The update_manager_file_get() function checks the argument url against a small hard-coded list of remote file schemes, and assumes that all other URIs may be resolved to paths on the local filesystem. This assumption is wrong and unnecessary. The code may be made simpler and more robust by simply passing the URI to \Drupal::service('file_system')->realpath() A return value of FALSE indicates that the scheme does not support translation to a local filesystem path.
The code should be modified as described above.
Review patch
Commit
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Needs work
9.5
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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