Opened 8 years ago

Closed 6 years ago

#8751 closed defect (fixed)

do something about TLS HELLO gmt_unix_time

Reported by: proper Owned by: tbb-team
Priority: High Milestone:
Component: Applications/Tor Browser Version:
Severity: Keywords: tbb-torbutton
Cc: proper Actual Points:
Parent ID: #3059 Points:
Reviewer: Sponsor:



When not using Tor...

  • For example, when using wget or Firefox, the user's ISP and destination server can watch TLS hello and thus learn about the client's clock.
  • Many updaters in background are also using TLS.

When using Tor...

  • For example, when using TBB, Tor exit nodes, the ISP's of Tor exit nodes and destination servers can see client's clock.

These are the assumptions. [3] Please tell me if they are wrong.


NTP server admins can willingly or if their server gets compromised and any man-in-the-middle can tamper with NTP replies and therefore introduce a unique clock skew.

Almost no one is using authenticated NTP, because there are no instructions in a forum or blog how to enable NTP authentication. Therefore almost everyone uses standard configuration and is at risk.

Also due to a clock defect, low battery, clock can skew without tampering with NTP.

Since the browser [1] transmits it in TLS HELLO (gmt_unix_time), it can be used to track individual users. For example, a clock skew of +/-30 minutes may not worry the user ("That damn clock is wrong again. I use my watch instead.") but could identify the user even when using Tor.

Also adversaries who didn't introduce the clock skew could use it to identify users. If the user visits a website under adversary control 2 without Tor for some non-anonymous activity, it knows the clock skew. Later, if the user visits another website under adversary control, it can see the same clock skew, which is at least a strong anonymity set reduction.


RFC 5245 says.

Clocks are not required to be set correctly by the basic TLS protocol;

So perhaps get ride of it entirely (replace it with some fixed time)?

higher-level or application protocols may define additional requirements.

Whatever that means.


I have no idea.




[1] Also #1517 "Provide JS with reduced time precision" wouldn't help much, since it wouldn't do something about bigger clock skews.
[2] Nowadays with services like google analytics and facebook like button, there are servers which are present on a high percentage of all websites.
[3] Haven't used wireshark, but read and

Child Tickets

Change History (7)

comment:1 Changed 7 years ago by nickm

I think Mike just merged a patch for this. Mike?

comment:3 Changed 7 years ago by mikeperry

Depends on if we want to backport to TBB-stable. That commit is only for 3.0-alpha. I also haven't tested it yet.

comment:4 Changed 7 years ago by nickm

If it works post-testing in 3.0-alpha, I'd suggest a backport before too much time passes.

comment:5 Changed 7 years ago by proper

TBB 2.x has been deprecated. Nevermind the backport.

Ticket solved?

comment:6 Changed 6 years ago by erinn

Component: TorBrowserButtonTor Browser
Keywords: tbb-torbutton added
Owner: changed from mikeperry to tbb-team

comment:7 Changed 6 years ago by mikeperry

Resolution: fixed
Status: newclosed

Yes, this was also fixed upstream in NSS 3.15.4.

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