The day that Mitchell Hurwitz starts making shallow humor is the day that we rip the mask off of Mitchell Hurwitz and find the real Mitchell Hurwitz bound in his own cellar.
Throughout the first three seasons of Arrested Development, we were treated to some ridiculous attention to details, hidden clues, foreshadowing, Easter eggs, and in-jokes. For instance, ever wonder what
was with the obsession with seals in the first few seasons? They were actually a metaphor for Lucille Bluth, the matriarch of the family, who kept poor baby Buster Bluth, a
juice-loving man child, terrified of ever leaving the nest (or entering the ocean). As an act of defiance, Buster enters the ocean and gets his hand bitten off by a “loose seal.”
Loose. Seal. “Lucille.” Do I need to add a “GET IT?!” there?
But that’s not even the crazy part. As we’ve talked about before, the hand being bitten off was foreshadowed throughout the entire season. In one episode in season three,
the U.S. Securities and Exchange Commission attempts to persuade Tobias Funke, a struggling actor, into
cooperating with them and becoming a mole in the Bluth family. He misunderstands the entire situation and inadvertently becomes a mole (in all senses of the word — by which I mean he
fucking dresses like a mole.) But the real mole in the family was right under our noses all along since season one …
Look at his shirt.
However, all of this is just a teaser for the grandest mystery in Arrested Development: that of “Nichael Bluth.”
I recently discovered a minor security vulnerability in mobile webcomic reading app Comic Chameleon, and I thought that it was interesting
(and tame) enough to share as a learning example of (a) how to find security vulnerabilities in an app like this, and (b) more importantly, how to write an app like this
without this kind of security vulnerability.
The nature of the vulnerability is that, for webcomics pushed directly into the platform by their authors, it’s possible to read comics (long) before they’re published. By way
of proof, here’s a copy of the top-right 200 × 120 pixels of episode 54 of the (excellent) Forward
Comic, which Imgur will confirm was uploaded on 2 July 2018: over three months ahead of its planned publication date.
I’m not going to spoil this comic for you, but if you follow it then when October comes I think you’ll be pleased.
How to hack a web-backed app
Just to be clear, I didn’t set out to hack this app, but once I stumbled upon the vulnerability I wanted to make sure that I was able to collect enough information that I’d be
able to explain to its author what was wrong and how to fix it. You’d be amazed how many systems I find security holes in almost-completely by accident. In fact, I’d just noticed that
the application supported some webcomics that I follow but for which I hadn’t been able to find RSS feeds (and so I was selfdogfooding my own tool, RSSey, to “produce” RSS feeds for my reader by screen-scraping: not the most-elegant solution). But if this app could produce a list of issues of the comic, it
must have some way of doing what I was trying to do, and I wanted to know what it was.
Comic Chameleon brings a lot of comics into a single slick Android/iOS app. Some of them you’ll even have heard of!
The app, I figured, must “phone home” to some website – probably the app’s official website itself – to get the list of comics that it supports and details of where to get their feeds
from, so I grabbed a copy of the app and started investigating. Because I figured I was probably looking for a URL, the first thing I
did was to download the raw APK file (your favourite search engine can tell you how to do this), decompressed it (APK files are just ZIP files, really) and ran strings on it to search for
likely-looking URLs:
As predicted, there are several hard-coded addresses. And all over unencrypted HTTP, eww!
I tried visiting a few of the addresses but many of them seemed to be API endpoints that were expecting additional parameters. Probably, I figured, the strings I’d extracted were
prefixes to which those parameters were attached. Rather than fuzz for the right parameters, I decided to watch what the app did: I spun up a simulated Android device using the official
emulator (I could have used my own on a wireless network that I control, of course, but this was lazier) and ran my favourite packet sniffer to
see what the application was requesting.
The web addresses are even clearer, here, and include all of the parameters I need.
Now I had full web addresses with parameters. Comparing the parameters that appeared when I clicked different comics revealed that each comic in the “full list” was assigned a numeric
ID which was used when requesting issues of that comic (along with an intermediate stage where the year of publication is requested).
Each comic is assigned an ID number, probably sequentially.
Interestingly, a number of comics were listed with the attribute s="no-show" and did not appear in the app: it looked like comics that weren’t yet being made available via
the app were already being indexed and collected by its web component, and for some reason were being exposed via the XML
API: presumably the developer had never considered that anybody but their app would look at the XML itself, but the thing about the Web is that if you put it on the Web, anybody can see it.
Still: at this point I assumed that I was about to find what I was looking for – some kind of machine-readable source (an RSS feed
or something like one) for a webcomic or two. But when I looked at the XML API for one of those webcomics I discovered quite a bit more than I’d bargained on finding:
Hey, what’s this? This feed includes titles for webcomics that haven’t been published yet, marked as ‘no-show’…
The first webcomic I looked at included the “official” web addresses and titles of each published comic… but also several not yet published ones. The unpublished ones were
marked with s="no-show" to indicate to the app that they weren’t to be shown, but I could now see them. The “official” web addresses didn’t work for me, as I’d expected,
but when I tried Comic Chameleon’s versions of the addresses, I found that I could see entire episodes of comics, up to three and a half months ahead of their expected
publication date.
