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Today at work, we brought out some PCs that were not used for a long time (were not in use when I joined this company so I have no idea about their history). After booting one of them (Windows XP Professional), I found that the system time is not set, so I tried to use NTP and sync the system to a time server. I used the default time server, i.e. time.windows.com. However, when I attempted to synchronize, the following error message was returned:

An error occurred while Windows was synchronizing with time.windows.com. For security reasons, Windows can not synchronize with the server because your date does not match. Please fix the date and try again.

What possible security reasons are meant here? What security threats are possible in the process of synchronizing a machine to a time server??

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2 Answers

up vote 7 down vote accepted

As one example, Kerberos requires time synchronization. If you connect to a time server that changes your time by a large amount, it could interfere with the operation of kerberos. There may be other protocols that depend on embedded timestamps for secure operation.

Also: some applications do not behave well if the time suddenly jumps. It may be possible that there are attacks based on changing the time. I don't know about Windows, but the NTP daemon on *nix systems tries to gradually adjust the time instead of jumping it -- for this reason.

A good way to sync with a time server (especially if your system clock is off by a lot) is to manually set the time "close enough" to what you can manage manually -- within a few seconds is best, within a couple of minutes is fine if you don't have a good time source. Then connect to the time server and the local time service / daemon can make smaller adjustments to the system time, possibly spreading the adjustments out over a longer time so that there are no jumps in the system clock.

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+1 (mentally cuz I don't have enough reputation here :) ) but where is the difference between 'jumping' the time manually or through the time server? It is the same risk to the system right? –  amyassin Dec 19 '11 at 20:42
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When the administrator makes a decision to jump the time, it happens under the control of the administrator. That's ok. When some unknown/untrusted server jumps the time, that's not good. The administrator can make a decision to do this when the system is not in use, or he can warn users, or he can force the system to reboot so that any affected applications will be restarted. –  bstpierre Dec 19 '11 at 20:48
    
Ahaa, I got what you mean... thank you –  amyassin Dec 19 '11 at 21:21
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Most NTP server daemons won't jump the clock significantly backwards or forwards. In some operating modes they'll refuse to jump at all, refusing to update to anything they can't reach by skewing (very minute adjustments over a long period).

Jumping messes up log continuity, comparison of files by timestamp, and a host of other issues. It also affects anything that depends on time -- software update checks, scheduled jobs, account expiry, etc.

Set your clock to something close to the current actual time (whatever's on your watch should be good enough) and it will bring itself into line.

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yes, I've actually just had to set the date and it was fixed... –  amyassin Dec 19 '11 at 20:46
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