CVE-2020-15025: NTP Denial of Service (DoS) Vulnerability

CVE-2020-15025 Overview

CVE-2020-15025 is a memory leak vulnerability affecting the ntpd service in NTP versions 4.2.8 before 4.2.8p15 and 4.3.x before 4.3.101. This denial of service vulnerability allows remote attackers to cause memory consumption by sending specially crafted packets when CMAC keys are configured in the ntp.keys file.

The vulnerability arises because memory is not properly freed in situations where a CMAC (Cipher-based Message Authentication Code) key is used and associated with a CMAC algorithm. An authenticated attacker with high privileges can exploit this flaw to gradually exhaust system memory resources, ultimately leading to service degradation or complete denial of service.

Critical Impact
Authenticated remote attackers can cause denial of service through memory exhaustion by sending packets to NTP servers configured with CMAC keys, potentially disrupting critical time synchronization services across enterprise networks.

Affected Products

NTP ntp versions 4.2.8 through 4.2.8p14 and 4.3.x before 4.3.101
openSUSE Leap 15.1 and 15.2
NetApp Cloud Backup and SteelStore Cloud Integrated Storage
NetApp Hardware (8300, 8700, A400, H410C, H300S, H500S, H700S, H300E, H500E, H700E, H410S series)
Oracle ZFS Storage Appliance Kit 8.8

Discovery Timeline

June 24, 2020 - CVE-2020-15025 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-15025

Vulnerability Analysis

This vulnerability (CWE-401: Missing Release of Memory after Effective Lifetime) represents a classic memory leak condition in the NTP daemon's packet processing logic. When the ntpd service processes incoming packets and the server is configured to use CMAC authentication keys, the allocated memory for handling these packets is not properly released after processing is complete.

The attack requires network access and elevated privileges (authenticated attacker), but once conditions are met, an attacker can systematically exhaust available system memory by sending a sustained volume of packets. This makes the vulnerability particularly concerning for critical infrastructure systems that rely on NTP for time synchronization, including financial systems, logging infrastructure, and distributed computing environments.

Root Cause

The root cause lies in the memory management code path within ntpd when handling CMAC key operations. Specifically, when a CMAC key is configured in the ntp.keys file and associated with a CMAC algorithm, the memory allocated for processing authentication-related operations is not deallocated after the packet handling completes. This leads to a gradual accumulation of unreleased memory blocks, eventually consuming all available system resources.

Attack Vector

The attack vector is network-based, requiring an authenticated attacker with elevated privileges to send packets to the vulnerable NTP server. The attack exploits the memory leak by sending repeated packets that trigger the CMAC authentication code path, gradually exhausting memory over time.

The exploitation mechanism involves:

Identifying an NTP server running a vulnerable version with CMAC keys configured
Establishing an authenticated session (requiring valid credentials)
Sending continuous packets that exercise the CMAC authentication pathway
Memory accumulates without being freed, eventually causing service degradation

Since this vulnerability requires authentication and high privileges, mass exploitation is less likely, but targeted attacks against specific infrastructure remain a concern.

Detection Methods for CVE-2020-15025

Indicators of Compromise

Abnormally high memory consumption by the ntpd process over time
Gradual increase in resident memory size for NTP daemon without corresponding increase in connections
System out-of-memory (OOM) events affecting ntpd or related services
Unusual volume of NTP authentication requests from specific sources

Detection Strategies

Monitor ntpd process memory usage trends over time using tools like ps, top, or process monitoring solutions
Implement alerting for ntpd memory consumption exceeding baseline thresholds
Review NTP access logs for unusual patterns of authenticated requests
Check for CMAC key configurations in /etc/ntp.keys or equivalent configuration files

Monitoring Recommendations

Configure system monitoring to track ntpd process RSS (Resident Set Size) memory metrics
Set up alerting thresholds for abnormal memory growth patterns in time synchronization services
Implement network flow analysis to detect anomalous NTP traffic patterns
Review and baseline normal NTP authentication activity to identify deviations

How to Mitigate CVE-2020-15025

Immediate Actions Required

Update NTP to version 4.2.8p15 or 4.3.101 or later immediately
Review ntp.keys configuration files to identify systems using CMAC authentication
Implement memory monitoring for ntpd processes to detect active exploitation attempts
Consider restricting NTP authentication to trusted sources only

Patch Information

The NTP Project has released patched versions addressing this vulnerability. Organizations should upgrade to NTP version 4.2.8p15 or 4.3.101 or later. Detailed patch information is available through the NTP Bug Report #3661 and the NTP Security Notice for June 2020.

Vendor-specific patches are also available from:

Workarounds

If immediate patching is not possible, consider temporarily disabling CMAC key authentication in the NTP configuration
Implement strict access controls limiting which hosts can authenticate to the NTP server
Configure firewall rules to restrict NTP access to only trusted and necessary clients
Implement automated restart procedures for ntpd if memory thresholds are exceeded as a temporary mitigation

bash

# Check current NTP version
ntpd --version

# Verify CMAC key usage in configuration
grep -i cmac /etc/ntp.keys

# Monitor ntpd memory consumption
ps aux | grep ntpd | awk '{print "Memory (RSS):", $6, "KB"}'