As of the end of July 2018, the Let’s Encrypt root, ISRG Root X1, is directly trusted by Microsoft products. Our root is now trusted by all major root programs, including Microsoft, Google, Apple, Mozilla, Oracle, and Blackberry.
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While Let’s Encrypt is now directly trusted by almost all newer versions of operating systems, browsers, and devices, there are still many older versions in the world that do not directly trust Let’s Encrypt. Some of those older systems will eventually be updated to trust Let’s Encrypt directly. Some will not, and we’ll need to wait for the vast majority of those to cycle out of the Web ecosystem. We expect this will take at least five more years, so we plan to use a cross signature until then.