Handling an unexpected Base64 string safely

Base64 is an encoding, not a trust signal. If an unknown sender gives you an encoded string, first ask what it is supposed to contain and why it needs decoding.

If you choose to inspect it, use a tool that treats the result as inert text; do not execute commands or open attachments derived from it. Be careful about pasting secrets into third-party sites. Invalid characters or unreadable output may mean the input is not Base64 text at all.

What kind of data do you expect after decoding?