unrar-free failed to open the files in 3 rar archives on 3 tested. It looks
like unrar-free doesn't fail gracefully with unsupported archives. I don't
know if unrar-free supports these files, but it failed with no explanation:

$ unrar-free test.rar

unrar 0.0.1  Copyright (C) 2004  Ben Asselstine, Jeroen Dekkers


Extracting from /home/chealer/test.rar

Extracting  dpkg                                                      Failed
1 Failed

In the worst case, unrar-free claimed success decompressing a
password-protected rar archive, even though it didn't decompress any file:

$ unrar-free attachment.rar

unrar 0.0.1  Copyright (C) 2004  Ben Asselstine, Jeroen Dekkers


Extracting from /home/chealer/attachment.rar

All OK

Unless I'm missing something, or unrar-free supports most archives and I
simply used bad examples, please consider upgrading this report's severity to
serious.
Debian Release: lenny/sid
  990 testing         security.debian.org
  990 testing         ftp.ca.debian.org
  500 unstable        ftp.ca.debian.org
--- Package information. ---
Depends       (Version) | Installed
=======================-+-===========
libc6        (>= 2.7-1) | 2.7-10

Attach sample encrypted RAR files created by WinRAR.
The password is "nyanyanyanyanyanyanya"