Microsoft Internet Explorer 11 MSHTML CView::CalculateImageImmunity Use-After-Free
Posted on 02 November 2016
Throughout November, I plan to release details on vulnerabilities I found in web-browsers which I've not released before. This is the second entry in that series. The below information is also available on my blog at http://blog.skylined.nl/20161102001.html. There you can find a repro that triggered this issue in addition to the information below. Follow me on http://twitter.com/berendjanwever for daily browser bugs. MSIE 11 MSHTML CView::CalculateImageImmunity UAF ================================================ (The fix and CVE number for this bug are not known) Synopsis -------- Setting the `listStyleImage` property of an Element object causes MSIE 11 to allocate 0x4C bytes for an "image context" structure, which contains a reference to the document object as well as a reference to the same `CMarkup` object as the document. When the element is removed from the document/document fragment, this image context is freed on the next "draw". However, the code continues to use the freed context almost immediately after it is freed. Known affected versions, attack vectors and mitigations ----------------------- + Microsoft Internet Explorer 11 An attacker would need to get a target user to open a specially crafted webpage. As far as can be determined, disabling JavaScript should prevent an attacker from triggering the vulnerable code path. Exploit ------- I tried a few tricks to see if there was an easy way to reallocate the freed memory before the reuse, but was unable to find anything. I do not know if there is a way to cause further reuse of the freed memory later on in the code. Running the repro as-is without page heap does not appear to trigger crashes. It does not appear that there is enough time between the free and reuse to exploit this issue. Repro.html: <script> var oDocumentFragment = document.createDocumentFragment(), oElement = document.createElement('x'); oDocumentFragment.appendChild(oElement); oElement.style.listStyleImage = "url(x)"; oDocumentFragment.removeChild(oElement); </script> Timeline -------- * May 2014: This vulnerability was found through fuzzing. * June 2014: This vulnerability was submitted to ZDI. * July 2014: ZDI rejects the submission. * November 2016: The issue does not reproduce in the latest build of MSIE 11. * November 2016: Details of this issue are released. Unfortunately, my records of what happened after ZDI rejected the issue are patchy. It appears that I did not pursue reporting the issue anywhere else, but Microsoft does appear to have patched the issue, as I can no longer reproduce it. Cheers, SkyLined
