Since November I have been releasing details on all vulnerabilities I found that I have not released before. This is the 37th entry in the series. This information is available in more detail on my blog at http://blog.skylined.nl/20161221001.html. There you can find a repro that triggered this issue in addition to the information below, as well as a Proof-of-Concept exploit that attempts to prove exploitability. If you find these releases useful, and would like to help me make time to continue releasing this kind of information, you can make a donation in bitcoin to 183yyxa9s1s1f7JBpPHPmzQ346y91Rx5DX. Follow me on http://twitter.com/berendjanwever for daily browser bugs. MSIE 11 MSHTML CPasteCommand::ConvertBitmaptoPng heap-based BOF =============================================================== (MS14-056, CVE-2014-4138) Synopsis -------- A specially crafted web-page can trigger an out-of-bounds write in Microsoft Internet Explorer 11. Code that handles pasting images from the clipboard uses an incorrect buffer length, which allows writing beyond the boundaries of a heap-based buffer. An attacker able to trigger this vulnerability can execute arbitrary code. Known affected software, attack vectors and potential mitigations ----------------------------------------------------------------- * **Microsoft Internet Explorer 11.0.9600.16521** An attacker would need to get a target user to open a specially crafted web-page. In order to trigger the issue, the web-page needs to either programmatically copy/paste an image using Javascript or get the user to do this (for instance by tricking the user into typing keyboard shortcuts such as CTRL+C/CTRL+V) . By default, MSIE prompts the user to allow or disallow programmatically copy/pasting the first time a website tries to do this, so user-interaction is normally required in such cases. Disabling the `Allow Programmatic clipboard access` setting in `Internet Options` -> `Security Settings` -> [Choose a zone] -> `Scripting` should prevent websites from programmatically copy/pasting an image. Disabling execution of scripts on web-pages altogether will have the same effect. Please note that neither option prevents a website from social engineering the user into typing a keyboard shortcut to copy/paste the image. Details ------- When an image is pasted in MSHTML, it gets converted from BMP format to PNG. This is done in the `MSHTML!CPasteCommand::ConvertBitmaptoPng` function. This function incorrectly uses the size of the original BMP image to allocate memory for storing the converted PNG image. The PNG image will be smaller than the BMP under most circumstances, but if a specially crafted image leads to the original BMP image being smaller than the converted PNG, the function will write PNG data beyond the bounds of the allocated memory. Exploit ------- An attacker looking to exploit this issue will commonly attempt to get the memory allocated to store the PNG image in a location that is followed by a pre-allocated memory block that contains information the attacker would like to modify. Using the buffer overflow, the attacker can overwrite this pre-allocated memory block with attacker controlled data. Depending on the type of the pre-allocated memory, this could allow the attacker to read or modify arbitrary information within the process and take control of execution flow. No attempt was made to create a Proof-of-Concept that shows this level of control. Time-line --------- * 8 May 2014: This vulnerability was submitted to ZDI. * 9 June 2014: This vulnerability was acquired by ZDI. * 23 June 2014: This vulnerability was disclosed to Microsoft by ZDI. * 14 October 2014: This vulnerability was address by Microsoft in MS14-056. * 21 December 2016: Details of this vulnerability are released. Cheers, SkyLined
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