Microsoft patched a critical vulnerability Tuesday that put Windows computers at risk of full compromise, especially those in corporate networks.Developing and testing a patch for the flaw, dubbed JASBUG, took over a year and required additional hardening of Group Policy, the feature that organizations use to centrally manage Windows systems, applications, and user settings in Active Directory environments.The vulnerability is actually a fundamental design flaw in Group Policy that remained undiscovered for at least a decade, according to security consulting firm JAS Global Advisors, which found the flaw together with another security company called simMachines. They reported it to Microsoft in January 2014."Unlike recent high-profile vulnerabilities like Heartbleed, Shellshock, Gotofail, and POODLE, this is a design problem not an implementation problem," JAS said in an advisory on its website. To fix it, Microsoft had to re-engineer core components of the operating system and add several new features, JAS said.Microsoft addressed the remote code execution flaw with the MS15-011 security bulletin, but also fixed a related Group Policy security bypass issue in MS15-014.Microsoft security engineers explained in a blog post that attackers would be most likely to exploit these vulnerabilities by using techniques like ARP spoofing on a local network in order to trick computers to accept and apply bad Group Policy configuration data from servers under their control.Successful exploitation could allow attackers to install programs, change data, or create new accounts on the vulnerable systems.While this on-LAN attack vector is certainly of concern because workers often connect their company-issued computers to untrusted networks like those in coffee shops and hotels, it is not the only attack scenario, JAS warned.JAS also identified and reported a completely over-the-Internet exploitation scenario. "There are a number of pre-requisites to get that to work -- it certainly doesn't work universally and it depends on some funky misconfigurations and happenstance. But it works frequently enough to be of concern," the company said.The company plans to release more technical details about the Internet-based attack method at a later date.In order to prevent attackers from successfully spoofing the domain controller when a targeted computer attempts to obtain Group Policy configuration data, Microsoft added a new feature called UNC (Universal Naming Convention) Hardened Access with the MS15-011 update.When enabled, this feature requires that both the client and server authenticate each other before the client accesses UNC resources like Group Policy data on the server. There are also settings for encrypting or checking the integrity of such connections. Microsoft provides instructions on how to use the feature in a knowledge base article.While the issue affects all supported versions of Windows, Microsoft decided not to release a patch for Windows Server 2003, which is scheduled to reach end of extended support on July 14.According to Microsoft, it wasn't feasible to build the fix for Windows Server 2003 because it would have required re-architecting not just the Group Policy component, but also other significant parts of the operating system. Such big architectural changes could have created incompatibility issues with applications that were designed to run on Windows Server 2003, the company said.While the explanation sounds reasonable, it's likely to be of little comfort to companies who were still expecting to receive Windows Server 2003 security patches until July -- and even beyond in the case of those who plan to pay for custom support.There are still millions of systems running Windows Server 2003 worldwide and analysts predict that migrating them to newer OS versions will be difficult, because an entire business software ecosystem has been built around the aging OS.