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Today I tried to start taskmgr but it said the verified publisher is unknown. I checked the taskmgr's signature and detected that the Microsoft Windows certificate has expired and the file does not have a timestamp. mmc.exe is same. I started it but it gave me an error that an admin has blocked it. I checked its digital signature and saw the same error. enter image description here

PS: No timestamp????

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The "Microsoft Development PCA" certificates are used for 'beta' versions and Insider Preview builds. The signatures aren't timestamped/countersigned because the entire build is usually marked to expire after a few months (e.g. winver says that build 18932 on my system expires on 2020-08), so there is no real need for signatures to outlast the intermediate CA.

Check the executable's version under "Properties" → "Details". Compare it to other (working) Windows components, and to the version shown in winver (or if that doesn't work, Explorer's "File" → "Help" → "About Windows", though it won't show all expiry information).

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