Hidden in plain sight: Brute-forcing Slack private files | Hacker News Reader

Hidden in plain sight: Brute-forcing Slack private files

ibuildings.nl

146 pointsrelaxnow51 comments

Summary from my read of this: (the article does a great job of couching the process of exploiting this, as well as motivating why these numbers are too low, but here are the vulnerabilities...)

- Slack chose to use a 6-hexadigit/24-bit "secret code" as the only/final code required to download "privately" shared files. That's way too short; people have botnets almost that big, such that even aggressive IP-based rate-limiting wouldn't stand a chance.

They might have also made these fairly common mistakes (which served to compound the vulnerability):

- Returning different/distinguishable error codes when the request matches correctly on some parts but not all. This allows attackers to guess each in turn.

- Considering values such as the "file ID" to provide additional security/entropy, when in fact these IDs are generated semi-sequentially, and thus a moderately-sophisticated attacker can narrow the search space dramatically.

- Considering values such as the "filename" to provide more security/entropy; however, you can make no guarantees about the length or uniqueness of filenames, so you shouldn't consider that a security feature at all.

Also important to note it took Slack over a year to fix this issue often with long stretches of silence even with requests from the bug hunter.

Interestingly, I put the same bug in at HackerOne 9 months ago. It was closed as not applicable. So they had at least two independent reports of the same bug and failed to understand it, acknowledge it and then fix it.

Way to go slack.

If you have any critical data passing through slack, when you get owned, you won't be able to say say it wasn't entirely preventable.

One thing I can add from my analysis is that there aren't seperate counters for files/teams/etc. there's only one. So if a given id is used by a team, it won't be used as a file id.

The correct answer for using URLs as capabilities (which is what a 'secret URL' really is: a capability to a resource, which can be handed out, copied &c.) is to use a 256-bit value as part of the URL. Thus, rather than 'http://example.invalid/TEAM-DOC-SHORT-RAND/' use 'http://example.invalid/w6uP8Tcg6K2QR905Rms8iXTlksL6OD1KOWBxT.... If you're really paranoid, double the length. I guarantee it won't be guessed, in either case.

Yes, exactly this. It's not rocket science; it's odd how much effort Slack put into implementing (and then reimplementing in a slightly less broken fashion) a clearly wrong solution.

Don't forget that it should ideally be cryptographically random. If the sequence is predictable (like based on an auto incrementing number or on time) then you might still be able to guess a 256-bit number.

> Don't forget that it should ideally be cryptographically random.

I thought that went without saying, nut yes, it must be cryptographically random (not ideally—it must be).

w3 has some guidelines on capability urls: http://www.w3.org/TR/capability-urls/

The guideline for unguessability in that TR just says to use a UUID. UUIDs are ugly, and quite long for the amount of randomness they contain. I prefer Slack's new 10*base36 solution.

UUIDs are often guessable, too, so that recommendation is just plain wrong (or at best, woefully incomplete).

Right, some kinds of UUIDs achieve their uniqueness precisely by leveraging predictability. The unpredictable kind are mostly just random bits formatted in a particular (not very efficient) way.

The random ones don't have to be unpredictable randomness, either. For example, you might seed a good RNG with the local MAC address and boot time and then use that to generate v4 UUIDs. As far as I can tell, this would be perfectly legal and should produce UUIDs that are as likely to be unique as any other, but they would also be easy for an attacker to predict.

Basically the whole "version" UUIDs is a stupid fever dream. Any sane implementation should return 16 random bytes and be done. The fact that there's a spec for this and it's longer than 2 sentences is just wrong.

I think they were just afraid of the birthday paradox. But I agree, just grabbing some random bytes really is the way to go.

Although, I'm a bit afraid of the birthday paradox, maybe we should use 20 bytes instead....

The idea that a UUID should have a "version" and the random one uses 122 bits instead of 128 bits sorta makes ya wonder how concerned they were.

Yeah, spot on. At least now in 2015 using 16 bytes for id's and hashes no longer seems like a lot of memory or storage.

It pains me to see people inventing weird id schemes for every app and api.

