CVE-2018-21029 in systemd
Summary
by MITRE
systemd 239 through 243 accepts any certificate signed by a trusted certificate authority for DNS Over TLS. Server Name Indication (SNI) is not sent, and there is no hostname validation with the GnuTLS backend.
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Analysis
by VulDB Data Team • 08/06/2024
The vulnerability identified as CVE-2018-21029 affects systemd versions 239 through 243 and represents a critical flaw in the implementation of DNS over TLS functionality. This issue stems from the improper handling of certificate validation within the GnuTLS backend, creating a significant security risk for systems relying on encrypted DNS resolution. The flaw specifically impacts how systemd validates certificates when establishing TLS connections for DNS queries, potentially allowing adversaries to intercept or manipulate DNS traffic.
The technical root cause of this vulnerability lies in the absence of Server Name Indication (SNI) transmission during DNS over TLS connections and the lack of hostname validation. SNI is a crucial extension to the TLS protocol that enables clients to indicate which hostname they are attempting to connect to, allowing servers to present the appropriate certificate. Without SNI, the server cannot properly identify which certificate to present, and the client cannot verify that the certificate belongs to the intended hostname. This vulnerability aligns with CWE-295, which addresses improper certificate validation and hostname verification in secure communications, making it particularly dangerous for DNS resolution where trust in the server identity is paramount.
The operational impact of this vulnerability is substantial as it creates a man-in-the-middle attack vector for DNS over TLS traffic. An attacker positioned within the network can potentially intercept DNS queries and respond with malicious DNS records while presenting a certificate signed by a trusted certificate authority. This attack becomes feasible because systemd accepts any certificate from a trusted CA without proper hostname validation, essentially undermining the security assurances that DNS over TLS is designed to provide. The vulnerability affects systems where systemd is responsible for managing DNS resolution, which is common in modern Linux distributions using systemd-resolved for DNS services.
The implications extend beyond simple certificate validation failures and represent a fundamental flaw in the security architecture of DNS over TLS implementations. This vulnerability demonstrates a critical gap in the ATT&CK framework's network infiltration techniques, specifically targeting the DNS resolution process as a means to establish persistent access or redirect traffic. The lack of hostname validation creates a trust boundary violation where the system accepts certificates without proper verification of the server's identity, making it susceptible to various attacks including DNS cache poisoning and traffic interception. Organizations relying on systemd for DNS resolution are particularly vulnerable, as this flaw affects the core functionality of secure DNS communication.
Mitigation strategies for this vulnerability include upgrading to systemd version 244 or later where the issue has been resolved through proper implementation of SNI and hostname validation. System administrators should also consider implementing additional network-level security measures such as DNSSEC to provide alternative protection layers against DNS-based attacks. The fix implemented in newer versions addresses the root cause by ensuring that SNI is properly transmitted and that certificate hostnames are validated against the expected server identity, thereby restoring the intended security properties of DNS over TLS. Organizations should also conduct thorough security assessments of their DNS infrastructure to identify any other potential vulnerabilities in their DNS resolution processes.