As part of a deliverable for two of our sponsors (Sponsor J, Sponsor L), I have been working on a forensic analysis of the Tor Browser Bundle. In this three part series, I will summarize the most interesting or significant traces left behind after using the bundle. This post will cover Debian Linux (#8166), part two will cover Windows 7, and part three will cover OS X 10.8.

Process

I set up a virtual machine with a fresh install of Debian 6.0 Squeeze, logged in once and shut it down cleanly. I then connected the virtual drive to another virtual machine and used dd to create an image of the drive. I also used hashdeep to compute hashes for every file on the drive, and rsync to copy all the files over to an external drive.

After having secured a copy of the clean virtual machine, I rebooted the system, connected an external drive, and copied the Tor Browser Bundle (version 2.3.25-6, 64-bit) from the external drive to my Debian home directory. I extracted the package archive and started the Tor Browser Bundle by running ./start-tor-browser inside the Tor Browser directory.

Once the Tor Browser was up and running, I browsed to a few pages, read a few paragraphs here and there, clicked on a few links, and then shut it down by closing the Tor Browser and clicking on the Exit-button in Vidalia. The Tor Browser did not crash and I did not see any error messages. I deleted the Tor Browser directory and the tarball using rm -rf.

I repeated the steps with dd, hashdeep, and rsync to create a copy of the tainted virtual machine.

Results

Using hashdeep, I compared the hashes from the tainted virtual machine against the hashes from the clean virtual machine: 68 files had a hash that did not match any of the hashes in the clean set. The most interesting files are:

~/.local/share/gvfs-metadata/home: contains the filename of the Tor Browser Bundle tarball: tor-browser-gnu-linux-x86_64-2.3.25-5-dev-en-US.tar.gz. GVFS is the virtual filesystem for the GNOME desktop, so this result will probably vary depending on the window manager used. I have created #8695 for this issue.

~/.xsession-errors: contains the following string: “Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x3800089 (Tor Browse)”. It is worth noting that a file named .xsession-errors.old could also exist. I have created #8696 for this issue.

~/.bash_history: contains a record of commands typed into the terminal. I started the Tor Browser Bundle from the command line, so this file contains lines such as ./start-tor-browser. I have created #8697 for this issue.

/var/log/daemon.log, /var/log/syslog, /var/log/kern.log, /var/log/messages: contains information about attached devices. I had an external drive attached to the virtual machine, so these files contain lines such as “Mounted /dev/sdb1 (Read-Write, label “THA”, NTFS 3.1)” and “Initializing USB Mass Storage driver…”.