The Linux Audit Framework

The Linux Audit
Framework
Gary Smith, EMSL, Pacific Northwest National
Laboratory

Agenda
! Who Am I?
! Capabilities of the Linux Audit Framework
! auditd.conf and audit.rules
! Types of Rules
! Examples of rules
! Creating Reports
! Advanced Rule Writing
! Visualizing Audit Data
2

A Little Context
! How do you think about Cyber Security?
! The Five Golden Principles of Security
! Know your system
! Principle of Least Privilege
! Defense in Depth
! Protection is key but detection is a must.
! Know your enemy.
3

Introduction
! Linux audit helps make your system more secure by
providing you with a means to analyze what is happening
on your system in great detail.
! It does not, however, provide additional security itself—it
does not protect your system from code malfunctions or
any kind of exploits.
! Instead, Audit is useful for tracking these issues and helps
you take additional security measures, like SELinux, to
prevent them.
! Audit consists of several components, each contributing
crucial functionality to the overall framework.
! The audit kernel module intercepts the system calls and
records the relevant events.
4

Introduction (cont.)
! The auditd daemon writes the audit reports to disk.
! Various command line utilities take care of displaying,
querying, and archiving the audit trail.
5

Linux Audit Framework Capabilities
! Audit enables you to do the following:
! Associate Users with Processes
! Audit maps processes to the user ID that started them.
! This makes it possible for the administrator or security officer
to exactly trace which user owns which process and is
potentially doing malicious operations on the system.
! Review the Audit Trail
! Linux audit provides tools that write the audit reports to disk
and translate them into human readable format.
! Review Particular Audit Events
! Audit provides a utility that allows you to filter the audit reports for
certain events of interest.
6

Linux Audit Framework Capabilities (1)
! You can filter for:
! User
! Group
! Audit ID
! Remote Hostname
! Remote Host Address
! System Call
! System Call Arguments
! File
! File Operations
! Session
! Success or Failure
7

Linux Audit Framework Capabilities (2)
! Apply a Selective Audit
! Audit provides the means to filter the audit reports for events of
interest and also to tune audit to record only selected events.
! You can create your own set of rules and have the audit daemon
record only those of interest to you.
! Prevent Audit Data Loss
! Audit provides several mechanisms to prevent the loss of audit
data in the event of a loss of system resources.
8

The Components of Linux Audit
9

Configuring The Linux Audit Framework
! Before you can actually start generating audit logs and
processing them, you must configure the audit framework.
! Julius Caesar said, “Gallia est omnis divisa in tres partes”,
and just like Gaul, the configuring the audit framework is
divided into three parts:
! The Audit Daemon Configuration
! The Audit Rules
! The Audispd Daemon Configuration
10

/etc/audit/auditd.conf
! The /etc/audit/auditd.conf configuration file determines
how the audit system functions once the daemon has
been started.
! The directives tell auditd where to put the audit log files,
flushing the audit records, managing the audit log files,
and error handling.
! For most use cases, the default settings shipped with the
package should suffice.
11

A Sample auditd.conf
log_file = /var/log/audit/audit.log!
log_format = RAW!
log_group = root!
priority_boost = 4!
flush = INCREMENTAL!
freq = 20!
num_logs = 5!
dispatcher = /sbin/audispd!
disp_qos = lossy!
name_format = hostname!
##name = mydomain!
max_log_file = 6!
max_log_file_action = ROTATE!
space_left = 75!
space_left_action = SYSLOG
action_mail_acct = root
admin_space_left = 50
admin_space_left_action = SUSPEND
disk_full_action = SUSPEND
12

/etc/audit/audit.rules
! audit.rules is a file containing audit rules that will be
loaded by the audit daemon’s startup script whenever the
daemon is started.
! The auditctl program is used by the startup script to
perform this operation.
! The audit rules come in 3 varieties: control, file, and
syscall.
13

