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Covering the Tracks on Mac OS X

Learn how attackers can hide files and manipulate log files on Mac OS X, and discover methods to identify these techniques.

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Covering the Tracks on Mac OS X

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  1. Covering the Tracks on Mac OS X Charlie Scott November 2010 GIAC GSEC Gold, GCIH Gold, GCPM SANS Technology Institute - Candidate for Master of Science Degree

  2. Objective • Apply "Covering the Tracks" from "Hacker Techniques, Exploits, and Incident Handling" to Mac OS X. • Demonstrate unique ways an attacker might hide files in Mac OS X. • Discuss the challenges of Mac OS X's unique log files. • Show how to identify some of the techniques in this presentation. SANS Technology Institute - Candidate for Master of Science Degree

  3. Hiding Files from the Command Line • Dot, dot-dot-space, and triple-dot work: $ mkdir .tmpx $ echo hidden data > ".. " $ echo more hidden data > "…" SANS Technology Institute - Candidate for Master of Science Degree

  4. Hiding Files from the Finder: SetFile • Dot-dot files are hidden from the command line, but not the Finder. • Use SetFile from the install DVD to make it invisible from the Finder. • Modify the "v" (invisible) attribute: $ SetFile –a V ".. " SANS Technology Institute - Candidate for Master of Science Degree

  5. Hiding Files from the Finder: xattr • Using xattr is another option. • The com.apple.FinderInfo attribute controls Finder visibility (and other things). • Benefit of xattr: It comes standard, you don't need dev tools. SANS Technology Institute - Candidate for Master of Science Degree

  6. Who Looks in the Trash? • /.Trashes folder temporarily stores files deleted by users. • Interesting permission set: d-wx-wx-wt • Anyone can write to this dir. • The creator of a file in this dir can modify, execute, or delete it, but not read; nobody but root can read it. SANS Technology Institute - Candidate for Master of Science Degree

  7. Editing Log Files • Attacker primarily concerned about: • system.log: notices, kernel debug, login • secure.log: authentication & authorization • You must stop and restart syslogd to modify them. • OS X uses the launchctl command to stop/start services. SANS Technology Institute - Candidate for Master of Science Degree

  8. The ASL Database • Syslog also logs to the Apple System Log (ASL): • /var/log/asl.db binary file on Leopard • /var/log/asl directory of binary files on Snow Leopard • A lot of duplication exists between the .log text files and ASL db, so an attacker needs to hit both. SANS Technology Institute - Candidate for Master of Science Degree

  9. The syslog Tool • In 10.5 the syslog tool prunes ASL db entries. • Syslogd must be shut down for pruning. • Use the "-db", "-p" and "-k" switches with a key/value expression. • Prune based on service, process ID, host, time, and other keys. SANS Technology Institute - Candidate for Master of Science Degree

  10. Examples of Removing ASL Entries with syslog • Remove all sshd entries: # syslog -db -p -k Sender sshd • Remove all authentication entries: # syslog -db -p -k Sender \ com.apple.SecurityServer • Remove all sudo entries: # syslog -db -p -k Sender sudo • Remember to restart syslogd! SANS Technology Institute - Candidate for Master of Science Degree

  11. The aslmanager Tool • In 10.6, aslmanager gives some syslog tool functionality. • You can delete logs based on size or age (in days). • Not likely a useful tool for intruders. SANS Technology Institute - Candidate for Master of Science Degree

  12. Identifying: The Find Command • Look for hidden files and directories. • E.g.: # find / -name "..." –print • Can be automated through "cron" SANS Technology Institute - Candidate for Master of Science Degree

  13. Host Based Intrusion Detection Integrity checking: Look for changes in system, Applications, and Developer directories. See if /etc/asl.conf has been modified. Rootkit detection: Look for hidden files and directories. Available at www.ossec.net Identifying: OSSEC HIDS SANS Technology Institute - Candidate for Master of Science Degree

  14. Summary • Attackers have unique ways to hide files on Macs. • An attacker may find removing log files on Macs challenging, but there are ways to do it. • A well-prepared sysadmin can still detect these attempts at stealth. SANS Technology Institute - Candidate for Master of Science Degree

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