Microsoft had relied on developers having to sign their app packages with "a third party paid certificate from a trusted certification authority," but evidently

Microsoft had relied on developers having to sign their app packages with "a third party paid certificate from a trusted certification authority," but evidently it put too much trust in such authorities. Following its decision to disable ms-appinstaller by default last week (in App Installer version 1.21.3421.0 or higher), Redmond announced it is working with certificate authorities "to revoke the abused code signing certificates utilized by malware samples we have identified."