Control
! Control commands generally involve configuring the audit
system rather than telling it what to watch.
! These commands, found at the top of the rules file,
typically include
! Deleting all rules (-D)
! Setting the size of the kernel's backlog queue (-b)
! Setting the failure mode (-f)
! Setting the event rate limit (-r)
! Tell auditctl to ignore syntax errors in the rules and continue
loading. (-i) and (-c)
! Enable/disable auditing (-e)
14

File System
! File System rules are sometimes called watches.
! These rules are used to audit access to particular files or
directories that you may be interested in.
! If the path given in the rule is a directory, then the rule
used is recursive to the bottom of the directory tree
excluding any directories that may be mount points.
! The syntax of these rules generally follow this format:
! -w path-to-file -p permissions -k keyname
! where the permission are any one of the following:
! r - read of the file
! w - write to the file
! x - execute the file
! a - change in the file's attribute
! keyname is an arbitrary string of text used to uniquely identify
the audit records produced by a rule
15

Integrity Checking
! Using file watches, write a list of rules to detect changes
to the audit rules files and the instantiated audit rules in
the kernel.
-w /etc/audit/auditd.conf –p wa -k audit-config!
-w /etc/audit/auditd.rules –p wa -k audit-config!
-w /sbin/auditctl –p x -k audit-config!
16

File Watches Gotchas
! When in need of detailed file-related records, enable
separate file watches for all files of interest.
! Directory watches produce less verbose logs than exact
file watches.
! Pathname globbing of any kind is not supported by audit.
Always use the exact pathnames.
! Auditing can only be performed on existing files.
! Any files added while the audit daemon is already running
are ignored until the audit rule set is updated to watch the
new files.
! Remember: First match wins!
17

Auditing the Execution of Setuid/Setgid Binaries
! Let’s say that as a matter of compliance, you have to
audit the execution of setuid/setgid binaries on your
system.
! How do you do set that up?
! First, run a script like this at boot time from /etc/rc.local
sending the output to a temp file, /tmp/snorf, for example.!
18

Auditing the Execution of Setuid/Setgid Binaries (1)
!
!
!
#!/bin/bash!
# Find all the file systems that are locally mounted!
for i in `/bin/egrep '(ext4|ext3|ext2)' /etc/fstab | /bin/awk '{print $2}'`!
do!
# Find all the files on the file system found above and print out!
# and audit rule for it!
/usr/bin/find $i -xdev -type f ( -perm -4000 -o -perm -2000 ) -print | !
/bin/sort | /bin/awk '{ print ”-w " $1 " -p x -k privileged -k ids-exec-high" }'!
done!
19

! And you get something like this (YMMV depending on
what’s installed).
-a -w /bin/cgclassify -p x -k privileged -k ids-exec-high!
-a -w /bin/cgexec -p x -k privileged -k ids-exec-high!
-a -w /bin/ping -p x -k privileged -k ids-exec-high!
-a -w /bin/ping6 -p x -k privileged -k ids-exec-high!
-a -w /bin/su -p x -k privileged -k ids-exec-high!
-a -w /sbin/mount.nfs -p x -k privileged -k ids-exec-high!
-a -w /sbin/netreport -p x -k privileged -k ids-exec-high!
-a -w /sbin/pam_timestamp_check -p x -k privileged -k ids-exec-high!
-a -w /sbin/unix_chkpwd -p x -k privileged -k ids-exec-high!
-a -w /usr/bin/chage -p x -k privileged -k ids-exec-high!
-a -w /usr/bin/chfn -p x -k privileged -k ids-exec-high!
-a -w /usr/bin/chsh -p x -k privileged -k ids-exec-high!
-a -w /usr/bin/crontab -p x -k privileged -k ids-exec-high!
-a -w /usr/bin/gpasswd -p x -k privileged -k ids-exec-high!
-a -w /usr/bin/ksu -p x -k privileged -k ids-exec-high!
20

