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Microsoft Edge Chakra Incorrect Parse

Microsoft Edge Chakra Incorrect Parse
Posted Sep 22, 2017
Authored by Google Security Research, lokihardt

Microsoft Edge Charka incorrectly parses object patterns.

tags | exploit
advisories | CVE-2017-8729
MD5 | adcfdebb66197fad09cf1524ba76fe89

Microsoft Edge Chakra Incorrect Parse

Change Mirror Download
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

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