10 base-36 digits is only around 52 bits, compared to the 112 random bits in a UUIDv4. 2^52 might be enough for particular use-cases, but it's nowhere near a UUID in terms of randomness. Even 10 base-64 digits is only 60 bits.

> We apologize for the delayed reply. We track these issues via our internal bug system, and only reply to the reporter once the bug is resolved internally. We generally ignore messages asking for updates, as we receive a high volume of these (even for non-issues).

This rationalization is illogical, which usually means someone is in conflict. From a logical standpoint, externally, it could be they are fixing something OR don't know about it OR don't care.

Given the conflicting rationalization, I'd say they didn't know about it and then made up an excuse instead of owning it.

It probably means that they're not prioritising vulnerability reports. Which is their prerogative honestly, but it doesn't make researchers happy to work with you.

The biggest 'fault' here I think lies squarely with HackerOne. They should've enforced their own guidelines and given me the option to publish in their system after 180 days. But I still don't have that option.

Great feedback, thanks.

The 180 day guidance you reference falls under a "Last Resort" clause when "... the Response Team [is] unable or unwilling to provide a disclosure timeline". (which, at first glance, might not have been the case here?)

These "Last Resort" scenarios have not yet been fully codified. As a safety precaution, the workflow is still initiated manually with support as these scenarios are extremely rare and littered with edge cases. We've been learning a lot from studying disclosures like this one and you can expect to see the "Last Resort" workflow codified in the product in the future.

Now that the report has been Resolved, you should see the normal disclosure options available. Please always feel free to send me a note if you have any questions or feedback on our disclosure workflows - especially if we don't support your preferred route.

This reminds me of my experience with Imgur's private images.

A few years ago, I wrote a little js tool to browse random Imgur images by guessing their urls (i.imgur.com/<5-digit code>) until it found one that succeeded. It would add the found image to an infinite-scrolling page. It was kinda fun to browse, and a lot of people seemed to enjoy playing with it.

After a couple years, though, Imgur suddenly started blocking access to their images on my site. It turned out they were blocking based on the referrer header.

I emailed them asking what was up, and apparently they were attempting to ensure the privacy of public-url images by manually going after any tools like mine (if you google 'random imgur', you'll find dozens).

I didn't bother circumventing this, I didn't want to be a jerk just to prove a point. I did try to point out that there were a number of ways to get around something as simple as a referrer block, but I don't think the customer support person I was dealing with was really interested in discussing the issue and I let it drop.

I had a similar experience, though I was on the other side. Under brute force login attack IT guy suggested I change login HTTP method from GET to POST (which is more appropriate anyway). While I agreed with him that this is better, I pointed out that this is very easy to circumvent. However he proved me wrong - the attacks stopped after that (and I am quite sure it is not because they gained access). Not all attackers are very determined...

What kills me is that this seems like such an unforced error. Just make 256-bit random tokens. They're private URLs. Who cares how ugly they are?

Not to mention making public file sharing explicit instead of implicit, or at least giving the option to exclude specific files from a public URL.

Damn, over a year to fix this? It's good that they did in the end, but the timeline is just crazy.

Maybe someone needed time to brute force all those tasty files hanging around ;)

The best part of the story actually comes at the end, back-and-forth messaging with slack about the bug report

definitely, I'm disappointed with slacks responses. We did a trial and have had some correspondence with their support team which has been excellent to date. So I assumed they were above some of this silicon valley elitism. I'm glad to see this kind of public disclosure. We have been a customer since that initial trial, we stopped using hipchat.

To be fair, most of the bad correspondence was from 2014. Their new representative 'Leigh' appears to be doing excellent work. Also we're still happy users of Slack, I would just never trust them with secrets :-).

If I were responsible for security at Slack, the thought of potentially leaking uploaded files like this would keep me up at night. Slack has gained such wide adoption-think of the things that people are sharing with their coworkers all day, every day. Someone with ill intent could have found so many valuable things.

Simple how-to instructions:

  1. log into slack

  2. share a private file

  3. go to:

> https://api.slack.com/web

  4. generate API token

  5. copy the link to your private file, and paste as plain text.

