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TUCoPS :: Web :: General :: web5124.htm

Web sites & CSS - CSS visited pages disclosure



25th Feb 2002 [SBWID-5124]
COMMAND

	CSS visited pages disclosure

SYSTEMS AFFECTED

	IE & Mozilla, maybe others, browsing CSS vulnerable sites.

PROBLEM

	Andrew Clover [http://and.doxdesk.com] found following :
	

	In  http://www.cs.princeton.edu/sip/pub/webtiming.pdf   ,   Felten   and
	Schneider outline a method for pages on an attacking  server  to  detect
	whether pages on another server have been visited, by trying to fetch  a
	URL from the target server and using the  time  taken  to  fetch  it  to
	guess whether the URL was in the browser\'s local cache.
	

	A method is also suggested to use the browser cache, read this  way,  as
	a store for persistent user data (\"cache cookies\").
	

	CSS has a feature that can be abused to exactly the  same  ends.  It  is
	simpler, more accurate, and more easily abused than the  timing  attacks
	described in the above paper.
	

	 Issue

	 =====

	

	The CSS :visited pseudo-class can be used to apply different on-  screen
	styling to links leading to pages the user has already visited.  However
	the styling  can  have  side  effects  which  can  be  detected  by  the
	attacking      server.      For      example,      the      page      at
	http://www.smith-widgets.foo/ could include the following markup:
	

	

	  <a id=\"jones\" href=\"http://www.jones-widgets.foo\"></a>

	

	

	with the style:
	

	

	  #jones:visited { background: url(/visited.cgi?site=jones); }

	

	

	In this case the side-effect of the style will be a call to the  CGI  at
	smith-widgets if the user has visited jones-widgets.  The  script  there
	could log this information, associate it with any cookies  passed,  then
	return a transparent background image set to expire soon.
	

	Any property that can be  given  with  a  <uri>  parameter  could  be
	abused  this  way.  CSS2  defines   background-image,   list-style-image
	(trickier to use without fiddling with display properties or using  CSS3
	selectors, as a list cannot normally go  inside  a  link),  content  and
	cursor (trickier to use due to poor browser support), and various  Aural
	CSS properties (again, terrible browser support).
	

	The simple answer to this problem would be to have all  URIs  associated
	with :visited conditions be fetched regardless of whether the  link  has
	been visited or not. However, apart from the  performance  penalty  this
	would incur, it does  not  solve  the  problem  for  browsers  with  the
	capability to read calculated styles. JavaScript can  then  be  used  to
	detect other side- effects, if it is enabled.
	

	IE gives each document element a \'currentStyle\' object  which  can  be
	queried to read which the calculated styles  applied  to  that  element,
	which can be used to determine whether a :visited rules was applied:
	

	

	  a         { color: blue; }

	  a:visited { color: red;  }

	

	  if (document.getElementById(\'jones\').currentStyle.color==\'red\')

	    document.writeln(\'<p>Hello! I see you\\\'ve been to Jones.\');

	    document.writeln(\'Don\\\'t buy from Jones - their widgets\');

	    document.writeln(\'are made from recycled babies.<\\/p>\');

	

	

	Mozilla\'s support of DOM Views should be able to do the  same  sort  of
	thing. Even without direct access to  calculated  style  objects,  there
	are ways to imply which rules have been  used,  for  example  using  the
	on-screen positions of elements:
	

	

	  #jones { position: absolute; top: 0; }

	  #jones:visited { top: 100px; }

	

	  if (document.getElementById(\'jones\').offsetTop>50)

	    ...

	

	

	IE\'s offsetFoo  properties  are  also  supported  by  Mozilla,  and,  I
	believe, Konqueror.
	

	The primitive one-bit-cache-storage \"cache cookies\" idea can  also  be
	used with one-bit-history-list-storage to get \"CSS cookies\". To  write
	to the history list would require an actual visit to the page, not  just
	an attempt to load it; this could be achieved using an invisible  frame.
	Mozilla also counts an <iframe> as being a visit, but  IE  does  not.
	using :visited rules then gives non-destructive read capability  to  the
	history list. Many single bits (documents) would  have  to  be  used  to
	store any practical  amount  of  user  data,  which  is  presumably  why
	\'cache cookies\' have not been exploited so far (as far as I know).

SOLUTION

	Possible solutions to the problem would be:
	

	  (a) as well as fetching all URIs independent of :visited

	      conditions, removing all access to calculated styles and

	      other run-time properties such as positioning.

	      Unfortunately these features can be very useful to web

	      authors! There is no practical way to limit access to

	      elements unaffected by :visited styles.

	

	  (b) as well as fetching :visited URIs, advising users to

	      turn scripting off in non-trusted sites. This is probably

	      a good idea in any case, but users never do it.

	

	  (c) something similar to Felten and Schneider\'s proposed

	      \'domain tagging\' - make \'visited\' links only look

	      \'visited\' when they point to documents in the same

	      domain as the current page. This would be a severe

	      blow to the functionality and usability expected of

	      visited links.

	


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