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Just days after the monthly Patch Tuesday Windows security update, unpatched system file zero-day vulnerabilities have been publicly disclosed. Every month, Microsoft fixes a bunch of security vulnerabilities across the product range on Patch Tuesday. The latest round of fixes has already been and gone, addressing a total of 111 security vulnerabilities. Some sixteen of these were rated as critical, and, crucially, there were no zero-days. A zero-day vulnerability is one that remains unpatched by the vendor, leaving a window of opportunity for those who would exploit it using a zero-day attack. That's good news. The bad news is that no less than four new zero-days affecting Microsoft Windows have now been publicly disclosed. Three of them impact a core Windows system file. Trend Micro's Zero Day Initiative (ZDI) is a bug bounty program founded in 2005 which encourages the reporting of zero-day vulnerabilities by financially rewarding security researchers. "We make every effort to work with vendors to ensure they understand the technical details and severity of a reported security flaw, which leaves researchers free to go find other bugs," the about ZDI page states. It also says that no technical details about any vulnerability are made public until the vendor has released a patch. ZDI gives vendors a 120-day window in which to address the vulnerability, after which a "limited advisory," which includes mitigation advice, is published if a patch has not been forthcoming. The Microsoft Windows zero-days that were publicly disclosed in such a fashion on May 19 mostly impact a core Windows system file called splwow64.exe, which is a printer driver host for 32-bit apps. The Spooler Windows OS (Windows 64-bit) executable enables 32-bit applications to be compatible with a 64-bit Windows system. CVE-2020-0915, CVE-2020-0916, and CVE-2020-0986 all impact that splwow64 Windows system file. All three are classified as high on the CVE severity scoring system with a 7.0 rating. If exploited by an attacker, these vulnerabilities would allow them to escalate privileges on the targeted Windows computer. "The specific flaw exists within the user-mode printer driver host process splwow64.exe. The issue results from the lack of proper validation of a user-supplied value prior to dereferencing it as a pointer," the ZDI advisory states, "An attacker can leverage this vulnerability to escalate privileges from low integrity and execute code in the context of the current user at medium integrity."Learn more about this zero-day vulnerability by visiting OUR FORUM.