# Fixing rewritten links in plaintext email

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Get urldec.py

Over the last couple of years, an increasing number of universities in the US123 have introduced a link rewriting system to their email services, the commonly used ones being URL Defense and Safelinks from Outlook (URLD and SL). These tools are hailed as the solution to scam and phishing emails, even though there apparently is no data4 to support such claims. Instead, it is implied that the university is better off paying an external company than educating its own employees. That, in spite of being the ultimate education tool. As a finishing touch of mild absurdity, link rewriting is often not applied to emails originating from another university address. What better way to encourage sharing kitten websites with colleagues.

One consequence of the above5 is that some university users will be comfortable embedding in their professional correspondence links redirecting to third-party trackers, which — however not malicious they might be — still have no business snooping on anyone. Alas — all this because a link is deemed not a transparent address, but a cryptic object, to be understood only by the “experts”.

Regardless of the efficacy of link rewriting, one thing is certain: it makes a mess out of plaintext email. And if that’s the kind of email you like reading — good old text in the terminal — you will find that a harmless homepage in your colleague’s signature will explode to several insanity-inducing lines in mutt (or your favorite plaintext MUA) after being rewritten first by URLD, and then SL.

Enter display_filter. From neomutt’s manual:

display_filter
Type: command
Default: (empty)

When set, specifies a command used to filter messages. When a message is viewed
it is passed as standard input to \$display_filter, and the filtered message is


That is, we should be able to pipe our emails through an external utility that will filter out rewritten links and fix them.

Fortunately, Proofpoint (the company behind URLD) have kindly provided a Python script to decode their links. The version of the script on their website contains a bug which causes it to break on URLs with non-Latin characters, such as this one:

https://uk.wikipedia.org/wiki/Остроградський_Михайло_Васильович

After fixing this bug and adding the ability to rewrite Safelinks URLs, we have a utility that restores the jumbled mess of a rewritten link to its original glory. To fix links, we pipe them through the script and collect stdout.

For incoming mail, we store urldec.py somewhere, make it executable, and then add

set display_filter="~/script/folder/urldec.py"


to muttrc. This still will not fix the outgoing emails, as display_filter is used only in the pager, and not for response. To rectify that, we can use vim’s ability to pipe buffers through external commands (naturally we use vim to edit our plaintext tirades), and add

set editor = "vim -c '%!~/script/folder/urldec.py'"


to muttrc. This way, vim will pipe the message through urldec.py every time it is invoked from mutt.

As an added bonus, we now get mutt to display links properly in any plaintext-only emails we may receive, while the web version of our email service (Outlook?) will show rewritten gibberish.

Conclusion:

• Running the script every time an email is opened is still very fast. In fact, rendering html emails in w3m is likely the slowest step in opening them.
• Even though using mutt to read email in the terminal is the sign of an invincible hacker, it is desirable not to follow obscure links, plaintext or otherwise.

1. There seem to be some studies on other anti-phishing techniques.

2. From personal experience.