Whoops.
Naturally, I compiled all of my findings into an email and contacted the app developer with all of the details they’d need to fix it – in hacker terms, I’m one of the “good guys”! – but
I wanted to share this particular example with you because (a) it’s not a very dangerous leak of data (a few webcomics a few weeks early and/or a way to evade a few ads isn’t
going to kill anybody) and (b) it’s very illustrative of the kinds of mistakes that app developers are making a lot, these days, and it’s important to understand why
so that you’re not among them. On to that in a moment.
Responsible disclosure
Because (I’d like to think) I’m one of the “good guys” in the security world, the first thing I did after the research above was to contact the author of the software. They didn’t seem
to have a security.txt file, a disclosure policy, nor a profile on any of the major disclosure management sites, so I sent an email. Were the
security issue more-severe, I’d have sent a preliminary email suggesting (and agreeing on a mechanism for) encrypted email, but given the low impact of this particular issue, I just
explained the entire issue in the initial email: basically what you’ve read above, plus some tips on fixing the issue and an offer to help out.
This is what stock photo sites think “hacking” is. Well… this, pages full of green code, or hoodies.
I subscribe to the doctrine of responsible disclosure, which – in the event of more-significant vulnerabilities –
means that after first contacting the developer of an insecure system and giving them time to fix it, it’s acceptable (in fact: in significant cases, it’s
socially-responsible) to publish the details of the vulnerability. In this case, though, I think the whole experience makes an interesting learning example about ways in which
you might begin to “black box” test an app for data leaks like this and – below – how to think about software development
in a way that limits the risk of such vulnerabilities appearing in the first place.
The author of this software hasn’t given any answer to any of the emails I’ve sent over the last couple of weeks, so I’m assuming that they just plan to leave this particular leak in
place. I reached out and contacted the author of Forward Comic, though, which turns out (coincidentally) to be probably the most-severely affected publication on the platform, so that
he had the option of taking action before I published this blog post.
Lessons to learn
When developing an “app” (whether for the web or a desktop or mobile platform) that connects to an Internet service to collect data, here are the important things you really, really
ought to do:
Don’t publish any data that you don’t want the user to see.
If the data isn’t for everybody, remember to authenticate the user.
And for heaven’s sake use SSL, it’s not the 1990s any more.
It’s a good job that nobody on the Web would ever try to view something easily-available but which they shouldn’t, right? That’s why screens like this have always worked so well.
That first lesson’s the big one of course: if you don’t want something to be on the public Internet, don’t put it on the public Internet! The feeds I found simply
shouldn’t have contained the “secret” information that they did, and the unpublished comics shouldn’t have been online at real web addresses. But aside from (or in addition to)
not including these unpublished items in the data feeds, what else might our app developer have considered?
Encryption. There’s no excuse for not using HTTPS these days. This alone wouldn’t have prevented a deliberate effort
to read the secret data, but it would help prevent it from happening accidentally (which is a risk right now), e.g. on a proxy server or while debugging something else on the same
network link. It also protects the user from exposing their browsing habits (do you want everybody at that coffee shop to know what weird comics you read?) and from having
content ‘injected’ (do you want the person at the next table of the coffee shop to be able to choose what you see when you ask for a comic?
Authentication (app). The app could work harder to prove that it’s genuinely the app when it contacts the website. No mechanism for doing this can ever be perfect,
because the user hasa access to the app and can theoretically reverse-engineer it to fish the entire authentication strategy out of it, but some approaches are better than others.
Sending a password (e.g. over Basic Authentication) is barely better than just using a complex web address, but
using a client-side certiciate or an OTP algorithm would (in conjunction
with encryption) foil many attackers.
Authentication (user). It’s a very-different model to the one currently used by the app, but requiring users to “sign up” to the service would reduce the risks and
provide better mechanisms for tracking/blocking misusers, though the relative anonymity of the Internet doesn’t give this much strength and introduces various additional burdens both
technical and legal upon the developer.
Fundamentally, of course, there’s nothing that an app developer can do to perfectly protect the data that is published to that app, because the app runs on a device that the user
controls! That’s why the first lesson is the most important: if it shouldn’t be on the public Internet (yet), don’t put it on the public Internet.
Hopefully there’s a lesson for you somewhere too: about how to think about app security so that you don’t make a similar mistake, or about some of the ways in which you might test the
security of an application (for example, as part of an internal audit), or, if nothing else, that you should go and read Forward, because it’s
pretty cool.
You think when you chain a container to an abandoned concrete block that’s been “just standing there” fire years that it’ll be safe, don’t you? Turns out: not necessarily! Looks like a
super-heavy vehicle managed to push this one over, damaging the cache!
I’ve replaced the container but for the time being it’s missing a pencil. Hint isn’t quite accurate any more either but it’s slightly-more visible anyway. Going to see what I can do
about all of the above, soon, but for now at least this cache is back in functional health. Happy wolf-hunting!
