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I use OpenVPN to connect to a server. I just found that in a series of emails I sent and received to and from the same person the following line appeared after every message I sent:

[cid:{a long hexadecimal number}@openvpn]

How come this is in the email? Should I be concerned about privacy?

  • Where does it appear? cid: in MIME is Content-ID not "Client Id". – techraf Jun 3 '16 at 14:18
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How come this is in the email?

OpenVPN does not care about application layer, which means that it did not introduce such content to the email. This means that this content is either introduced by the sender or by the mail server.

While you offer only a few information about the mail itself let me just guess:

  • there is a hostname openvpn.
  • there is some attachment (image or whatever) in the mail.
  • this attachment/image is included in the mail like this (raw mail/MIME):

    --some-boundary
    Content-type: image/png (or something else)
    Content-ID: {a long hexadecimal number}@openvpn
    Content-Transfer-Encoding: base64
    ...
    

And then this Content-Id is referenced somewhere in the mail with something like

 <img src="cid:{a long hexadecimal number}@openvpn" ..>

If this is the case then you simply deal with normal HTML mails with inline images or similar, see RFC 2392. And the long hexidecimal number is just a unique id for this attachment.

Should I be concerned about privacy?

Probably not about this. But if you look at the source code of the email and especially at the Received-Headers you might find other privacy relevant information to worry about.

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