! Then, point auditctl at the temp file to add the newly
created audit rules.
! The auditctl program is used to control the behavior, get
status, and add or delete rules into the kernel’s audit
system.
/sbin/auditctl –R /tmp/snorf
! A couple of things about auditctl:
! auditctl is not a filter, so output cannot be piped into it.
! Rules files for auditctl must be owned by root.
21

System Call
! The system call rules are loaded into a matching engine
that intercepts each syscall that all programs on the
system makes.
! Therefore, it is very important to only use syscall rules
when you have to since these affect performance. The
more rules, the bigger the performance hit.
! You can help the performance, though, by combining
syscalls into one rule whenever possible.
! Remember: First match wins!
! Syscall rules take the general form of:
! -a action,list -S syscall -F field=value -k keyname
22

System Call (1)
! The -a option tells the kernel's rule matching engine that
we want to append a rule and the end of the rule list.
! But we need to specify which rule list it goes on and what
action to take when it triggers.
! The action and list are separated by a comma but no
space in between. Valid lists are: task, entry, exit, user,
and exclude.
! Valid actions are:
! always - always create an event
! never - never create an event
23

System Call (2)
! Next in the rule is the -S option specifying either a syscall
name or number. Usually, the name is almost always
used. You may give more than one syscall in a rule by
specifying another -S option.
! After the syscall is specified, you would normally have
one or more -F options that fine tune what to match
against.
! The audit system considers uids to be unsigned numbers.
The audit system uses the number -1 to indicate that a
loginuid is not set. This means that when its printed out, it
looks like 4294967295.
! The last thing about syscall rules is that you can add a
keyname which is a free form text string that you want
inserted into the event to help identify its meaning.
24

Syscall Audit Rules Examples
! To see files opened by a specific user:
-a exit,always -S open -F auid=l337!
! To see unsuccessful open calls:
-a exit,always -S open -F success=0!
25

Record Attempts to Alter System Time
! Hackers frequently tinker with the system time to hid their
actions. Use system call auditing to create audit rules to
record changes to the system time.
-a always,exit -F arch=b32 -S adjtimex -S settimeofday -S stime -k audit_time_rules!
-a always,exit -F arch=b64 -S adjtimex -S settimeofday -k -audit_time_rules!
-a always,exit -F arch=b32 -S clock_settime -k audit_time_rules!
-a always,exit -F arch=b64 -S clock_settime -k audit_time_rules!
26

Creating Audit Reports
! The audit records are stored in /var/log/audit/audit.log.
! grep is your friend and you can pull stuff out of the audit
log and get stuff like this:
! type=SYSCALL msg=audit(1365719016.212:333043): arch=c000003e
syscall=171 success=yes exit=0 a0=7fff86310c37 a1=6 a2=d
a3=7fff8630f3b0 items=0 ppid=22300 pid=22311 auid=0 uid=0 gid=0
euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=37491
comm="domainname" exe="/bin/hostname"
subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
key=73797374656D2D6C6F63616C65016964732D7379732D6C6F77!
! Maybe grep isn’t your friend, after all.
! The raw audit data auditd stores in the /var/log/audit
directory is quite complex.
! To find what you want, you might have to sift through
bazillions of other events before you locate the one that
you want.
27

Creating Audit Reports (1)
! Use aureport to create concise, human-readable reports.
! Some of the useful options are:
! --summary
! --failed
! --start and --end (aureport understands today, yesterday, now,
recent, this-week, this-month, and this-year)
! --auth, --avc, --login, --user, --executable, --syscall
! To get started, do aureport –summary and you get
something like this:
28

