This also applies to:
Microsoft .NET Framework 3.5
This security update resolves a vulnerability in Microsoft .NET Framework that could allow remote code execution when .NET Framework processes untrusted input. An attacker who successfully exploits this vulnerability in software by using .NET Framework could take control of an affected system. The attacker could then install programs; view, change, or delete data; or create new accounts that have full user rights. Users whose accounts are configured to have fewer user rights on the system could be less affected than users who operate with administrative user rights.
To exploit the vulnerability, an attacker would first have to convince the user to open a malicious document or application.
This security update addresses the vulnerability by correcting how .NET Framework validates untrusted input.
To learn more about this vulnerability, see Microsoft Common Vulnerabilities and Exposures CVE-2018-8421.
All updates for Windows 8.1, Windows RT 8.1 and Windows Server 2012 R2 require update KB 2919355 to be installed. We recommend that you install update KB 2919355 on your Windows 8.1-based, Windows RT 8.1-based or Windows Server 2012 R2-based computer so that you receive updates in the future.
If you install a language pack after you install this update, you must reinstall this update. Therefore, we recommend that you install any language packs that you need before you install this update. For more information, see Add language packs to Windows.
After the September 2018 .NET Framework security update is applied to resolve CVE-2018-8421 (.NET Framework remote code execution vulnerability), SharePoint out-of-the-box workflows stop working.
For more information about this known issue, see the following article in the Microsoft Knowledge Base:
4465015 SharePoint Workflows stop working after you install .NET security updates for CVE-2018-8421
Additional information about this security update
The following articles contain additional information about this security update as it relates to individual product versions. The articles may contain known issue information.
4457045 Description of the Security and Quality Rollup for .NET Framework 3.5 for Windows 8.1, RT 8.1, and Server 2012 R2 (KB 4457045)
4457036 Description of the Security and Quality Rollup for .NET Framework 4.5.2 for Windows 8.1, RT 8.1, and Server 2012 R2 (KB 4457036)
4457034 Description of the Security and Quality Rollup for .NET Framework 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, and 4.7.2 for Windows 8.1, RT 8.1 and Server 2012 R2 (KB 4457034)
How to obtain help and support for this security update
Help for installing updates: Windows Update FAQ
Help for protecting your Windows-based products and services from viruses and malware: Microsoft Secure
Local support according to your country: International Support