referer header is a HTTP request header that is sent by the browser whenever you navigate to a different page - the URL of the page that contained the link you clicked is sent to the new page within this header (even if it is a different domain).
The referer isn't sent if you type in a URL, and it is not sent by most browsers if your current page is protected by TLS (
This page can be used to find out if you referer is leaked to it, but this of course will depend on where you link to it from.
The Dropbox vulnerability is when a file is shared via the web and a public URL is generated. This URL is in the form
123456qwerty appears to be a randomly generated token.
Normally these links only present a download link to the file, but you can modify the URL from
https://dl.dropbox.com/s/... which will then become
https://dl.dropboxusercontent.com/s/... and would present the HTML file as being served from this domain. Also, the protocol can be changed to
http which would disable the protection of the referer normally enabled by using the https protocol.
If you share this
dl link with somebody, and the HTML file contains links to 3rd parties, the
referer could be sent to the third party when links are clicked, exposing your private share URL with the third party. They would be able to view the
referer header in their server logs or site analytics reports. This has also caused a second problem when users enter the generated links in a search engine instead of the browser's address bar, as when ads are displayed the PPC report to the advertiser shows the search term that was used, which happens to be the "private" URL to the file.
I'm not sure what the official fix is, bar the temporary deactivation of some shared links (although I haven't been notified of any disabled on my account). Computer Weekly reports:-
Dropbox has now disabled access to links that have been previously shared and implemented a patch to prevent shared links from being exposed from now on.
Possible fixes to this issue could include:
- Parsing of the page to change links such as
http://www.example.com to use a service such as https://href.li so the link becomes
- Force HTTPS for all shared HTML pages. This won't work on all browsers.
- Make HTML pages downloadable only instead of displayed inline - this could break some user setups though if they are using Dropbox as a static file server rather than simply sharing.
- An auth cookie generated when a shared link is followed by the original recipient. This would set the cookie and then redirect to a new URL. e.g.
https://www.example.com/s/123456qwerty/file.htm sets the cookie then redirects to
https://www.example.com/s/auth/zxcvbn76543/file.htm which is only accessible if the auth cookie is present. This would make any leaked referer data useless as the URL can only be used in combination with the cookie. This would break any bookmarking of the shared links though, unless the original link was saved manually.
- Setting a
referrer-policy header to prevent the referer from being leaked in supported browsers.
You could test the final fix (whatever that may be) by this linking to a site from a shared Dropbox HTML file, checking the
referer and finding out if you get the same result as the hidden version. At the moment of writing, these are still being sent by my Dropbox shared HTML page when accessed over http.
The next paragraph in their statement says that they have patched the vulnerability. How was this done? It looks like if someone managed to get ahold of your shared link, they're still going to have full access to it. At least, when I created new links to my shared folder, I didn't see anything different.
Only recently have Dropbox added Passwords for Shared Links and Shared Link Expiration to their Pro accounts only.
If you’re a Dropbox Pro or Dropbox for Business user, you can set a password for any shared link that you create. This helps to ensure that only collaborators with the password can access your shared content.
If you’re a Dropbox Pro or Dropbox for Business user, you can set an expiration for any shared link that you create. This enables you to provide temporary access to content that you want to share.
These two features help resolve the vulnerability of files being accessible due to
referer or PPC reports.
As per https://www.dropbox.com/help/files-folders/public-folder:
As of September 1, 2017 public links have been disabled for all users.