CVE-2026-70452 in Rsync
Summary
by MITRE • 08/13/2026
rsync 3.1.0 before 3.5.0 contains an access control bypass vulnerability that allows remote attackers to circumvent hosts deny rules by inducing DNS resolution failures during hostname-based access control evaluation. When a DNS lookup for a hostname-based deny rule fails, the daemon skips the rule rather than defaulting to a deny decision, enabling attackers who can trigger DNS failures to bypass module-level IP access controls and gain unauthorized access to restricted module file trees.
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Analysis
by VulDB Data Team • 08/13/2026
This vulnerability exists in rsync versions prior to 3.5.0 and represents a critical access control flaw that undermines the integrity of hostname-based security policies. The issue stems from how the rsync daemon handles DNS resolution failures during access control evaluation, creating a dangerous bypass mechanism that can be exploited by remote attackers. When the rsync daemon encounters a hostname-based deny rule where DNS lookup fails, instead of enforcing a default deny behavior, it simply skips the rule entirely, effectively rendering the access control policy ineffective.
The technical flaw operates at the core of rsync's host validation mechanism, specifically within the module access control evaluation process. During normal operation, rsync evaluates access rules based on both IP addresses and hostnames to determine whether connections should be permitted or denied. When DNS resolution fails for a hostname-based deny rule, the daemon's logic incorrectly treats this as a non-applicable condition rather than a security risk, allowing malicious actors to bypass intended restrictions. This behavior creates a fundamental weakness in the authorization framework where network-level security controls can be circumvented through simple DNS manipulation techniques.
The operational impact of this vulnerability extends beyond simple unauthorized access, potentially enabling attackers to gain read or write privileges to restricted file trees and module directories that should remain protected. Attackers can exploit this by either directly manipulating DNS resolution for targeted hostnames or by leveraging network conditions that cause DNS failures, such as DNS cache poisoning or network partitioning. The vulnerability affects all rsync modules that implement hostname-based access controls, making it particularly dangerous in environments where multiple modules with varying security requirements are configured. This bypass allows unauthorized file access that could include sensitive data exposure, privilege escalation, or even remote code execution through write operations.
Security professionals should prioritize this vulnerability as a high-risk issue requiring immediate remediation, particularly in environments where rsync serves critical data repositories and network security is paramount. The solution involves upgrading to rsync version 3.5.0 or later, which implements proper fallback behavior for DNS resolution failures by enforcing default deny decisions when hostname validation cannot be completed. Organizations should also consider implementing additional network-level controls such as DNS filtering, access control lists, and monitoring for unusual DNS resolution patterns to detect potential exploitation attempts. From a compliance perspective, this vulnerability may violate security standards including those outlined in cisecurity controls and nist cybersecurity framework requirements for access control enforcement. The flaw aligns with CWE-284 access control weaknesses and could be leveraged by threat actors following ATT&CK techniques related to privilege escalation and lateral movement through network services. Network administrators should also review their existing rsync configurations to identify any hostname-based access controls that might be susceptible to this bypass, ensuring that IP-based restrictions are properly implemented as secondary security layers while the primary vulnerability is patched through version updates.