The praecis_parse function in ntpd/refclock_palisade.c in the Network Time Protocol (NTP) version 4.2.8p15 contains an out-of-bounds (OOB) write vulnerability. This vulnerability leaves NTP installations open to attacks via a complex method, such as those using a manipulated GPS receiver. In this article, we will examine the vulnerability in detail, walk through a code snippet, and provide links to the original references.


The Network Time Protocol (NTP) is a protocol used to synchronize the system time of computers over a network. It is an essential tool for modern communication and system administration. The vulnerability CVE-2023-26555 stems from an issue in the praecis_parse function of the ntpd/refclock_palisade.c source file within NTP 4.2.8p15.

The function has a flawed way of handling input received from the GPS receiver, which could lead to a buffer overflow condition. If an attacker could manipulate the received input through the GPS receiver, they might write to addresses beyond the intended buffer in the system memory. This behavior could result in compromising the system's security by executing arbitrary code, causing a crash, or rendering the application unresponsive.

Code Snippet

A relevant portion of ntpd/refclock_palisade.c showcasing the flaw can be observed in lines 57-60 as follows.

for (i = ; i < buflen; i++) {
	if (buf[i] == ',') {
		buf[i] = '\';

Here, the issue arises from the loop iterating over the buffer, searching for commas, and replacing them with null characters. However, there is no boundary check on i, and no bounds checking is performed as the loop iterates through the entire buffer. If the buffer is not null-terminated or contains more commas than it should, a buffer overflow may occur.

Exploit Details

Due to the complexity of the attack, exploiting this vulnerability successfully requires intricate and careful manipulation of GPS-receiver-generated input. An attacker would need specialized equipment, including a custom GPS receiver, and perhaps knowledge of potential target systems. Even if the attacker can forge or replay GPS signals, they face significant hurdles to control the manipulated data to trigger the buffer overflow flaw in a precise way, potentially allowing them to execute arbitrary code.

Original References

1. The original advisory on this vulnerability can be found on the NTP 4.2.8p15 Release page, under ChangeLog Cl-0007:
2. The Common Vulnerabilities and Exposures (CVE) entry for this issue, CVE-2023-26555, can be found at:


Despite its complexity, CVE-2023-26555, an out-of-bounds write vulnerability in NTP 4.2.8p15, remains a potential threat to the integrity and security of NTP installations. Understanding the affected code, the potential exploit details, and the original references should encourage those responsible for maintaining NTP installations to apply the appropriate patches and ensure their systems' ongoing security.


Published on: 04/11/2023 21:15:00 UTC
Last modified on: 04/24/2023 13:58:00 UTC