Microsoft Windows 7/8.1 FON Font File win32k.sys divide by zero

Summaryinfo

A vulnerability was found in Microsoft Windows 7/8.1 and classified as problematic. This affects an unknown function in the library win32k.sys of the component FON Font File Handler. Executing a manipulation can lead to divide by zero. The attack may be launched remotely. Furthermore, there is an exploit available.

Detailsinfo

A vulnerability was found in Microsoft Windows 7/8.1 (Operating System). It has been declared as problematic. This vulnerability affects an unknown code in the library win32k.sys of the component FON Font File Handler. The manipulation with an unknown input leads to a divide by zero vulnerability. The CWE definition for the vulnerability is CWE-369. The product divides a value by zero. As an impact it is known to affect availability.

The weakness was shared 08/17/2016 by Mateusz Jurczyk (j00ru) with Google as Issue 855 / MSRC-33949 Issue 855 as confirmed bug report (Bug Tracker). The advisory is available at bugs.chromium.org. The vendor cooperated in the coordination of the public release. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Technical details and also a public exploit are known. This vulnerability is assigned to T1499 by the MITRE ATT&CK project. The advisory points out:

It is a Denial of Service issue caused by an unhandled divide by zero exception triggered in the win32k.sys driver while processing malformed .FON fonts. We suspect feasible remote attack vectors might exist through GDI clients which pass non-validated fonts directly to the kernel for loading (Microsoft Office?). Most importantly, the crash is extremely easy to hit, which renders any dumb .FON fuzzing quite ineffective and may thus mask other, more serious vulnerabilities.

A public exploit has been developed by Mateusz Jurczyk (j00ru) and been published immediately after the advisory. It is possible to download the exploit at bugs.chromium.org. It is declared as proof-of-concept. The vulnerability was handled as a non-public zero-day exploit for at least 54 days. During that time the estimated underground price was around $5k-$25k.

The bug report contains the following remark:

Microsoft has responded that the bug does not meet the bar for a security bulletin, but a fix will be evaluated for upcoming next-version releases.

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Productinfo

Type

• Operating System

Vendor

• Microsoft

Name

• Windows

Version

• 7
• 8.1

License

• commercial

Support

• end of life (old version)

Website

• Vendor: https://www.microsoft.com/
• Product: https://www.microsoft.com/en-us/windows

CPE 2.3info

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