Thursday, May 3, 2018

Bash sessions in macOS (and why you need to understand its working)

While all versions of macOS have provided bash_history for users, since macOS 10.11 (El Capitan), we get even more information on terminal history through the bash sessions files. This is not a replacement for the old .bash_history file which is still there.

There are several problems with bash_history - you cannot tell when any command in that file was run, the sequence of commands may not be right, and so on. For more on that, refer Hal Pomeranz's excellent talk - You don't know jack about Bash history

Even if there were no anomalies and only a single terminal was always in use, there is still the issue of how do I know which command was run when? With Bash sessions, macOS gives us more data to work with. Since El Capitan, every new terminal window will be tracked independently with a TERM_SESSION_ID which appears to be a randomly generated UUID.

Figure 1 - Fetching terminal's session id

Each session can also be restored when you shutdown and restart your machine with the "Reopen windows when logging back in" option set. Perhaps for this purpose, session history (a subset of bash history) is tracked and saved separately on a per session basis.

Figure 2 - Restored session

Show me the artifacts!

The location you want to go to is  /Users/<USER>/.bash_sessions

You will find 3 files for each session as seen in screenshot below.

Figure 3 - .bash_sessions folder contents

TERM_SESSION_ID.history    --> Contains session history
TERM_SESSION_ID.historynew --> Mostly blank/empty
TERM_SESSION_ID.session    --> Contains the last session resume date and time

Figure 4 - Sample .session file

Figure 5 - Sample .history file showing commands typed at terminal 

How this helps?

Some (but not all) of the problems associated with reading .bash_history are now gone.
Theoretically, as bash history is now also stored on a per session basis, this should make it trivial to track commands run in different windows (sessions). If you were expecting history for a single session in its .history file, then you thought wrong. The .history file contains all previous history (from earlier sessions) and then appended at the very end, the history for this session.

So can we reliably break apart commands per session? Is the sequence of commands intact? Let's run a small experiment to find out.

We create two sessions (2 terminal windows) and run a few commands in each session. Commands are interspersed, so we run a command in Session-1, then another in Session-2 and then again something in Session-1. We will try to see if order is maintained.

Session-1 started 9:44
Session-2 started 9:51
Figure 6 - Commands run with their sequence

Session-1 closed 9:57
Session-2 closed 9:59

Session-1 is closed first, followed by Session-2.  Here is a snippet of relevant metadata from the resulting files:

Figure 7 - Relevant metadata from stat command

Fun Facts

The start and stop time for a session is available if you look at the crtime (File Created time) for the .history and .historynew files. These are in bold in the screenshot above.

Created Time of TERM_SESSION_ID.historynew = Session created time
Created Time of TERM_SESSION_ID.history        = Session end time

Isolating session data

By comparing the data in various .history files (from different sessions), you can find out exactly which commands belong to a particular session. See pic below, where lines 1-181 (not shown) are from older history (other past sessions). Lines 182-184 are from Session-1 and are seen in its history file at the end. Session-2 (closed after Session-1) has the same format, ie, old session history with this session's history appended (lines 185-189).

Figure 8- .history files from Session-1 (Left) and Session-2 (Right)

This is easily done in code and the mac_apt BASHSESSIONS plugin parses this information to break out the individual commands per session, along with session start and stop time.

While you still cannot get the exact time when an individual command was run, the sessions functionality does give you a very good narrowed time frame to work with. While we do not have the absolute order of commands ("cp -h" was run before "printenv"), we do have a narrowed time-frame for the set of commands ("cp-h" run between 9:51-9:59 and "printenv" run between 9:44-9:57). This is a big thing for analysts and investigators!

1 comment:

  1. Thanks for this post! I did some housecleaning and wanted to know what ~/.bash_sessions/ is. And on the other hand in the past I wondered how macOS 's "Reopen windows when logging back in" feature magically also restored my Terminal windows. This post brought clarity to me 🙂 in both regards. Thanks!

    And with the move to .zsh in macOS 11 Big Sur Apple re-applied that same concept. There's now a ~/.zsh_sessions/ directory. It also containing "Random-UID.session" files. But they are all 52bytes with the content "echo Restored session: "$(/bin/date -r NNNNNNNNNN)". I guess in the GUI they somehow reconstruct it from .zsh_history where the command input is prefixed with a timestamp. But then again from where is the session-context reconstructed? In .zsh_history each line only has ": timestamp:cmdDuration:cmd". No session ID anywhere.