CVE-2026-70463 in Rsync
Summary
by MITRE • 08/13/2026
rsync 3.1.0 before 3.5.0 contains an authorization bypass in auth users directive parsing. The auth users parser uses comma-only tokenization when splitting the user list, which fails to correctly handle entries of the form @Group Name where the group name contains a space. The space within the group name causes the parser to split the entry at the space boundary, discarding the deny rule associated with the group. An authenticated user whose username or group membership would be denied by an @Group Name auth users entry can connect to a restricted module because the deny rule is silently discarded during parsing.
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Analysis
by VulDB Data Team • 08/13/2026
The vulnerability in rsync versions prior to 3.5.0 represents a critical authorization bypass flaw that undermines the security controls designed to restrict access to sensitive modules. This issue stems from a fundamental parsing error in how the auth users directive processes group names containing spaces, creating a pathway for unauthorized access that directly contradicts the intended security posture of the application.
The technical root cause lies in the comma-only tokenization approach used by the auth users parser when processing user lists. When encountering entries formatted as @Group Name where the group name contains spaces, the parser incorrectly splits these entries at the space boundary rather than preserving them as single logical units. This parsing failure results in the systematic discarding of deny rules associated with group memberships, effectively neutralizing access controls that were explicitly configured to restrict certain user groups from accessing specific modules.
This authorization bypass has significant operational implications for systems relying on rsync for file synchronization and transfer operations. Administrators who configure rsync with @Group Name entries to deny access to particular user groups find their security policies silently undermined, potentially allowing authenticated users to establish connections to restricted modules that should have been inaccessible. The vulnerability affects any environment where rsync is configured with group-based access controls and where group names contain spaces, creating a substantial attack surface for malicious actors seeking unauthorized access.
The flaw aligns with CWE-284 Access Control Issues, specifically addressing improper access control mechanisms in software applications. From an adversarial perspective, this vulnerability maps to ATT&CK technique T1078 Valid Accounts, as it allows attackers to leverage legitimate authentication credentials to gain access to restricted resources through a parsing error rather than direct credential compromise. The vulnerability also relates to T1566 Phishing, as attackers could potentially exploit the misconfigured access controls to establish connections to sensitive modules through social engineering or by leveraging compromised accounts that would otherwise be blocked.
Mitigation strategies should prioritize immediate upgrades to rsync version 3.5.0 or later, which contains the corrected parsing logic that properly handles group names with spaces. System administrators should conduct thorough audits of existing rsync configurations to identify any @Group Name entries that might be vulnerable to this parsing issue. Additionally, implementing monitoring for unauthorized access attempts to restricted modules can help detect potential exploitation of this vulnerability before it results in data compromise. Organizations should also consider implementing additional layers of access control beyond rsync's built-in mechanisms to provide defense-in-depth against similar authorization bypass scenarios.
The vulnerability demonstrates the importance of robust input validation and parsing logic in security-critical applications, particularly when dealing with complex user and group management systems. It underscores the need for careful attention to edge cases in configuration parsing, as seemingly minor implementation details can have major security implications. Proper testing of edge cases in authentication and authorization systems remains crucial for preventing similar vulnerabilities in other software components that handle user access controls and privilege management.
This authorization bypass vulnerability represents a serious threat to organizations relying on rsync for secure file transfers, as it allows attackers to circumvent access controls through a parsing error rather than exploiting weaknesses in cryptographic implementations or authentication mechanisms. The security implications extend beyond simple unauthorized access to include potential data exfiltration, system compromise, and violations of organizational security policies that depend on proper access control enforcement.