CVE-2026-53786 in rsync
Summary
by MITRE • 08/13/2026
rsync before 3.5.0 contains a filter rule bypass vulnerability that allows authenticated clients to override module-level filter restrictions by supplying malicious --filter merge file directives. Attackers can inject client-side merge file directives during filter evaluation to introduce rules that supersede daemon module-level restrictions, gaining access to files the module filter was intended to exclude.
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Analysis
by VulDB Data Team • 08/13/2026
The rsync vulnerability present in versions prior to 3.5.0 represents a critical security flaw that undermines the integrity of file synchronization operations through a sophisticated filter rule bypass mechanism. This vulnerability specifically targets the daemon module configuration where administrators define access controls and file filtering rules to restrict client access to sensitive directories and files. The flaw manifests when authenticated clients exploit a design oversight in how merge file directives are processed during filter evaluation, allowing them to inject malicious filter rules that override existing module-level restrictions.
The technical implementation of this vulnerability stems from insufficient validation of filter merge directives within the rsync daemon context. When clients provide --filter merge file parameters during synchronization operations, the system fails to properly sanitize or validate these directives against the module's configured access controls. This processing gap creates an attack surface where maliciously crafted merge rules can be interpreted by the daemon as legitimate filter instructions, effectively bypassing all protection mechanisms established at the module level. The vulnerability operates through a direct manipulation of the filter evaluation pipeline, where client-supplied directives are processed with the same privileges as system-level configuration elements.
The operational impact of this vulnerability extends beyond simple unauthorized file access, creating potential pathways for data exfiltration and privilege escalation within environments relying on rsync for secure file transfers. Attackers can leverage this flaw to access files that should be restricted based on module configurations, potentially exposing sensitive data such as configuration files, system logs, or proprietary information. The authenticated nature of the attack requires an existing user account with legitimate access rights, but once exploited, it provides a mechanism to circumvent administrative controls and gain access to resources beyond what was originally intended. This vulnerability directly violates security principles of least privilege and access control enforcement that are fundamental to secure system design.
The exploitation of this vulnerability aligns with several established attack patterns documented in the attack framework, particularly those involving privilege escalation and data access bypass techniques. From a cybersecurity perspective, this flaw represents a significant concern for organizations using rsync as part of their file synchronization infrastructure, especially in enterprise environments where proper access controls are critical for maintaining data security boundaries. Organizations should consider implementing immediate mitigations including upgrading to rsync version 3.5.0 or later, where the vulnerability has been addressed through enhanced validation of filter merge directives and stricter enforcement of module-level access controls.
This vulnerability maps directly to CWE-284, which addresses improper access control in software systems, and demonstrates how inadequate input validation can create security holes that bypass established authorization mechanisms. The fix implemented in rsync 3.5.0 involves strengthening the validation process for filter merge directives to ensure that client-supplied rules cannot supersede module-level restrictions, thereby maintaining the integrity of access control policies. Security practitioners should also consider implementing network-level controls and monitoring for unusual rsync activity patterns that might indicate exploitation attempts, while ensuring proper patch management procedures are in place to address similar vulnerabilities across the organization's infrastructure.