Summary

A denial of service vulnerability was discovered in ntpd-rs where an attacker can induce a message storm between two NTP servers running ntpd-rs.

Details

Since ntpd-rs version 1.2.0, when configured as a server, incorrectly responded to all NTP messages sent to the server's port with a time reply, including to responses from other servers. As a consequence, a message with a spoofed IP address of another server could cause two servers running ntpd-rs to continually respond to each other, consuming significant amounts of resources.

Impact

Any time server running ntpd-rs with version between 1.2.0 and 1.6.1 inclusive which allows non-NTS traffic is affected. Client-only configurations are not affected. Affected users are recommended to upgrade to version 1.6.2 as soon as possible.

Workarounds

Should upgrading not be possible, the impact of the issue can be mitigated by:

• Whitelisting access to only IP addresses of clients using the server, using the ignore filter method.
• Blocking incoming non-request traffic on the NTP server port using a firewall.
• Disabling public access to the vulnerable NTP server
• Disabling the server functionality by removing any [server] sections from the configuration.

Acknowledgements

The ntpd-rs authors thank Eric Sesterhenn from X41 D-Sec GmbH for finding and reporting this issue.