Bugtraq: CVE-2016-0199 / MS16-063: MSIE 11 garbage collector attribute type confusion

CVE-2016-0199 / MS16-063: MSIE 11 garbage collector attribute type confusion

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This information is available in an easier to read format on my blog at

http://ift.tt/1UAOsAs

With [MS16-063] Microsoft has patched [CVE-2016-0199]: a memory

corruption bug

in the garbage collector of the JavaScript engine used in Internet

Explorer 11.

By exploiting this vulnerability, a website can causes this garbage

collector

to handle some data in memory as if it was an object, when in fact it

contains

data for another type of value, such as a string or number. The garbage

collector code will use this data as a virtual function table (vftable)

in order

to make a virtual function call. An attacker has enough control over

this data

to allow execution of arbitrary code.

Known affected software and attack vectors

------------------------------------------

+ **Microsoft Internet Explorer 11** (all versions before the June 2016

patch)

An attacker would need to get a target user to open a specially crafted

webpage. Disabling JavaScript should prevent an attacker from

triggering the

vulnerable code path.

Repro

-----

I've created two separate html files that can be used to reproduce this

issue

and shows control over a 32-bit vftable pointer in x86 versions of MSIE or a

partial control over a 64-bit vftable pointer in x64 versions.

````HTML

oElement = document.createElement("IMG");

var oAttr = document.createAttribute("loop");

oAttr.nodeValue = oElement;

oElement.loop = 0x41424344; // Set original value data to 44 43 42 41

oElement.setAttributeNode(oAttr); // Replace oElement with original

value data

oElement.removeAttributeNode(oAttr);

CollectGarbage(); // Use original value data as address 0x41424344 of

a vftable

````

(I've had to use xcript rather than script because Gmail refused to send it

otherwise, see http://ift.tt/1kvi7LG for the reason.)

Description

-----------

When `setAttributeNode` is used to set an attribute of a HTML element,

and the

`Attr` node's `nodeValue` is not a valid value, this `nodeValue` is set

to the

value the attribute had before the call. This can happen for instance

when you

try to set an attribute that must have a string or number value by using an

`Attr` node with a HTML element as its `nodeValue` (as this is not a

string or

number). The HTML element in `nodeValue` is replaced with the string or

number

value the attribute had before the call to `setAttributeNode`.

If the `Attr` node is then removed using `removeAttributeNode` and the

garbage

collector runs, the code appears to assume the nodeValue still contains an

object, rather than the string or number it has been changed into. This

causes

the code to use the data for the string or number value as if it was a C++

object. It attempts to determine a function pointer for a method from the

object's virtual function table before calling this function using the

pointer.

If the previous value is a string, the character data from the string is

used

to calculate the function pointer. If the previous value is a number,

the value

of the number is used. This provides an attacker with a large amount of

control

over the function pointer and may allow execution of arbitrary code.

Scanner

-------

I build a "scanner" to analyze this issue and help create two

proof-of-concept

files that show control over the vftable pointer. More details and the

source

for these can be found on my blog at http://blog.skylined.nl.

Cheers,

SkyLined

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