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From Brian Clapper <>
Subject Fwd: Risks of multiple HTTP standards
Date Wed, 04 Sep 1996 17:22:41 GMT
More M$-related stuff.
Brian Clapper .............................................. ............. PGP public key available on request
If you build something a fool can use, only a fool will want it.

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From: Pete Bentley <>
Subject: Risks of multiple HTTP standards
Date: Tue, 03 Sep 1996 15:37:16 +0100

The Microsoft Network offers a free service allowing people to create their
own custom 'start' page at which may contain personal
information including the individual's address.  All of this information is
encoded into a 'cookie' which is stored by the user's own web browser and so
in theory cannot be seen by other people.  Or can it?

MSN uses Microsoft's own product, Microsoft Internet Information Server 1.0,
as its web server and it tries to send a response which will ensure that is not cached by any proxy servers (to prevent people
seeing each others information by mistake), but is cached by the users web
browser for a short time (for speed). It does this by sending an HTTP/1.0
reply with Date and Expires headers which indicate that the information is
valid for half an hour and adds a "cache-control: private" header to prevent
any proxy servers from caching the information.  However, cache-control is a
header from the draft HTTP/1.1 specification which is not interpreted by
many HTTP/1.0 proxy servers in use today (verified by myself with the CERN
and Squid proxies) and so is ignored.  The remaining headers indicate to the
proxy that it may cache the page itself for half an hour.  If the proxy then
caches a page containing a user's personal information then any other user
accessing via that proxy in the next half hour will
receive that page (and personal information).  This scenario has indeed been
observed in practice with a busy web proxy run by a large ISP and a detailed
analysis has been sent to Microsoft for their attention.

The risk here seems to mostly be mixing HTTP standards and assuming that
HTTP/1.0 servers will understand some HTTP/1.1 headers. It raises issues for
both server coders and cache coders as the Net starts gradually migrating to
HTTP/1.1 at the same time as 'smart' web servers (generating pages on the
fly) want finer control over caching proxies.

  [Aside: whilst making HTTP requests by telnetting to
  (to compare headers), I noticed that either restarts
  (connection refused) or goes missing (routing loops between
  routers) quite frequently. ]

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