We recently made a change to how provisioners upload files in order to
address an unintended remote code execution vector when using SSH, which
revealed that we had not previously documented well enough the expected
contract for how provisioners upload files to remote systems, and so some
users were depending on unintended consequences of the the bug now fixed.
We are retaining the fix on security-related grounds, but this is a good
prompt to be clearer in the docs about what exactly Terraform is doing
when asked to upload files over SSH and WinRM, so users can understand
what is supported and write their configurations accordingly.
This also includes an additional section to the v1.1 upgrade guide, since
we apparently neglected to document this intentional breaking change in
the first draft of that page.
Of course, provisioners as a whole remain a last resort, and so we're
documenting this as hopefully a helpful aid to those who have no other
option, and not meaning in any way to recommend their use for any new
use-cases.
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