CVE-2026-53792 in rsync
Summary
by MITRE • 08/13/2026
rsync before 3.5.0 contains an out-of-bounds read vulnerability in the sender-side block matching logic that allows a malicious receiver to trigger memory access before the start of an allocated buffer by sending a crafted checksum block with a length of zero. Attackers can send a specially crafted checksum set containing a zero-length block to cause a negative offset calculation during delta computation, resulting in an out-of-bounds read of file data buffer memory on the sender side.
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Analysis
by VulDB Data Team • 08/13/2026
The rsync vulnerability identified in versions prior to 3.5.0 represents a critical out-of-bounds read flaw that fundamentally compromises the integrity of the file synchronization protocol. This vulnerability resides within the sender-side block matching logic where the software fails to properly validate checksum block lengths during delta computation processes. The flaw specifically manifests when a malicious receiver crafts and transmits a checksum block with a length parameter set to zero, creating a scenario where memory access occurs before the allocated buffer boundaries.
The technical implementation of this vulnerability stems from insufficient input validation within the block matching algorithm that rsync employs to efficiently transfer files between systems. During normal operation, rsync calculates delta values to determine what portions of files need to be transmitted, but when processing a zero-length checksum block, the mathematical calculations used for offset determination produce negative values that cause memory access violations. This occurs because the software assumes all blocks will have positive length values and does not account for edge cases where block lengths could be zero or negative during the delta computation phase.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can potentially expose sensitive data from the sender's file buffer memory through the out-of-bounds read mechanism. Attackers exploiting this flaw can craft malicious checksum sets that trigger memory access patterns which may reveal contents of adjacent memory regions, including potentially confidential information stored in the application's memory space. The vulnerability affects the core synchronization functionality and undermines the trust model between communicating rsync instances.
This issue aligns with CWE-129, which addresses improper validation of length parameters in buffer operations, and demonstrates characteristics consistent with ATT&CK technique T1059.007 for command and scripting interpreter execution through protocol manipulation. The vulnerability's exploitation requires minimal privileges since it operates at the network protocol level during normal rsync communication flows. Organizations using older rsync versions face significant risk exposure as this flaw can be triggered without authentication requirements, making it particularly dangerous in environments where untrusted parties might establish connections.
Mitigation strategies should prioritize immediate upgrade to rsync version 3.5.0 or later where the buffer validation has been implemented to prevent zero-length block processing. Network administrators should also consider implementing firewall rules that restrict rsync traffic between untrusted networks and deploy intrusion detection systems capable of identifying malicious checksum block patterns. Additionally, organizations should conduct thorough security assessments of their rsync configurations and implement monitoring for unusual checksum block length patterns that might indicate exploitation attempts. The fix addresses the root cause by introducing proper validation checks that ensure all checksum blocks maintain positive length values before processing begins, thereby preventing the mathematical overflow condition that leads to memory access violations.