full-disclosure - Re: Firefox + popup blocker + XMLHttpRequest + srand() = oops

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Date: Mon, 5 Feb 2007 12:38:07 +0000
From: "pdp (architect)" <pdp.gnucitizen@...glemail.com>
To: "Michal Zalewski" <lcamtuf@...ne.ids.pl>
Cc: full-disclosure@...ts.grok.org.uk, bugtraq@...urityfocus.com
Subject: Re: Firefox + popup blocker + XMLHttpRequest +
	srand() = oops

Hi Michal,

Nice read! Very complicated though and with too many "If"s, but very
interesting. I just want to sum up. As long as the user has a
malicious html file stored on their system you know the path to it,
the attacker can read local files. You don't need to do this pop-up
trick at all. You may as well use a QuickTime .mov/.qtl or a PDF
document to open a file:// link . I think it is easier.

Nice work.

On 2/5/07, Michal Zalewski <lcamtuf@...ne.ids.pl> wrote:
> 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 1.5.0.9.
>
> 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"
>
>      <html><body><script>
>      x = new XMLHttpRequest;
>      x.open("GET", "file:///c:/BOOT.ini", false);
>      x.send(null);
>      alert("The script attempted to read your C:/BOOT.ini:\n\n"
>            + x.responseText);
>      </script>
>
>   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:
>      window.open('file:///c:/windows/temp/xxxxxxx.htm','new2',''),
>      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).
>
> (Ta-dah!)
>
> /mz
>
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-- 
pdp (architect) | petko d. petkov
http://www.gnucitizen.org

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