Skip to main content

Of Auto-Authenticating URLs, Shortlinks, and Danger

The road to hell is paved with good intentions, they say.

So, it was a good intention when someone (not me) decided to install a link shortener and send password reset links through it, before producing the printed newsletters that would be sent out to individual members. They would need to type the URLs in by hand, so a shorter URL was a good idea. At least, it must have seemed like it, at the time.

Today I took a look at this system, which I was asked to clean up before it gets used again after several months of being ignored. I admit it didn't click in my mind immediately, but after producing some newsletter content in our staging system and verifying the shortlinks were being recorded properly, it suddenly jumped out of the screen and bit me on the nose:

The shortener was producing sequential links for a bunch of password reset links.

What this meant in practical terms is that two newsletters sent out with password reset links for two different users would send them URLs like http://foo.com/z1 and http://foo.com/z2 and while these were very short for the user to type in they were not easy to type in, if you require "correctly" as part of the measurement of how easy it is. They are so short and the space so condensed that mistyping won't get you a 404, but someone else's entirely valid password reset link. This is terrible. There is a reason password resets give you links with long randomized sequences of characters, and all of those reasons were being thrown out the window.

Turns out, a shortened URL can be too short.

Comments

cool-RR said…
Nice observation!
ashwoods said…
Not only is a short url too easy too mistype, but it is also very easy to hack. Say you only use a link with 6 random characters, upper-lower case letters and numbers. Than it would take a botnet to test all combinations, with only 5000 requests per second:

((((26 + 26 + 10)^6) / 5 000) / 3 600) / 24 = 131.482027

131 days to go through all combinations. So if you have maybe 50 users at a given time resetting passwords, you'll be finding links every 2.6 days on average.
Anonymous said…
In the old days, we used check digits to catch the vast number of common typos. (The block mode terminals could validate the check digit without round tripping to the mainframe.)

While not addressing the predictability problem, it would address the "oops" problem.

Popular posts from this blog

CARDIAC: The Cardboard Computer

I am just so excited about this. CARDIAC. The Cardboard Computer. How cool is that? This piece of history is amazing and better than that: it is extremely accessible. This fantastic design was built in 1969 by David Hagelbarger at Bell Labs to explain what computers were to those who would otherwise have no exposure to them. Miraculously, the CARDIAC (CARDboard Interactive Aid to Computation) was able to actually function as a slow and rudimentary computer.  One of the most fascinating aspects of this gem is that at the time of its publication the scope it was able to demonstrate was actually useful in explaining what a computer was. Could you imagine trying to explain computers today with anything close to the CARDIAC? It had 100 memory locations and only ten instructions. The memory held signed 3-digit numbers (-999 through 999) and instructions could be encoded such that the first digit was the instruction and the second two digits were the address of memory to operate on

The Range of Content on Planet Python

I've gotten a number of requests lately to contribute only Python related material to the Planet Python feeds and to be honest these requests have both surprised and insulted me, but they've continued. I am pretty sure they've come from a very small number of people, but they have become consistent. This is probably because of my current habit of writing about NaNoWriMo every day and those who aren't interested not looking forward to having the rest of the month reading about my novel. Planet Python will be getting a feed of only relevant posts in the future, but I'm going to be honest: I am kind of upset about it. I don't care if anyone thinks it is unreasonable of me to be upset about it, because the truth is Planet Python means something to me. It was probably the first thing I did that I considered "being part of the community" when I submitted my meager RSS feed to be added some seven years ago. My blog and my name on the list of authors at Plan

Interrupting Coders Isn’t So Bad

Here’s a hot take: disrupting coders isn’t all that bad. Some disruptions are certainly bad but they usually aren’t. The coder community has overblown the impact. A disruption can be a good thing. How harmful disruption might be a symptom of other problems. There are different kinds of disruptions. They are caused by other coders on your team, managers and other non-coders, or meetings throughout the day. The easiest example to debunk is a question from a fellow developer. Imagine someone walks over to your desk or they ping you on Slack, because they have “one quick question.” Do you get annoyed at the interruption when you were in the middle of something important? You help out your teammate quickly and get back to work, trying to pick up where you left off. That’s a kind of interruption we complain about frequently, but I’m not convinced this is all that bad. You are being disrupted but your team, of which you are only one member of the whole unit, is working smoothly. You u