TOCTOU Race Condition in rsync File Write Handling

Executive Summary

This is a time-of-check to time-of-use (TOCTOU) race condition in rsync versions before 3.5.0. It occurs in the non-daemon receiver's handling of destination directories. An attacker can manipulate parent components of the destination path by replacing a symlink between path resolution and the chdir() call. This redirects the receiver's working directory outside the intended destination tree, causing subsequent file writes using relative paths to land in unintended filesystem locations.

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Detection Guidance

To detect this vulnerability, check the installed rsync version on your system. Run: rsync --version. If the version is below 3.5.0, the system is vulnerable. Additionally, monitor for unusual file writes or symlink manipulations in destination directories during rsync operations.

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Impact Analysis

This vulnerability allows attackers to write files to unintended locations on your system. If you use rsync in non-daemon mode with versions before 3.5.0, an attacker with local access could exploit this to overwrite sensitive files, escalate privileges, or disrupt system operations by redirecting file writes outside the intended directory.

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Mitigation Strategies

Upgrade rsync to version 3.5.0 or later immediately. If upgrading is not possible, avoid using rsync in non-daemon mode for sensitive operations or restrict write permissions to prevent symlink attacks. Monitor filesystem changes closely.

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