If you’re currently using a Windows system, Microsoft highly recommends you immediately install the patches they released late Tuesday, November 11th.

The tech firm has released 14 security updates to fix 33 critical vulnerabilities in almost all versions of Windows as well as Office and Internet Explorer. The most dangerous flaw of them all is the so-called Unicorn bug (MS14-066), which affects all supported versions of the operating system.

IBM’s cybersecurity researchers first discovered the flaw in May and have since worked with Microsoft to patch the security hole before it goes public. The experts found that the bug is present in the code for the older Windows 95 versions and has been there for 19 years.

“The buggy code is at least 19 years old and has been remotely exploitable for the past 18 years,” said Robert Freeman, IBM X-Force Research manager, in a blog post. “We’ve reported this issue with a working proof-of-concept exploit back in May 2014, and today, Microsoft is patching it.”

“Looking at the original release code of Windows 95, the problem is present,” Freeman added.

The Unicorn bug could allow remote code execution, which means hackers can remotely take control over any Windows device if they send “specially crafted packets to a Windows server.”

IBM says the vulnerability could be exploited since the Internet Explore (IE) version 3.0.

“This complex vulnerability is a rare, ‘unicorn-like’ bug found in code that IE relies on but doesn’t necessarily belong to it.”

“The bug can be used by an attacker for drive-by attacks to reliably run code remotely and take over the user’s machine – even sidestepping the Enhanced Protected Mode (EPM) sandbox in IE 11 as well as the highly regarded Enhanced Mitigation Experience Toolkit (EMET) anti-exploitation tool Microsoft offers for free.”

Freeman said the may be multiple techniques for exploiting the Unicorn bug that could lead to possible remote code execution.

“Typically, attackers use remote code execution to install malware, which may have any number of malicious actions, such as keylogging, screen-grabbing and remote access.”

The flaw will allegedly fetch more than six figures if it had been sold to cyber criminals. Considering that a lot of computers around the world run on Windows, this could lead to major casualties.