NTP BUG 3071: Client rate limiting and server responses
Last update: June 27, 2022 20:45 UTC (51d68a4aa)
Summary
Description
When ntpd is configured with rate limiting for all associations (restrict default limited in ntp.conf), the limits are applied also to responses received from its configured sources. An attacker who knows the sources (e.g., from an IPv4 refid in server response) and knows the system is (mis)configured in this way can periodically send packets with spoofed source address to keep the rate limiting activated and prevent ntpd from accepting valid responses from its sources.
While this blanket rate limiting can be useful to prevent brute-force attacks on the origin timestamp, it allows this DoS attack. Similarly, it allows the attacker to prevent mobilization of ephemeral associations.
Mitigation
- Implement BCP-38.
- If you choose to use
restrict default limited ..., be sure to use restrict source ... (without limited) to avoid this attack.
- Properly monitor your
ntpd instances, and auto-restart ntpd (without -g) if it stops running.
Credit
This weakness was discovered by Miroslav Lichvar of Red Hat.
Timeline