Google Chrome Accessibility blink::Node Corruption

Credit: SkyLined
Risk: Medium
Local: No
Remote: Yes

Throughout November, I plan to release details on vulnerabilities I found in web-browsers which I've not released before. This is the twenty-first entry in that series. Unfortunately I won't be able to publish everything within one month at the current rate, so I may continue to publish these through December and January. The below information is available in more detail on my blog at There you can find a repro that triggered this issue and relevant code snippets in addition to the information below. Follow me on for daily browser bugs. Google Chrome Accessibility blink::Node corruption ================================================== (The fix and CVE number for this issue are unknown) Synopsis -------- A specially crafted web-page can trigger an unknown memory corruption vulnerability in Google Chrome Accessibility code. An attacker can cause code to attempt to execute a method of an object using a vftable, when the pointer to that object is not valid, or the object is not of the expected type. Successful exploitation can lead to arbitrary code execution. Known affected software and attack vectors ------------------------------------------ * Chrome 48.0.2540.0 dev-m An attacker would need to get a target user to open a specially crafted webpage. Renderer accessibility must be enabled through the "--force-renderer-accessibility" command-line option. Disabling JavaScript will not prevent an attacker from triggering the vulnerable code path. Description ----------- Repeatedly loading two different pages in an iframe can cause the accessibility code to crash. This crash can happen in two different code paths, which are similar and both end up crashing because of a corrupt `blink::Node` instance. The first code path calls `blink::isDisabledFormControl` with the corrupt `blink::Node` instance as an argument from `AXNodeObject::canSetFocusAttribute`. This causes an access violation when `blink::isDisabledFormControl` attempts to call the `isDisabledFormControl` method on the corrupt `blink::Node` instance. The second code path calls `blink::Element::fastGetAttribute` with the corrupt `blink::Node` instance as an argument from `blink::AXObject::getAttribute`. This can cause an access violation at various locations along the code path, but almost certainly does so if the code reaches the part where it attempts to match the attribute name, as the `blink::AttributeCollectionGeneric<...>` was taken from a corrupt `blink::Node` instance and that data is therefore almost certainly completely invalid. Exploit ------- Is is unclear to me why the `blink::Node` instance was corrupted. During analysis, I was having trouble running Google Chrome with Page Heap enabled, which severely limited my ability to reliably crash the application and find out what information on the heap belongs to what object. Then, before I could get my debugging environment fixed, the issue appears to have been fixed, as I was no longer able to reproduce it. Any information on exploitability is therefore based on speculation. An attacker who is able to trigger the issue reliably, and has some control over the corrupted `blink::Node` instance that is returned, or heap memory in this area, may be able to control execution flow through the `blink::isDisabledFormControl` call, as this uses information from the corrupted `blink::Node` instance as a pointer to a vftable. Time-line --------- * October 2015: This vulnerability was found through fuzzing. * November 2016: Details of this issue are released. (This issue was never reported, as I was struggling with my debugging environment, as described above. At some point after I discovered it, this issue appears to have been fixed, as evidenced by the repro no longer working. However, I have no exact date, nor a fix number to provide here). Cheers, SkyLined Repro.html <iframe id=x></iframe> <script> var u = 0; onload = x.onload = function () { x.src = "Target" + (u++ % 2) + ".html" } </script> Target0.html <form> Target1.html <canvas><object id=a>x

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