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Anonymous Inbox

This allows employees to anonymously send emails to their bosses. In essence, this is a very basic web mailer (send only).

How to protect from Spam?

The URI and the basic auth credentials are well-known inside the company, but not published outside. Occasionally change the credentials.

How does it work?

You type in an email. It is never stored anywhere, but immediately sent to an SMTP server. You can see that while it happens.

Why is it anonymous?

  • Everybody uses the same basic auth credentials.

  • No cookies, sessions, external resources.

  • The application never logs the IP of the incoming mail submission (If you don't trust that, use a coffee shop WiFi, some friend's computer, an internet cafe, or Tor).

  • To guard against spoofing by admins or man-in-the-middle, we do not log the text submitted, and also that text is encrypted towards the recipients' S/MIME keys. For the same reason, the subject-line is fixed (as that's S/MIME unencrypted in S/MIME-mails), rather than set-able by the user.

  • However, we don't do HTTPS ourselves (maybe some time we will). We leave that to a reverse proxy in front of us. So when transiting the last yard of network, your data is unencrypted.

Prerequisites (installation)



With Docker in place, run something like

docker build -t registry.invalid/anonymous_inbox .

or, if you are like me and use a local http cache to speed up repeated Docker builds:

docker build --build-arg=http_proxy=  -t registry.invalid/anonymous_inbox .

You may need to replace the with the Docker host's IP used on your machine.

Environment variables for configuration

ano_inbox.title The title to be displayed. Default: "Sending anonymous email."

ano_inbox.from_addr What you want to set as the sender address of the mails. Must be ASCII, or sending will fail.

ano_inbox.subject The subject line that'll be seen by the recipient(s), default "Incoming anonymous email.". Must be ASCII, or sending will fail.

ano_inbox.user ano_inbox.passwd The crendentials required (basic auth, not needed for /health).

ano_inbox.smtp_host The SMTP host we'll use. Make sure it listens on port 25 and supports starttls.

ano_inbox.key0, ano_inbox.key1, ano_inbox.key2, ano_inbox.key3, ... (at least one): The X.509 certificates of the people you want to receive the emails. Their recipient addresses will be extracted automatically.

We need PEM format, including line breaks and all. If you have problems getting line breaks into the environment variables, here are some ways out:

  • If there is no initial -----BEGIN CERTIFICATE----- line in such a variable's value, but it starts with https:// (or http://, but, generally speaking, don't do that), the program will assume a URI, will issue a GET request to retrieve the value behind that URI, and, if that succeeds, assume what it now has is the key in PEM format (again with line breaks and all). Caveat: The keys are retrieved each time a new worker is started. So you should keep that key server up and running beyond the start of this service. (You may prefer the next option, and use this one only as a means of last resort.)

  • If the value starts with neither -----BEGIN CERTIFICATE-----, nor https://, nor http://, the program will attempt to base64-decode the value and assume the result is the key in PEM format (again with line breaks and all). So this is the recommendation what to use, if you can't put the bare certificate with line breaks into the environment variable.

For base64 encoding, you can use the output of the base64 tool. This tool is contained in the Docker image you've built. Run something like

docker run --rm -i registry.invalid/anonymous_inbox base64 -w 0 < my-public-cert.pem; echo

and copy the output into the environment variable. (If you want to run base64 locally on a Mac instead, replace -w 0 with -b 0.)


Something like

docker run --env=... --publish 80:14505 registry.invalid/anonymous_inbox

(or choose another port instead of the 80 if you already run an HTTP server on that box).

Point your browser to that server and check the root page shows all the email addresses, one for each key you provided.


A simple web application that allows a closed user group to send anonymous email to someone responsible.







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