TUCoPS :: Browsers :: c07-2238.htm

Firefox + popup blocker + XMLHttpRequest + srand() = oops
Firefox + popup blocker + XMLHttpRequest + srand() = oops
Firefox + popup blocker + XMLHttpRequest + srand() = oops

There is an interesting vulnerability in the default behavior of Firefox
builtin popup blocker. This vulnerability, coupled with an additional
trick, allows the attacker to read arbitrary user-accessible files on the
system, and thus steal some fairly sensitive information.

This was tested on

For security reasons, Firefox does not allow Internet-originating websites
to access the file:// namespace. When the user chooses to manually allow a
blocked popup however, normal URL permission checks are bypassed. The
attacker may fool the browser to parse a chosen HTML document stored on
the local filesystem, and because Firefox security manager treats all
file:/// URLs as having "same origin", such a document could read other
local files at its discretion with the use of XMLHttpRequest, and relay
that information to a remote server.

Now, to make the attack effective, the attacker would need to plant a
predictably named file with exploit code on the target system.  This
sounds hard, but isn't: Firefox sometimes creates outright deterministic
temporary filenames in system-wide temporary directory when opening files
with external applications; even if we ignore this possibility (since it
requires the user to take an additional step that may be difficult to
justify), "random" temporary files are created using a flawed algorithm in
nsExternalAppHandler::SetUpTempFile and other locations.

The problem here is that stdlib linear congruential PRNG (srand/rand) is
seeded immediately prior to file creation with current time in seconds
(actually, microseconds, but divided by 1e6); rand() is then used in
direct succession to produce an "unpredictable" file name. Normally, were
the PRNG seeded once on program start and then subsequently invoked,
results would be deterministic, but difficult to blindly predict in the
real world; but here, the job is much easier: we know when the download
start, we know what the seed would be, and how many subsequent calls to it
are made - we know the output.

In a different setting, there would be a level of uncertainty caused by
the fact that system clocks tend to drift or have imprecise settings
(although today, most Windows systems either synchronize with Windows
Time, or domain time services, so this is less of a factor). Still,
there's a yet another nail to the coffin: on first call, Javascript
Math.random() is seeded using the same call in the same manner, PR_Now()
* 1e-6. The seed, and hence a time very close to the moment of file
creation, can be trivially computed by analyzing Math.random() output. But
wait, why bother at all - Javascript can be used to directly read system
clock with a 1-second resolution.

One of several attack scenarios I could think of might look as follows:

  1) Have user click on a link on a malicious page. The link would point
     to "evil.cgi", and have onClick handler set to function foo().
     This function would acquire current system time, and use setTimeout to
     invoke window.open("p2.html?" + curtime,"new",""); in 100 ms. The
     aforementioned cgi script would return:

     Content-type: text/html
     Content-disposition: attachment; filename="foo.html"


  2) After user clicks the link, a download prompt will appear, and a copy
     of evil.cgi output would be saved in - for example -
     C:\WINDOWS\TEMP\c3o89nr7.htm. The download prompt will be immediately
     hidden under the newly created p2.html window (this, by default,
     bypasses popup blocker. because the window is created in response to
     user action).

  3) The page currently displayed on top, p2.html, instructs the user to
     accept the popup to open a movie player or whatnot; since unsolicited
     popups are an annoyance, not a security risk, even an educated user
     is likely to comply.

     To create a popup warning, a script embedded on the page calls:
     with a name calculated by repeating a procedure implemented in
     SetUpTempFile() with a seed calculated by the server based on
     reported system time (p2.html?time).

  4) When the user opens that particular popup, attacker-supplied HTML
     file is loaded and executed with local file read privileges (in the
     aforementioned example, the contents of BOOT.ini file would be
     reported back to the victim).



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