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Is it possible to prevent an email from being forwarded, or to have sensitive information automatically removed when reply-all/forward is pressed?

For example, a Google Calendar invitation includes a link to directly accept/reject the event. However, if someone clicks reply-all, and they keep the HTML formatting, the link will be shared with everyone else who received the invitation, and they can change the user's RSVP. (Specifically, the email is sent only to you, but has a "To:" header showing all invitees, so the problem remains.)

What can be done to ensure that the recipient does not accidentally share the sensitive link? Would it be possible with some MIME trickery or with a semi-standard header? Or is the best practice to refrain from including such "direct action" links when someone might be tempted to reply-all?

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migrated from Sep 23 '10 at 15:17

This question came from our site for professional and enthusiast programmers.

up vote 17 down vote accepted

It's not possible to prevent forwarding. If you have sensitive content, it should be locked behind credentials. Force the user to login to view the content.

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please see my reply regarding the credentials topic. – lajuette Sep 23 '10 at 15:38

You can't prevent an e-mail from being forwarded. The receiver will have a copy of the data the email consists of. He can do with it whatever he/she wants.

Securing the data with credentials is not really effective. If you share the credentials with the receiver along with the sensitive data, who says he/she wont share or forward the credentials/key/etc.?

If you can't trust him to not forward the data, you can't trust him with the credentials either.

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And data can be copied and sent out too. Even if copy/paste is disabled, a print screen can capture the data, and then it is out of your control. – thursdaysgeek Sep 23 '10 at 15:56
that's right. That's why the multimedia industry tried to protect videos with HDCP. Every device needs a key to decrypt the video stream. Now the key has been disclosed, the system is useless. – lajuette Sep 23 '10 at 19:18
At the end of the day, everybody can take a digital picture of their screen and forward that. DRM and security in general is much like physical security - it's all about raising the bar. You can't stop someone from doing it, but you can make it enough of a hassle that nobody can claim to have done it by accident. – Mark Allen Sep 23 '10 at 21:06

If you are in control of a Corporate network, and can control the email clients receiving the emails, then you might want to consider Information Rights Management. You'd have to have all the email clients running Outlook (2003 or above), but it does seem to prevent data being forwarded, copied, printed etc.

I'd only use it as a warning feature though (i.e. to stop someone accidentally forwarding emails). It's not going to be hacker proof.

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A screenshot will defeat IRM if you have an untrustworthy trusted user. – Broam Sep 23 '10 at 19:13
Agreed - where there's a will there's a way. IRM is only any good for users who don't know any better, or can't be bothered to find a workaround. – misterjaytee Sep 23 '10 at 20:48

No such thing exists. You can't stop people from forwarding emails; it's just data.

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Of course nothing will stop someone determined. My meaning was to supply a hint for what the client should do by default, if such a thing exists. – Josh Lee Sep 23 '10 at 15:22

Use something like BigString

  • Self-Destructing Email
  • Recallable/Erasable Email
  • Non-Forwardable Email
  • Non-Printable/Savable Email Advanced
  • Email Tracking

Its free :D Big String

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This is just really DRM on email. Once the user has the content (screenshot if copy/paste "disabled)'s easily forwarded – Broam Sep 23 '10 at 19:12
possible spam. Also, doesn't work for RFC-compliant mail – knitti Sep 23 '10 at 21:12
@knitti: I really have no intention of spamming because I've got nothing to do with BigString. It's just free so use it or leave it. @Broam: I understand. It just makes it a lil bit hard... – Ranhiru Cooray Sep 27 '10 at 18:00

Lotus Notes allows senders to place a forwarding block on email. Its simply a feature for honest recipients so they won't accidently forward sensitive information. Just like locks on doors only keep honest people out. Blocks and locks makes violations very intentional rather than an oversight.

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In Lotus notes you can add the header

Sensitivity: Private\r\n

This only works in notes, but it seems to work pretty well.

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