Rsync Integer Overflow Vulnerability Leading to Information Disclosure (CVE-2026-43618)

Rsync, a widely used utility for synchronizing files between computer systems, is susceptible to an integer overflow vulnerability (CVE-2026-43618) within its compressed-token decoder. Specifically, versions 3.4.2 and earlier fail to adequately validate a 32-bit signed counter, leading to an overflow condition. A malicious rsync sender can exploit this flaw by crafting a specially designed data stream that triggers the overflow during decompression on the receiving end. This overflow can cause the receiver process to read data outside of the intended buffer boundaries. Successful exploitation results in the disclosure of sensitive process memory contents.

Attack Chain

1. Attacker crafts a malicious data stream designed to exploit the integer overflow in the rsync compressed-token decoder.
2. The attacker initiates an rsync session with a vulnerable rsync server (version 3.4.2 or prior).
3. During data transfer, the malicious data stream is sent to the rsync server.
4. The rsync server attempts to decompress the data stream using the vulnerable compressed-token decoder.
5. The 32-bit signed counter overflows due to the crafted data stream.
6. The overflow causes the rsync server process to read data from memory locations outside the intended buffer.
7. Sensitive information, such as environment variables, passwords, heap data, stack data, and library memory pointers, are exposed.
8. The attacker gains access to the disclosed memory contents, potentially facilitating further exploitation and bypassing ASLR.

Impact

Successful exploitation of CVE-2026-43618 leads to information disclosure on the affected system. An attacker can potentially access sensitive data residing in the rsync process memory, including environment variables, passwords, and memory addresses. This leaked information can be leveraged to bypass ASLR, escalate privileges, and perform lateral movement within the network. The vulnerability poses a significant risk to the confidentiality and integrity of the affected systems.

Recommendation

• Upgrade rsync to a version higher than 3.4.2 to patch CVE-2026-43618.
• Deploy the Sigma rule Detect Rsync CVE-2026-43618 Integer Overflow Attempt to detect potential exploitation attempts by monitoring process command-line arguments.
• Review systems running vulnerable rsync versions for suspicious network connections and memory access patterns.