CVE-2017-11501 in NixOS
Summary
by MITRE
NixOS 17.03 and earlier has an unintended default absence of SSL Certificate Validation for LDAP. The users.ldap NixOS module implements user authentication against LDAP servers via a PAM module. It was found that if TLS is enabled to connect to the LDAP server with users.ldap.useTLS, peer verification will be unconditionally disabled in /etc/ldap.conf.
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Analysis
by VulDB Data Team • 12/13/2022
The vulnerability identified as CVE-2017-11501 represents a critical security flaw in NixOS versions 17.03 and earlier, specifically within the users.ldap module that governs user authentication against LDAP servers through PAM integration. This issue fundamentally undermines the security posture of systems relying on LDAP authentication by disabling essential SSL certificate validation mechanisms. The flaw manifests when administrators enable TLS connectivity to LDAP servers using the users.ldap.useTLS configuration parameter, yet the system fails to properly validate server certificates during the TLS handshake process.
The technical implementation of this vulnerability stems from the improper handling of TLS peer verification within the LDAP configuration file generated by NixOS. When TLS is enabled through the configuration module, the system unconditionally disables peer verification in the /etc/ldap.conf file, effectively removing the cryptographic validation that ensures clients are communicating with legitimate LDAP servers. This configuration bypasses the fundamental security principle of certificate validation that prevents man-in-the-middle attacks and ensures authentication integrity. The vulnerability directly maps to CWE-295 which addresses improper certificate validation, and represents a significant failure in secure communication protocol implementation.
The operational impact of this vulnerability extends far beyond simple authentication failures, creating substantial risks for organizations relying on NixOS for their infrastructure. Attackers can exploit this weakness to perform man-in-the-middle attacks against LDAP authentication processes, potentially gaining unauthorized access to sensitive systems and user credentials. The absence of certificate validation means that malicious actors can intercept LDAP traffic and present fake certificates to authenticate users, effectively bypassing the entire authentication mechanism. This vulnerability aligns with ATT&CK technique T1550.001 which covers use of valid accounts through legitimate authentication protocols, but in this case the authentication is compromised through certificate validation bypass rather than credential theft.
Organizations affected by this vulnerability face significant risk exposure, particularly in environments where LDAP serves as a primary authentication mechanism for critical systems. The impact is compounded by the fact that the vulnerability affects the default configuration behavior, meaning that systems configured with TLS enabled are inherently insecure unless administrators manually override the default settings. This represents a classic case of insecure default configuration that violates security best practices outlined in various industry standards including NIST SP 800-53 and ISO 27001. The vulnerability demonstrates the critical importance of proper cryptographic implementation and the dangers of disabling security controls even when they appear to be optional configuration parameters.
Mitigation strategies for CVE-2017-11501 require immediate action from affected organizations, beginning with upgrading to NixOS versions 17.09 or later where the vulnerability has been addressed. System administrators should also implement manual configuration overrides to ensure proper certificate validation is enforced even when TLS is enabled. The recommended approach involves explicitly configuring certificate validation parameters in LDAP client configurations and monitoring for unauthorized changes to LDAP authentication settings. Additionally, organizations should conduct comprehensive security assessments of their LDAP infrastructure to identify any potential exploitation attempts and implement network-level monitoring to detect suspicious authentication patterns that might indicate active attacks leveraging this vulnerability.