CVE-2026-70457 in rsync
Summary
by MITRE • 08/13/2026
rsync 3.2.3 before 3.5.0 contains an out-of-bounds write in parse_size_arg() where the return value of snprintf() is used directly as an index into a .bss-segment array without bounds checking. When snprintf truncates the formatted size string, the return value equals the number of characters that would have been written including the truncated portion, and this value may exceed the array length. The subsequent indexed write targets memory outside the intended array bounds, corrupting .bss memory.
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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 out-of-bounds write flaw within the parse_size_arg() function that demonstrates poor input validation and memory management practices. This issue arises from the improper handling of snprintf return values when processing size arguments, creating a scenario where the function writes data beyond the allocated array boundaries in the .bss segment. The vulnerability specifically occurs when the formatted size string exceeds the maximum buffer capacity, causing snprintf to return a value equal to the number of characters that would have been written if the buffer were large enough. This return value is then directly used as an index into an array without any bounds checking mechanism, leading to memory corruption in adjacent memory regions. The .bss segment, which contains uninitialized global and static variables, becomes particularly vulnerable to this type of attack due to its predictable memory layout and lack of protection mechanisms.
The technical exploitation of this vulnerability follows a well-established pattern that aligns with common software security weaknesses classified under CWE-129 and CWE-787. Attackers can manipulate size arguments passed to rsync commands to trigger the out-of-bounds write condition, potentially leading to arbitrary code execution or system compromise. The flaw operates through a classic buffer overflow scenario where the return value of snprintf is treated as a valid array index without proper validation against the actual array dimensions. This type of vulnerability falls under the ATT&CK technique T1059.007 for Command and Scripting Interpreter and T1566 for Phishing, as attackers can exploit this weakness to execute malicious payloads through crafted rsync commands or by manipulating configuration files that utilize size parameters. The impact extends beyond simple memory corruption since the .bss segment contains critical application state variables that may be overwritten, potentially leading to denial of service conditions or more sophisticated attacks depending on the specific memory layout.
The operational impact of this vulnerability is significant for any system running affected rsync versions, particularly in environments where rsync is used for file synchronization across network boundaries. Systems that rely on rsync for backup operations, file distribution, or automated synchronization processes face potential compromise through this out-of-bounds write vulnerability. The attack surface expands when considering that rsync is commonly used in enterprise environments, cloud deployments, and automated infrastructure management systems where attackers could leverage this weakness to gain unauthorized access to sensitive data or disrupt critical services. Organizations using rsync for regular file transfers, incremental backups, or remote synchronization tasks are particularly at risk since the vulnerability can be triggered through normal command-line usage or configuration file parsing. The vulnerability also affects systems that integrate rsync into larger automation frameworks where size parameters might be dynamically generated or sourced from untrusted inputs.
Mitigation strategies for this vulnerability should focus on immediate patching to rsync version 3.5.0 or later, which addresses the out-of-bounds write issue through proper bounds checking of snprintf return values before array indexing operations. System administrators should implement comprehensive monitoring for any suspicious rsync activity that might indicate exploitation attempts, particularly around size parameter usage in command-line invocations or configuration files. Network segmentation and access controls should be enhanced to limit rsync exposure to trusted networks and users, reducing the potential attack surface for this vulnerability. Additional defensive measures include implementing input validation at multiple layers, including validating size parameters before they reach the vulnerable function, and utilizing memory protection mechanisms such as stack canaries, address space layout randomization, and data execution prevention. Organizations should also consider deploying intrusion detection systems that can identify anomalous rsync usage patterns or attempts to manipulate size arguments in ways that could trigger the vulnerability. Regular security audits of rsync configurations and command-line usage should be conducted to ensure that all systems are properly updated and that no legacy installations remain vulnerable to this critical flaw.