CVE-2023-26551: NTP Buffer Overflow Vulnerability

CVE-2023-26551 Overview

CVE-2023-26551 is an out-of-bounds write vulnerability in the mstolfp function located in libntp/mstolfp.c within NTP version 4.2.8p15. The vulnerability occurs in the cp<cpdec while loop, where improper boundary checking allows an adversary to write data beyond the intended buffer limits. While the vulnerability cannot be used to attack the ntpd daemon directly, it can potentially be exploited to compromise ntpq client processes.

Critical Impact

An attacker with network access may be able to exploit this out-of-bounds write vulnerability to attack NTP client (ntpq) processes, potentially leading to memory corruption, information disclosure, or limited code execution within the client context.

Affected Products

• NTP version 4.2.8p15
• Systems running ntpq client processes with vulnerable NTP library
• Applications linking against vulnerable libntp library

Discovery Timeline

• 2023-04-11 - CVE-2023-26551 published to NVD
• 2025-02-11 - Last updated in NVD database

Technical Details for CVE-2023-26551

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), a memory corruption issue that occurs when the mstolfp function in the NTP library fails to properly validate buffer boundaries during string parsing operations. The function is designed to convert millisecond timestamp strings to NTP's internal fixed-point format, but the cp<cpdec while loop contains insufficient bounds checking.

The attack requires network access and involves high complexity to exploit successfully. When successfully exploited, an attacker may achieve limited confidentiality, integrity, and availability impacts on the affected ntpq client process. Importantly, the ntpd daemon is not vulnerable to this specific attack vector, limiting the overall exposure to client-side exploitation scenarios.

Root Cause

The root cause of this vulnerability lies in the improper handling of input string boundaries within the mstolfp function. During the parsing of timestamp strings, the while loop comparing cp<cpdec does not adequately validate that write operations remain within the allocated buffer space. This allows specially crafted input to cause writes beyond the intended memory region, corrupting adjacent memory structures.

Attack Vector

The vulnerability can be exploited remotely over the network, though the attack complexity is high. An adversary would need to craft malicious NTP responses or manipulate network traffic to deliver a specially crafted timestamp string to an ntpq client process. When the client parses this malicious input using the vulnerable mstolfp function, the out-of-bounds write occurs.

The attack is limited to ntpq client processes and cannot be used to compromise ntpd server daemons. This restricts the attack surface to scenarios where an attacker can influence the NTP data received by client applications. Successful exploitation could result in memory corruption, potential information leakage, or limited code execution within the context of the ntpq process.

Technical details and proof-of-concept information are available in the GitHub CVE-2023-26551 PoC repository, with additional discussion in the GitHub Issue Comment Discussion.

Detection Methods for CVE-2023-26551

Indicators of Compromise

• Unexpected crashes or abnormal termination of ntpq client processes
• Memory access violations or segmentation faults in NTP client applications
• Unusual NTP response packets containing malformed timestamp strings
• Core dumps from ntpq processes showing corruption in libntp memory regions

Detection Strategies

• Monitor for unusual NTP traffic patterns, particularly responses with abnormally long or malformed timestamp fields
• Implement application crash monitoring for ntpq processes and correlate with NTP network activity
• Deploy network intrusion detection signatures to identify potentially malicious NTP response packets
• Use memory safety tools like AddressSanitizer during testing to detect out-of-bounds write attempts

Monitoring Recommendations

• Enable detailed logging for NTP client operations to capture parsing errors
• Configure SIEM rules to alert on repeated ntpq process crashes or restarts
• Monitor network flows for anomalous NTP traffic originating from untrusted sources
• Implement endpoint detection to identify memory corruption attempts in NTP-related processes

How to Mitigate CVE-2023-26551

Immediate Actions Required

• Identify all systems running NTP version 4.2.8p15 and prioritize for remediation
• Restrict network access to NTP services using firewall rules to trusted time sources only
• Consider temporarily disabling ntpq client usage in sensitive environments until patches are applied
• Monitor for vendor security advisories and update to patched versions when available

Patch Information

Users should check for updated NTP versions that address this out-of-bounds write vulnerability. Consult the official NTP project resources and your operating system vendor for security patches. The vulnerability is specific to NTP version 4.2.8p15, and upgrading to a later patched version is the recommended remediation.

Additional technical context and discussion regarding this vulnerability can be found in the GitHub Issue Comment Discussion.

Workarounds

• Configure firewall rules to restrict NTP traffic to known, trusted time servers only
• Implement network segmentation to isolate systems running vulnerable NTP clients
• Use alternative NTP client implementations that are not affected by this vulnerability
• Deploy application sandboxing for ntpq processes to limit the impact of potential exploitation

# Restrict NTP client traffic to trusted time servers using iptables
iptables -A OUTPUT -p udp --dport 123 -d <trusted_ntp_server_ip> -j ACCEPT
iptables -A OUTPUT -p udp --dport 123 -j DROP

# Verify NTP version to identify vulnerable installations
ntpq --version