Microsoft Edge Chakra Incorrect Parse
Posted on 24 September 2017
Microsoft Edge: Chakra incorrectly parses object patterns CVE-2017-8729 When the Chakra's parser meets "{", at first, Chakra treats it as an object literal without distinguishing whether it will be an object literal(i.e., {a: 0x1234}) or an object pattern(i.e., {a} = {a: 1234}). After finishing to parse it using "Parser::ParseTerm", if it's an object pattern, Chakra converts it to an object pattern using the "ConvertObjectToObjectPattern" method. The problem is that "Parser::ParseTerm" also parses ".", etc. using "ParsePostfixOperators" without proper checks. As a result, an invalid syntax(i.e., {b = 0x1111...}.c) can be parsed and "ConvertObjectToObjectPattern" will fail to convert it to an object pattern. In the following PoC, "ConvertObjectToObjectPattern" skips "{b = 0x1111...}.c". So the object literal will have incorrect members(b = 0x1111, c = 0x2222), this leads to type confusion(Chakra will think "c" is a setter and try to call it). PoC: function f() { ({ a: { b = 0x1111, c = 0x2222, }.c = 0x3333 } = {}); } f(); This bug is subject to a 90 day disclosure deadline. After 90 days elapse or a patch has been made broadly available, the bug report will become visible to the public. Found by: lokihardt
