Hidden in plain sight: Brute-forcing Slack private files

[dantillberg]

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.

[joshstrange]

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.

[wtbob]

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.

[Lazare]

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.

[relaxnow]

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.

[evancordell]

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

[kordless]

> 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.

[relaxnow]

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.

[fishtoaster]

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.

[Drdrdrq]

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...

[tptacek]

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?

[qyv]

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.

[NKCSS]

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

[elcct]

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

[thmpp]

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

[braum]

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.

[macNchz]

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.

[the_wolf]

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!

[wzm]

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.

[thom_nic]

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...

[sebak]

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.

[willvarfar]

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.

[delinka]

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.

[jkern]

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?

[rhuddleston]

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

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

[relaxnow]

Heh, well it is an old picture, I'll see if I can't get a better one made. I didn't join IT for my looks though so hopefully it doesn't stand in the way of your enjoyment of the content.