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Microsoft Edge Chakra InitializeNumberFormat / InitializeDateTimeFormat Type Confusion

Microsoft Edge Chakra InitializeNumberFormat / InitializeDateTimeFormat Type Confusion
Posted Aug 17, 2018
Authored by Google Security Research, lokihardt

The InitializeNumberFormat function in Intl.js is used to initialize an Intl.NumberFormat object, and InitializeDateTimeFormat is used for an Intl.DateTimeFormat object. There are two versions of each initializer. One is for WinGlob and the other is for ICU. The problem is that the versions for ICU don't check whether the given object has been initialized. This allows to initialize the same object multiple times which can lead to type confusion.

tags | exploit
advisories | CVE-2018-8298
MD5 | 1b3261f5867fe61b3069b230e5d96d54

Microsoft Edge Chakra InitializeNumberFormat / InitializeDateTimeFormat Type Confusion

Change Mirror Download
Microsoft Edge: Chakra: Bugs in InitializeNumberFormat and InitializeDateTimeFormat 

CVE-2018-8298


The InitializeNumberFormat function in Intl.js is used to initialize an Intl.NumberFormat object, and InitializeDateTimeFormat is used for an Intl.DateTimeFormat object. There are two versions of each initializer. One is for WinGlob and the other is for ICU. The problem is that the versions for ICU don't check whether the given object has been initialized. This allows to initialize the same object multiple times which can lead to type confusion.

It seems the recent version of Edge in Windows Insider Preview has started to use ICU. Tested on Microsoft Edge 42.17672.1000.0 and Microsoft EdgeHTML 17.17672.

The initializer for ICU has no check:
<a href="https://github.com/Microsoft/ChakraCore/blob/bc2e55a7d80338ee4c9c63b76893f6d816dfe70b/lib/Runtime/Library/InJavascript/Intl.js#L1151" title="" class="" rel="nofollow">https://github.com/Microsoft/ChakraCore/blob/bc2e55a7d80338ee4c9c63b76893f6d816dfe70b/lib/Runtime/Library/InJavascript/Intl.js#L1151</a>

The initializer for WinGlob has a check:
<a href="https://github.com/Microsoft/ChakraCore/blob/bc2e55a7d80338ee4c9c63b76893f6d816dfe70b/lib/Runtime/Library/InJavascript/Intl.js#L3046" title="" class="" rel="nofollow">https://github.com/Microsoft/ChakraCore/blob/bc2e55a7d80338ee4c9c63b76893f6d816dfe70b/lib/Runtime/Library/InJavascript/Intl.js#L3046</a>

PoC:
let object = {};
Intl.NumberFormat.apply(object);
Intl.DateTimeFormat.apply(object);
Intl.DateTimeFormat.prototype.formatToParts.apply(object);

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.




Found by: lokihardt

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