Windows Defender Application Guard DoS via Long Hostname

[Jonathan Gregson via Fulldisclosure <fulldisclosure () seclists org>]

Windows Defender Application Guard (also known as "WDAG", Microsoft Defender Application Guard, and "MDAG") can be 
closed by any script or website loaded in WDAG by redirecting the browser to a URL with a long hostname (e.g, 10,000 
characters long). This can cause a denial-of-service condition.

Impact: 4.3
CVSS 3.1: AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L/E:H/RL:U/RC:C

## Details

Application Guard will immediately close if there is any attempt by a user or website to load a URL where the hostname 
exceeds a certain length. The exact length needed is difficult to determine, but the issue occurs reliably with 
hostnames longer than 5,100 characters. A Microsoft engineer on the WDAG team indicated that this is working as 
designed, stating "This doesn't appear to be a crash. We quickly, but gracefully, tear down the container browser if 
container<->host messages are too large."

To exploit this issue, an attacker first needs to determine that the user is viewing the website in WDAG. This is easy 
to determine on the server side as WDAG sends an HTTP header named `x-ms-applicationguard-initiated` with a value of 
`1` with all requests.

Using these details, a website loaded inside of WDAG is able to close WDAG on the visitor's host operating system with 
the following PHP code:

    if (isset($_SERVER['HTTP_X_MS_APPLICATIONGUARD_INITIATED'])) {
        header('Location: https://'.str_repeat('a', 10000).'.example.com');
    }

JavaScript executed in WDAG is able to redirect to a similar URL and cause WDAG to close, but a method of detecting 
WDAG on the client side has not yet been found.

You can find a Proof-of-Concept here: https://research.jdgregson.com/poc/wdag/dos1/
You can find this writeup online here: 
https://github.com/jdgregson/Disclosures/tree/master/microsoft/wdag-dos-long-hostname

## Impact

The impact of this vulnerability is Denial-of-Service. A malicious website is able to close WDAG at will, which can be 
used to prevent users from loading certain pages in WDAG. This may cause the user to load the page outside of WDAG on 
the host operating system.

The denial-of-service condition is short-lived as the user can simply open WDAG again. However, all of their tabs will 
be closed. Frequent closing of WDAG resulting in data and research loss may harm user adoption of WDAG.

## Disclosure Timeline

- 01/26/2021: Issue reported to WDAG team at Microsoft.
- 01/27/2021: Microsoft engineer responded indicating that WDAG is working as designed and the issue is not a buffer 
overflow.
- 08/31/2021: Full disclosure.

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