Monday, December 1st, 2014 by arvid
A .torrent file is very flexible in what it allows a path or filename to contain. Each directory name in a path is a length-prefixed utf-8 string. It can be an empty string, and it can have any character imaginable in it. The way a .torrent file represents filenames and paths is also agnostic to the way your OS/filesystem may represent paths (it does assume a directory hierarchy though).
A file is represented by a list of directory entries, where the last one is the name of the file. Like this:
On a unix system, this path would be
and on windows the forward slashes would be backslashes. Typical filesystems are more restrictive. They may have restrictions on:
On windows, a file may not have a trailing period (“.”). It also may not have a trailing space (” “).
In order to map the directory structure from a .torrent file to a filesystem, filenames and directory names need to be transformed into something that’s allowed and supported by it. These are some of the transformations:
The kinds of attacks one can launch against a client lacking these kinds of tests may not be obvious. Consider a maliciously crafted torrent where one of the files path is:
and that your download directory is /Users/arvid/downloads. The file may end up downloading at “/Users/arvid/downloads/../.profile”, which resolves to “/Users/arvid/.profile”. The creator of that torrent can now control the contents in your terminal startup script and essentially do anything the next time you launch a shell.
Windows supports a more modern way of specifying paths with some nice properties. UNC paths are more general paths that can refer to windows shares, but also files on the local computer (using “?” as the hostname). The benefit of UNC paths is that many of the old DOS restrictions no longer apply to them:
By default, libtorrent uses UNC paths on windows.
All strings in .torrent files are UTF-8 encoded, or at least they are treated as UTF-8. One of my favorite attacks is generating invalid UTF-8 sequences in attempts to circumvent security checks.
First, a brief review. UTF-8 is a scheme to encode any unicode codepoint (each representing a character) with variable length sequences. There are about 1,1 million unicode codepoints, and UTF-8 encodes the first 127 as a single bytes, the next 1920 characters as 2 bytes and so on. The higher the value of the codepoint, the more bytes you need to encode it in UTF-8.
A naive UTF-8 decoder may accept a 2 or 3 byte sequence encoding a codepoint <= 127. Just encode redundant leading zeros. For instance, imagine encoding the character “.” as a 2 byte sequence, instead of the single byte it’s supposed to be. This may trick security checks looking for a “..” directory entry, but still be allowed, and be decoded into a “.”. Such encodings are called overlong and are not legal UTF-8.
libtorrent verifies all encoding by decoding all characters and re-encodes them. If any illegal sequence is encountered, it’s replaced by a replacement character.
All the rules of how the filename mappings are done cause compatibility issues. Between:
The last one may be the most surprising. libtorrent has experienced this once and will experience it again. For security reasons, it may be important to update how filenames are formed from torrents, and when upgrading from one version to the next, you may no longer be seeding the files you thought you were.
Specifically what has happened in libtorrent was turning empty folder names into “_”. Previous versions would leave the empty string, let it end up as a path like this:
["foo", "", "bar"] -> "foo//bar" -> (OS collapses) "foo/bar"
Relying on the operating system to collapse empty folders seemed inelegant and potentially dangerous. Also, UNC paths on windows don’t allow for this. This was the main motivation behind the change.
Another upcoming change for the next major version of libtorrent is that path elements will be preserved as specified in the torrent, rather than split. For example, current libtorrent makes this transform:
["foo/bar", "abc", "def"] -> "foo/bar/abc/def"
The next major version will instead make the following transform:
["foo/bar", "abc", "def"] -> "foo_bar/abc/def"
The good news though, is that only edge cases and borderline exploit attempts are affected by these changes. The majority of torrents presumably honor the typical naming restrictions of modern operating systems.