CVE-2005-0337 in Red Hat
Summary
by MITRE
Postfix 2.1.3, when /proc/net/if_inet6 is not available and permit_mx_backup is enabled in smtpd_recipient_restrictions, allows remote attackers to bypass e-mail restrictions and perform mail relaying by sending mail to an IPv6 hostname.
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Analysis
by VulDB Data Team • 06/06/2019
The vulnerability described in CVE-2005-0337 represents a significant security flaw in Postfix email server software version 2.1.3 that specifically affects systems lacking IPv6 network interface support. This issue arises from the software's handling of email routing decisions when IPv6 functionality is unavailable, creating an unexpected path for malicious actors to circumvent email security controls. The vulnerability is particularly concerning because it leverages the absence of IPv6 support in the underlying operating system to bypass established email restriction mechanisms that are designed to prevent unauthorized mail relaying.
The technical root cause of this vulnerability lies in Postfix's implementation of the permit_mx_backup restriction within smtpd_recipient_restrictions. When the /proc/net/if_inet6 file is not accessible or available on the system, Postfix fails to properly evaluate IPv6 address resolution capabilities, leading to incorrect assumptions about the recipient's network configuration. This flaw occurs specifically when systems are configured to use permit_mx_backup as part of their email validation process, which should normally prevent unauthorized relaying by verifying that mail is being sent to legitimate recipients with proper MX records. The absence of IPv6 interface support causes the software to incorrectly process IPv6 hostnames, allowing attackers to exploit this misconfiguration to relay mail through the server without proper authentication or authorization.
The operational impact of this vulnerability extends beyond simple mail relay bypass, as it fundamentally undermines the email server's ability to enforce security policies in environments where IPv6 support is limited or non-existent. Attackers can exploit this weakness to send spam or malicious emails through the compromised Postfix server, potentially using it as an open relay point for various malicious activities. The vulnerability affects systems that are configured to permit MX backups while simultaneously lacking IPv6 network capabilities, creating a dangerous intersection where the software's defensive mechanisms fail. This issue particularly impacts organizations running older Postfix versions on systems where IPv6 has not been properly implemented or enabled, creating a security gap that can be easily exploited by threat actors.
Organizations should immediately implement mitigation strategies that include updating to Postfix versions that address this specific vulnerability, typically those released after 2005, and ensuring proper IPv6 network interface configuration. The recommended approach involves disabling permit_mx_backup when IPv6 support is not available, or configuring systems to properly handle IPv6 address resolution even when the /proc/net/if_inet6 file is inaccessible. Additionally, administrators should implement comprehensive monitoring of mail relay activities and consider implementing additional email security controls such as sender verification, recipient verification, and proper access control lists. This vulnerability demonstrates the importance of thorough testing of security controls in environments with mixed network protocol support and highlights the need for robust configuration management practices. The issue aligns with CWE-284, which addresses improper access control, and represents a specific case where inadequate input validation and network configuration handling creates security exposure. From an ATT&CK perspective, this vulnerability maps to privilege escalation and command and control activities, as it allows attackers to establish unauthorized mail relay capabilities that can be used for phishing campaigns, spam distribution, and other malicious email activities.