NTP BUG 2956: Small-step/big-step
Last update: April 22, 2024 18:49 UTC (7e7bd5857)
Summary
Description
If ntpd is always started with the -g option, which is common and against long-standing recommendation, and if at the moment ntpd is restarted an attacker can immediately respond to enough requests from enough sources trusted by the target, which is difficult and not common, there is a window of opportunity where the attacker can cause ntpd to set the time to an arbitrary value. Similarly, if an attacker is able to respond to enough requests from enough sources trusted by the target, the attacker can cause ntpd to abort and restart, at which point it can tell the target to set the time to an arbitrary value if and only if ntpd was re-started against long-standing recommendation with the -g flag, or if ntpd was not given the -g flag, the attacker can move the target system’s time by at most 900 seconds' time per attack.
Mitigation
- Configure
ntpd to get time from multiple sources.
- Upgrade to 4.2.8p5 or later.
- As we’ve long documented, only use the
-g option to ntpd in cold-start situations.
- Monitor your
ntpd instances.
Credit
This weakness was discovered by Aanchal Malhotra, Isaac E. Cohen, and Sharon Goldberg at Boston University.
Timeline