable the PARANOID ACL option when it is not essential for security operations, as this removes the vulnerable code path entirely. System administrators should also ensure that DNS resolution is properly secured through DNSSEC implementation and other DNS security measures to prevent DNS spoofing attacks. Additionally, network segmentation and firewall rules should be implemented to provide defense-in-depth measures that do not rely solely on TCP Wrappers for access control. The vulnerability aligns with CWE-20, which describes improper input validation, and can be mapped to ATT&CK technique T1071.004 for application layer protocol: DNS, as it exploits weaknesses in DNS resolution validation. Organizations should also consider upgrading to newer FreeBSD versions where this vulnerability has been addressed, as the affected versions are outdated and may contain additional unpatched security issues. Regular network monitoring and intrusion detection systems should be deployed to detect potential DNS spoofing activities that could be used to exploit this vulnerability.