Re: [tcpm] Privacy problems of TCP Fast Open

Erik Sy <> Tue, 21 May 2019 07:52 UTC

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To: Michael Tuexen <>
References: <> <> <>
From: Erik Sy <>
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Date: Tue, 21 May 2019 09:52:09 +0200
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Subject: Re: [tcpm] Privacy problems of TCP Fast Open
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Hi Michael,

thanks for this question!

Yes, TFO cookies are bound to the clients (local) IP address. However, a
client with a static local IP address in a home network will use the
same TFO cookie independently of it's publicly visible IP address. As a
result, TFO cookies present an independent tracking mechanism, which
does not necessarily rely on the client's publicly visible IP address.

Returning to your example, onion routing does not necessarily protect
you against tracking via TFO cookies.

Best regards,

On 5/21/19 09:13, Michael Tuexen wrote:
>> On 20. May 2019, at 23:19, Erik Sy <> wrote:
>> I think it is important to warn users about the privacy risks of RFC
>> 7413. For example, Mozilla reacted to the privacy problems of TCP Fast
>> Open by deprecating this protocol on all it's Firefox branches. In
>> total, TCP Fast Open has significant issues with respect to user
>> privacy, performance and deployment on the real-world Internet. From my
>> point of view, it is about time to deprecate RFC 7413.
> Hi Eric,
> my understanding is that a cookie is specific to a client address, a server
> address and a server port. So it would make sense for a client to remove
> entries from the cookie cache on an address change. Assuming that, how
> does your described host based attacks relate to the server just using
> the client IP address for tracking? If you are trying to hide you IP-address
> (like using a TOR browser) you don't want to use TFO, but you are not
> optimising for small RTTs in that case, so it makes no sense in that case.
> Best regards
> Michael
>> Regards,
>> Erik
>> On 5/10/19 14:14, Erik Sy wrote:
>>> Hi everyone,
>>> TCP Fast Open has significant privacy problems which are not considered
>>> in RFC 7413.
>>> For example, this protocol allows a passive network observer to
>>> correlate connections established by the same client, which protocols
>>> such as TLS 1.3 and QUIC actively protect against. Furthermore, Fast
>>> Open cookies present a kernel-based tracking mechanism which is quite
>>> persistent. Amongst others, they can be used to conduct cross-browser
>>> tracking on the same operating system.
>>> For further details please refer to this article:
>>> I suggest, that the working group takes steps to highlight these privacy
>>> problems of RFC 7413.
>>> Regards,
>>> Erik
>>> _______________________________________________
>>> tcpm mailing list
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