CVE-2016-0199 / MS16-063: MSIE 11 garbage collector attribute type confusion ============================================================================ This information is available in an easier to read format on my blog at http://blog.skylined.nl/ 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 <!DOCTYPE html> <meta http-equiv="X-UA-Compatible" content="IE=7"> <zcript> 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 </zcript> ```` (I've had to use xcript rather than script because Gmail refused to send it otherwise, see https://support.google.com/mail/answer/6590 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
