A spoofing vulnerability exists in the way Windows CryptoAPI (Crypt32.dll) validates Elliptic Curve Cryptography (ECC) certificates.
An attacker could exploit the vulnerability by using a spoofed code-signing certificate to sign a malicious executable, making it appear the file was from a trusted, legitimate source. The user would have no way of knowing the file was malicious, because the digital signature would appear to be from a trusted provider.
A successful exploit could also allow the attacker to conduct man-in-the-middle attacks and decrypt confidential information on user connections to the affected software.
The security update addresses the vulnerability by ensuring that Windows CryptoAPI completely validates ECC certificates.
On Tuesday the 14th of January 2020, in the frame of their first Patch Tuesday of 2020, Microsoft addressed a critical flaw discovered by the NSA in the Windows 10, Windows Server 2016 and 2019 versions of crypt32.dll, the library implementing Windows’ CryptoAPI. It didn’t take too long until it got branded “ChainOfFools” by Kenn White in a blog post. (And was then later rebranded “CurveBall” by Tal Be’ery.)
TL;DR: test if you are vulnerable using our test website!