CVE-2023-26551 is a recently discovered vulnerability in the Network Time Protocol (NTP) version 4.2.8p15. This vulnerability is an out-of-bounds write that occurs within the mstolfp function in the libntp/mstolfp.c file, which can potentially be exploited by an adversary. In this long-read post, we will delve deep into the technical details of the vulnerability, explore its exploitability, and discuss possible mitigations for affected systems.

Background on NTP and libntp

Before diving into the vulnerability, let's provide a brief refresher on what NTP is and its significance. NTP is a protocol designed to synchronize the clocks of computers over a network. It is widely adopted and implemented across various platforms and devices – from servers to embedded systems.

Libntp is a library that provides the core functionality for NTP implementations. It contains a variety of utility functions and data structures required for NTP's efficient operation. One such function, which concerns us in this post, is mstolfp – a function that converts text-formatted timestamps into binary representation.

The Vulnerability: CVE-2023-26551

An out-of-bounds write occurs when data is written to a buffer, and that buffer's bounds are exceeded, potentially leading to memory corruption, crashes, or code execution. In mstolfp function, this vulnerability happens in the 'cp

Here is a code snippet from libntp/mstolfp.c where the vulnerability exists

...
while (cp < cpdec) {
    fst_adjust(d, L_SCALE);
    cp *= 10;
    d.minutes *= 10;
...

The issue lies in the fact that 'cp' and 'cpdec' variables do not have proper bounds checks within the loop, which allows for an uncontrolled increase in the value of 'cp', eventually causing the buffer to be overwritten.

Exploit Details and Limitations

An adversary can exploit this vulnerability by sending a carefully crafted text-formatted timestamp to a vulnerable client ntpq process. By causing the out-of-bounds write, the attacker may achieve arbitrary code execution, memory corruption, or crash the client process altogether.

However, it is essential to note that this vulnerability does not impact the ntpd server process, which has an entirely separate codepath for parsing NTP packets. Therefore, the potential attack surface for this vulnerability lies solely within the client ntpq processes.

Original References and Further Reading

For more details about this vulnerability, we highly recommend reading the official CVE entry for CVE-2023-26551:

- CVE-2023-26551 - MITRE

Additionally, the NTP project's source code repository contains the affected source file, allowing you to examine the vulnerability in context:

- libntp/mstolfp.c - NTP Source Code Repository

Mitigation and Conclusion

To mitigate the risks posed by this vulnerability, administrators should apply security updates as soon as they become available. It is also advisable to monitor for any unusual activity or crashes in the ntpq processes on affected systems.

In conclusion, CVE-2023-26551 highlights the importance of proper bounds checking and secure coding practices in widely adopted software, such as NTP. While the impact of this vulnerability is notably limited to client ntpq processes, it serves as a reminder that even well-established protocols can contain subtle flaws waiting to be discovered and exploited.

Timeline

Published on: 04/11/2023 21:15:00 UTC
Last modified on: 04/20/2023 14:37:00 UTC