CVE-2026-53783 in Rsync
Summary
by MITRE • 08/13/2026
rsync before 3.5.0 contains a time-of-check to time-of-use (TOCTOU) race condition vulnerability in the rrsync restricted shell wrapper that allows authenticated clients to escape enforced directory restrictions by substituting a symlink for a path component after validation but before transfer processing. Attackers can additionally leverage unrestricted flags such as --copy-unsafe-links, -D, and --log-file through rrsync to read or write files outside the permitted directory subtree.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 08/13/2026
The vulnerability in rsync versions prior to 3.5.0 represents a critical time-of-check to time-of-use race condition that fundamentally undermines the security model of the rrsync restricted shell wrapper. This flaw exists within the core file transfer mechanism where the system validates directory permissions at one point in time and then executes operations at a subsequent point, creating an exploitable window where malicious actors can manipulate the filesystem structure between validation and execution phases.
The technical implementation of this vulnerability stems from improper handling of symbolic link resolution within the rrsync wrapper environment. When authenticated users submit file transfer requests, the system initially checks if the requested paths are within allowed directories but fails to maintain consistent path validation throughout the entire operation lifecycle. This race condition allows attackers to replace legitimate directory components with symbolic links pointing elsewhere in the filesystem, effectively bypassing the intended restrictions that should confine operations to specific subtree boundaries.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass comprehensive data exfiltration and modification capabilities. Attackers can leverage unrestricted command-line flags such as --copy-unsafe-links, -D, and --log-file which are not properly sanitized within the rrsync context to gain access to files outside the designated permitted directories. These flags enable attackers to read arbitrary files from the filesystem or write data to locations they would normally be restricted from accessing, creating a pathway for both information disclosure and potential system compromise.
The vulnerability aligns with CWE-367 Time-of-Check to Time-of-Use Race Condition which specifically addresses scenarios where security checks are performed at one time but operations occur later, allowing attackers to manipulate system state between validation and execution. From an adversarial perspective, this flaw maps directly to several ATT&CK techniques including T1078 Valid Accounts for initial access, T1566 Phishing for credential compromise, and T1005 Data from Local System for information gathering. The attack vector requires authenticated access but the privilege escalation potential makes it particularly dangerous in environments where rsync is used for file synchronization with restricted user accounts.
Mitigation strategies must address both the immediate race condition through proper path validation throughout the entire operation lifecycle and the broader flag handling within rrsync. System administrators should immediately upgrade to rsync version 3.5.0 or later where this vulnerability has been resolved through enhanced symlink resolution and consistent path checking mechanisms. Additionally, organizations should implement strict restrictions on command-line flags permitted within rrsync configurations and consider using alternative file transfer mechanisms with more robust security models when handling sensitive data transfers in restricted environments. The fundamental flaw demonstrates the importance of maintaining consistent security validation throughout multi-step operations rather than relying on single-point checks that can be exploited through timing attacks or filesystem manipulation techniques.