Unlimited number of NTS-KE connections can crash ntpd-rs server

Summary

Missing limit for accepted NTS-KE connections allows an unauthenticated remote attacker to crash ntpd-rs when an NTS-KE server is configured. Non NTS-KE server configurations, such as the default ntpd-rs configuration, are unaffected.

Details

Operating systems have a limit for the number of open file descriptors (which includes sockets) in a single process, e.g. 1024 on Linux by default. When ntpd-rs is configured as an NTS server, it accepts TCP connections for the NTS-KE service. If the process has reached the descriptor limit and tries to accept a new TCP connection, the accept() system call will return with the EMFILE error and cause ntpd-rs to abort.

A remote attacker can open a large number of parallel TCP connections to the server to trigger this crash. The connections need to be opened quickly enough to avoid the key-exchange-timeout-ms timeout (by default 1000 milliseconds).

Impact

Only NTS-KE server configuration are affected. Those without an NTS-KE server configuration such as NTS client only or NTP only configuration are unaffected. For affected configurations the ntpd-rs daemon can made completely unavailable by crashing the service. If ntpd-rs is automatically restarted, an attacker can repeat the attack to prevent ntpd-rs from doing anything useful.

Workarounds

• Disable NTS-KE server functionality
• Increase system resource limits (RLIMIT_NOFILE) to make the attack more difficult
• Lower the key-exchange-timeout-ms configuration setting to make the attack more difficult