CVE-2023-26555: NTP Buffer Overflow Vulnerability

CVE-2023-26555 Overview

CVE-2023-26555 is an out-of-bounds write vulnerability affecting the praecis_parse function in ntpd/refclock_palisade.c within NTP version 4.2.8p15. This memory corruption flaw could potentially allow an attacker to corrupt memory by exploiting the improper handling of data from GPS receiver hardware. The attack complexity is notably high, requiring physical access to manipulate a GPS receiver device that interfaces with the vulnerable NTP daemon.

Critical Impact

Successful exploitation could lead to memory corruption affecting confidentiality, integrity, and availability of systems running the vulnerable NTP daemon, though exploitation requires physical access and manipulation of GPS receiver hardware.

Affected Products

• NTP 4.2.8p15
• Systems using the Palisade/Praecis reference clock driver
• Linux distributions shipping vulnerable NTP versions (including Fedora)

Discovery Timeline

• April 11, 2023 - CVE-2023-26555 published to NVD
• February 11, 2025 - Last updated in NVD database

Technical Details for CVE-2023-26555

Vulnerability Analysis

The vulnerability resides in the praecis_parse function within the reference clock driver code (refclock_palisade.c). This driver is responsible for parsing timing data received from Praecis GPS receivers, which are precision timing devices used for NTP synchronization. The out-of-bounds write condition occurs when the function processes malformed or crafted input data from the GPS receiver without proper bounds checking. Due to the physical attack vector requirement, exploitation would necessitate direct access to manipulate the GPS receiver hardware or its communication channel to the NTP daemon.

Root Cause

The root cause is improper bounds checking in the praecis_parse function (CWE-787: Out-of-bounds Write). When processing timing data from the Praecis GPS reference clock, the function fails to adequately validate the length or content of input data before writing to memory buffers. This allows carefully crafted input to write beyond the intended buffer boundaries, potentially corrupting adjacent memory structures.

Attack Vector

The attack vector is classified as Physical, requiring an attacker to have direct access to manipulate the GPS receiver hardware connected to the target system. A potential attack scenario would involve:

1. Gaining physical access to the GPS receiver connected to the NTP server
2. Manipulating the receiver to send malformed timing data to the NTP daemon
3. Triggering the out-of-bounds write condition in praecis_parse
4. Exploiting the memory corruption to achieve code execution or denial of service

The vulnerability affects the reference clock parsing mechanism. The praecis_parse function in ntpd/refclock_palisade.c processes incoming data from GPS timing receivers without adequate validation of input boundaries. When maliciously crafted data is received from a manipulated GPS device, the function may write beyond allocated buffer limits, leading to memory corruption. For detailed technical analysis, see the GitHub CVE-2023-26555 Info page.

Detection Methods for CVE-2023-26555

Indicators of Compromise

• Unexpected crashes or restarts of the ntpd service
• Anomalous behavior in systems using Praecis GPS reference clocks
• Memory corruption indicators in system logs related to the NTP daemon
• Unusual timing synchronization errors or inconsistencies

Detection Strategies

• Monitor NTP daemon logs for parsing errors or crashes related to reference clock drivers
• Implement process monitoring for unexpected ntpd terminations or restarts
• Deploy memory protection mechanisms (ASLR, stack canaries) to detect exploitation attempts
• Use integrity monitoring on NTP configuration and binary files

Monitoring Recommendations

• Enable verbose logging for the NTP daemon to capture reference clock parsing events
• Monitor system stability of servers using GPS-based time synchronization
• Implement physical security monitoring for GPS receiver hardware access
• Review system logs for memory-related errors in NTP processes

How to Mitigate CVE-2023-26555

Immediate Actions Required

• Verify NTP version installed on systems (ntpq -c version)
• Assess whether Praecis GPS reference clock drivers are in use
• Apply physical security controls to GPS receiver hardware
• Consider migrating to alternative NTP implementations if patches are unavailable

Patch Information

Organizations should update NTP to a patched version when available from their distribution vendor. Fedora has released package announcements addressing this vulnerability. Check the Fedora Package Announcement and Fedora Package Announcement Update for distribution-specific updates. Additional technical details are available in the GitHub Issue Comment on CVE-2023-26555.

Workarounds

• Disable the Palisade/Praecis reference clock driver if not required for operations
• Implement network segmentation to isolate NTP infrastructure
• Consider using alternative time synchronization methods or NTP implementations
• Enhance physical security controls for GPS receiver hardware access

# Configuration example - Disable Praecis reference clock if not needed
# In ntp.conf, remove or comment out any Praecis/Palisade refclock entries:
# server 127.127.29.x  # Type 29 = Palisade/Praecis
# fudge 127.127.29.x ...

# Verify no Palisade driver is configured
grep -i "127.127.29" /etc/ntp.conf

# Restart NTP service after configuration changes
systemctl restart ntpd