Insecure time synchronization and the leaking of time data exposes the user to a subset of advanced deanonymization attacks. The attack vectors and possible mitigations are described below.
Denial of Service
The UDP-based NTP protocol can be abused to cause much larger replies than normal, causing systems to be overwhelmed. These are known as amplification attacks. 
Locating Onion Services
Timers can leak CPU data. Under some circumstances, related activity data can lead to deanonymization of an onion service. 
Remote Code Execution
NTP is a buggy and ancient protocol. Flaws in NTP clients can be remotely exploited to allow an adversary control over the system. The unencrypted and unauthenticated nature of NTP makes this attack trivial for network adversaries of any size.  
Remote Device Fingerprinting
Clock leaks arising from either software on the host or application-level protocols on Whonix-Workstation (
anon-whonix) allow a passive adversary to easily link anonymous and non-anonymous traffic to the same machine. Active clock skew attacks can be trivially mounted to deanonymize users. 
Replaying an older time allows an adversary to:
- Feed an old Tor consensus. 
- Provide updates and (https) certificates which are old, outdated, or have known vulnerabilities.
This is possible because cryptographic verification depends on an accurate system clock. For example, a clock set to two years in the past will accept certificates or updates which have already expired or been revoked.
Clock Leak Vectors
Certain properties of protocols leak clock information: 
- ICMP Timestamps: Leak host time in query replies. 
- NTP Clients: Leak host time  and expose the system to every attack outlined above.
- TCP Initial Sequence Numbers (ISNs): Even when timers do not fully leak the host's clock, they can allow side-channel attacks because sensitive information about a system's CPU activity is leaked.  This information is leaked in any traffic sent over clearnet. It can therefore be linked to maliciously induced load patterns on an onion site, resulting in deanonymization. 
- TCP Timestamps: Included in every TCP packet, these leak system information down to the millisecond, as well as system uptime. They also permit fingerprinting of devices behind a router. 
It is practically possible to block all of the clock leak vectors in the preceding section. Users running onion services or those who require very high-level security are strongly recommended to apply the measures below.
- Uninstall any NTP clients and disable systemd's timdatectl NTP synchronization feature. 
- Disable TCP timestamps via kernel sysctl. This boolean disables both IPv4 and IPv6 TCP timestamps since they are controlled by the same sysctl option.
The following line must be added to /etc/sysctl.conf or /etc/sysctl.d/tcp_timestamps.conf
net.ipv4.tcp_timestamps = 0
To make this change, run.
echo "net.ipv4.tcp_timestamps = 0" > /etc/sysctl.d/tcp_timestamps.conf
To apply the sysctl settings without rebooting, run.
Check the setting was applied correctly.
- Block incoming ICMP messages and any other incoming traffic with iptables or any of its frontends, such as ufw. 
- TCP Initial Sequence Numbers mitigation.
An artificially induced CPU-load temperature influences the timer crystal and skews its frequency. TCP ISNs are a permanent feature and are necessary from a security standpoint to prevent arbitrary TCP connection hijacking by non-global network adversaries.  See the following reference about removing the timer output function from Linux's TCP ISN code. 
- Application-level mitigation.
For application-level leak mitigation, avoid sending any clearnet traffic. Without clock information leaking from the host, network adversaries do not have non-anonymous timestamp sources to match this data with, even if software on Whonix-Workstation (
Whonix Solutions and Limitations
Whonix has implemented sdwdate as a secure time synchronization mechanism to replace NTP.  sdwdate-gui  is the GUI front-end. sdwdate was written with safety in mind and to avoid the many security pitfalls in NTP. Furthermore, NTP is UDP-based and cannot work over Tor, and onion services must have an accurate clock to be reachable.
sdwdate fetches its time exclusively from reputable sources -- whistle-blowing and privacy-friendly onion sites -- that are very likely to be hosted on different hardware. sdwdate also benefits from the security of Tor's end-to-end encryption.
sdwdate can only protect against passive timestamp linkage of data leaking from both the host and Whonix-Workstation (
anon-whonix). It cannot defend against a skilled adversary that is able to compromise Whonix-Workstation (
anon-whonix). Via a clock correlation attack, the adversary would discover the host clock when the VM is rebooted, and then link the time readings with any host clock leaks. The only way to prevent this and similar attacks is to stop the leaks in the first place.
- See Don't update NTP - stop using it and The Rising Sophistication of Network Scanning.
- See http://www.caida.org/publications/papers/2005/fingerprinting/ and Tor Browser upstream bug #3059 for the kind of application-level leaks that can happen: Find some way to deal with time-based fingerprints.
- - slide 9
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