Microsoft's system patches for January 2020 have a doozy hiding in there, one that makes patching an affected Windows system mandatory.
In an unprecedented move, the National Security Agency (NSA) advised Microsoft about a bug (CVE-2020-0601) in one of the CryptoAPI libraries used since NT 4.0 days. This is the first time the NSA got reporting credit about a bug from Microsoft, and shows that the NSA may be changing how it does things.
MSFT says the problem affects Windows 10, Windows Server 2019, and Windows Server 2016 OS versions. Windows 8.1 and prior versions, as well as the Server 2012 R2 and their prior counterparts, do not support ECC keys with parameters and would not be directly affected by this vulnerability.
Microsoft described the vulnerability as follows: "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."
This means the bug could directly bypass one of the major methods of authentication used on the Internet, and allow a malicious file to have unimpeded access to any area they wished.
Carnegie-Mellon's Software Engineering Institute noted that, "Any software, including third-party non-Microsoft software, that relies on the Windows CertGetCertificateChain() function to determine if an X.509 certificate can be traced to a trusted root CA may incorrectly determine the trustworthiness of a certificate chain."
The US government is taking this very seriously. The Department of Homeland Security issued an Emergency Directive requiring the entire government to patch within ten days. They say that, "Aside from removing affected endpoints from the network, applying this patch is the only known technical mitigation to these vulnerabilities."
The NSA shows it is taking this seriously as well. In an advisory, they say that "NSA assesses the vulnerability to be severe and that sophisticated cyber actors will understand the underlying flaw very quickly and, if exploited, would render the platforms as fundamentally vulnerable.The consequences of not patching the vulnerability are severe and widespread. Remote exploitation tools will likely be made quickly and widely available. Rapid adoption of the patch is the only known mitigation at this time and should be the primary focus for all network owners."
Ed Bishop, CTO and co-founder of email security company Tessian, is also concerned on how the flaw could be used for phishing. He observes that "Adversaries may send emails attaching a program that Windows shows as 'legitimate,' but is actually malicious. Attackers could also send users a link to a website where the malicious file sits. To protect their people from falling for these scams, businesses need to make employees aware of the threat and urge them to question the legitimacy of emails offering security patches for the vulnerability."
This is pretty much a no-brainer. If you use an affected version of Windows, patch it. Period.
— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.