  6. a file id is in the link, somewhere

  7. try out the file id by visiting links like:

> https://slack.com/api/files.info?token=#secret!&file=???????...?

  8. see also:

> https://api.slack.com/methods/files.info

  9. in the JSON output you will CTRL+F to see an address like:

> https://slack-files.com/#########-?????????-!!!!!!!!!

  10. slackbot warns you, and only you, once and only once 
      that someone found that link. this notification may
      get buried, or forgotten about. it is your only chance
      to revoke the public link.

  11. if you forget about that link, and it goes viral with 
      millions of visits, lots of luck gentlemen!

Do a google search for site:https://s3.amazonaws.com/uploads.hipchat.com/ , Atlassian does the same thing, and hasn't really addressed customer questions about it for a long time. https://help.hipchat.com/forums/138883-suggestions-ideas/sug... is a long running Atlassian support ticket for this.

Github does something similar, if you drag an image into the textarea in their issue tracker, it uploads the image to (I think) a public URL. I've considered what this could mean for teams with private projects who might e.g. attach screenshots with sensitive information.

Here's an example of an image uploaded via the GH issue tracker. Definitely public.

https://cloud.githubusercontent.com/assets/95562/7319912/200...

The good news is that github puts a uuid in the url, so its unguessable.

Slack, on the other hand, didn't have a big unguessable number... they had a very small number you could brute-force.

UUIDs are (usually) generated in a systematic fashion, so large parts of them are often possible to determine ahead of time.

Not in guid v4

Unfortunately that is not necessarily true, as nothing requires a UUIDv4 generator to use a cryptographically secure source of randomness.

What popular implementations in 2015 are using the braindead UUID generation methods?

Careful, being unguessable is not one of the properties required of a UUID. All a UUID guarantees is that two UUIDs generated by the specified procedure will never match, even if they're generated by different computers not in communication with each other. But it does not guarantee that an attacker cannot generate the same UUID generated by somebody else if they follow a different procedure.

It's the difference between avoiding collisions between cooperating entities, and avoiding collisions from malicious entities.

For example, a version 1 UUID is just the combination of the computer's MAC address and the current time. This is sufficient to guarantee uniqueness (as long as you don't duplicate MAC addresses, and you wait at least 100 nanoseconds between generating successive UUIDs) but will be pretty easy to guess if you have a rough idea of when the UUID was generated and which manufacturer might have made the NIC on the machine where it was generated.

More abstractly, a version 4 UUID is just a couple of identifying bits and 122 random bits, but the nature of the random bits is not specified. Your UUID generator may well use a PRNG that is not cryptographically secure, which could still be good enough for cooperative uniqueness, but not good enough to avoid predictability. (For example, a PRNG seeded with the machine's MAC address and boot time would fit this.)

In short, don't rely on a UUID being secret unless you know exactly how it's being generated, all the way down to the underlying random number generator. And if you're going that far, you might as well just read 16 bytes from /dev/random and be done with it.

The github sourcecode is not open-source, but they've open-sourced a lot of other code in which they seem to be using Ruby's `SecureRandom::uuid()` function in the same kind of context for generating what they term 'opaque ids'.

The problem is not with having public urls. The problem is with public urls that don't have enough random numbers or if these numbers aren't generated by a CSPRNG.

Doesn't help how complex your URL is if legitimate users can pass it to anyone else who can then access the file without proper authorization.

It does help. Malicious users intending to share files can do so without having a public URL.

Preventing legitimate users from sharing the data with malicious users is essentially what DRM is, and as we all know DRM is never perfect and rarely any good at all.

It's much more important to prevent malicious users from being able to access these files without the help of legitimate users. Which seems like an obvious thing to do, but it's what Slack has failed at here. It's impossible to tell from that one GitHub URL whether they get this right or not.

This should indeed inform potential users of additional security risks of using Slack, especially for communication of corporate confidential information. That said, this isn't a realistic threat to my own social use of Slack.

Similarly to using a hash function that is purposefully slow, wouldn't it be a good idea to introduce some artificial latency when responding to requests for urls like this?

I see slack now has the option "Disable public file URL creation"

Now no-one can share files publicly in our companies slack channels

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