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.
Labels:

Comments

cool-RR said…
Nice observation!
4:30 AM
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.
4:57 AM
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.
11:05 PM
Post a Comment

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 operat...
1 comment
Read more

Statement Functions

At a small suggestion in #python, I wrote up a simple module that allows the use of many python statements in places requiring statements. This post serves as the announcement and documentation. You can find the release here . The pattern is the statement's keyword appended with a single underscore, so the first, of course, is print_. The example writes 'some+text' to an IOString for a URL query string. This mostly follows what it seems the print function will be in py3k. print_("some", "text", outfile=query_iostring, sep="+", end="") An obvious second choice was to wrap if statements. They take a condition value, and expect a truth value or callback an an optional else value or callback. Values and callbacks are named if_true, cb_true, if_false, and cb_false. if_(raw_input("Continue?")=="Y", cb_true=play_game, cb_false=quit) Of course, often your else might be an error case, so raising an exception could be useful...
1 comment
Read more

How To Teach Software Development

How To Teach Software Development Introduction Developers Quality Control Motivation Execution Businesses Students Schools Education is broken. Education about software development is even more broken. It is a sad observation of the industry from my eyes. I come to see good developers from what should be great educations as survivors, more than anything. Do they get a headstart from their education or do they overcome it? This is the first part in a series on software education. I want to open a discussion here. Please comment if you have thoughts. Blog about it, yourself. Write about how you disagree with me. Write more if you don't. We have a troubled industry. We care enough to do something about it. We hark on the bad developers the way people used to point at freak shows, but we only hurt ourselves but not improving the situation. We have to deal with their bad code. We are the twenty percent and we can't talk to the eighty percent, by definition, so we need to impro...
4 comments
Read more