[websec] HSTS: Infinite max-age to address NTP spoofing attack?

Xiaoyin Liu <xiaoyin.l@outlook.com> Fri, 07 November 2014 06:56 UTC

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From: Xiaoyin Liu <xiaoyin.l@outlook.com>
To: "websec@ietf.org" <websec@ietf.org>
Date: Fri, 7 Nov 2014 01:56:57 -0500
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Subject: [websec] HSTS: Infinite max-age to address NTP spoofing attack?
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 I recently read the slides "Bypassing HTTP Strict Transport Security" by 
Jose Selvi.[1] It seems to me that one way to address NTP spoofing attack 
on HSTS is to allow sites to specify HSTS policies that never expire (i.e. 
infinite max-age), so that the enforcement of HSTS does not depend on the 
system time.
 
So I want to propose a update to RFC 6797 to define a new directive called 
"infinite" (or something else). When a UA sees this directive, max-age 
should be ignored and HSTS should always be enforced until users clear the 
cache or the server sends a valid STS header without "infinite" directive.
 
The new header field will look like:
  Strict-Transport-Security: max-age=31536000; infinite
 
Of course, many websites will be unwilling to set infinite max-age, so this 
attack is not completely addressed. However, I think this new directive 
should help a lot, because some websites, especially those that need to 
send and receive sensitive information, such as online banking, are very 
unlikely to revert to HTTP in the future. Also, a very long max-age, such 
as 20 years used by Twitter, is effectively infinite, but long max-age is 
subject to the NTP attack, while an explicit "infinite" is not.
 
Any comments on this? Thanks!
 
Best,
Xiaoyin
[1] https://www.blackhat.com/docs/eu-14/materials/eu-14-Selvi-Bypassing-HTTP-Strict-Transport-Security-wp.pdf