Summary Report!
======================!
Range of time in logs: 08/09/2015 02:28:36.498 - 10/02/2015 18:07:38.737!
Selected time for report: 08/09/2015 02:28:36 - 10/02/2015 18:07:38.737!
Number of changes in configuration: 0!
Number of changes to accounts, groups, or roles: 4!
Number of logins: 16!
Number of failed logins: 13134!
Number of authentications: 59!
Number of failed authentications: 46567!
Number of users: 9!
Number of terminals: 18!
Number of host names: 1139!
Number of executables: 14!
Number of files: 0!
Number of AVC's: 0!
Number of MAC events: 0!
Number of failed syscalls: 0!
Number of anomaly events: 0!
Number of responses to anomaly events: 0!
Number of crypto events: 50419!
Number of keys: 0!
Number of process IDs: 25791!
Number of events: 129193!
29

! Lets look at some of the failed logins with aureport –auth
–failed:
Login Report!
============================================!
# date time auid host term exe success event!
============================================!
1. 09/23/2015 19:53:04 send 195.4.234.9 sshd /usr/sbin/sshd no 2469819!
3. 09/23/2015 19:53:10 root 195.4.234.9 sshd /usr/sbin/sshd no 2469825!
4. 09/23/2015 19:53:12 ts 195.4.234.9 sshd /usr/sbin/sshd no 2469828!
9. 09/23/2015 21:34:52 ubnt 222.186.21.154 sshd /usr/sbin/sshd no 2470589!
10. 09/23/2015 21:34:54 ubnt 222.186.21.154 sshd /usr/sbin/sshd no 2470591!
12. 09/24/2015 00:25:17 pi 218.4.117.26 sshd /usr/sbin/sshd no 2471858!
13. 09/24/2015 00:25:19 pi 218.4.117.26 sshd /usr/sbin/sshd no 2471860!
14. 09/24/2015 00:25:21 test 218.4.117.26 sshd /usr/sbin/sshd no 2471863!
15. 09/24/2015 00:25:23 test 218.4.117.26 sshd /usr/sbin/sshd no 2471865!
30

Drilling Deeper with ausearch
! aureport lets you to create overall summaries of what is
happening on the system, but if you want to drill deeper
into the details of a particular event, ausearch is the tool
to use.
! ausearch allows you to search the audit logs using
special keys and search phrases that relate to most of the
flags that appear in event messages in /var/log/audit/
audit.log
! A methodology to use is find an event class of interest
with aureport and then drill down into the nitty-gritty with
ausearch.
! For instance, you use aureport –syscall –failed to see
the failed system calls. Use ausearch and one of the
event ids to get more information.
31

Drilling Deeper with ausearch (1)
! From aureport –syscall –fail we get:
Syscall Report!
=======================================!
# date time syscall pid comm auid event!
=======================================!
1. 10/02/2015 15:26:16 2 5630 cp 25016 306512!
! From ausearch –i –a 306512 we get:
----!
type=PATH msg=audit(10/02/2015 15:26:16.299:306512) : item=0 name=/etc/shadow
inode=394070 dev=fd:00 mode=file,000 ouid=root ogid=root rdev=00:00
obj=system_u:object_r:shadow_t:s0 nametype=NORMAL!
type=CWD msg=audit(10/02/2015 15:26:16.299:306512) : cwd=/home/dr-horrible!
type=SYSCALL msg=audit(10/02/2015 15:26:16.299:306512) : arch=x86_64 syscall=open
success=no exit=-13(Permission denied) a0=0x7ffde2dd8739 a1=O_WRONLY|O_TRUNC a2=0x0
a3=0x7ffde2dd6bd0 items=1 ppid=27491 pid=5630 auid=dr-horrible uid=dr-horrible gid=users
euid=dr-horrible suid=dr-horrible fsuid=dr-horrible egid=users sgid=users fsgid=users
tty=pts0 ses=16292 comm=cp exe=/bin/cp subj=unconfined_u:unconfined_r:unconfined_t:s0-
s0:c0.c1023 key=identity!
!
32

Drilling Deeper with ausearch (2)
! A useful feature, if you tagged your audit rules with keys,
is to search for events based on those keys.
! For instance, ausearch –ts today –i –k identity and we
get:
type=PATH msg=audit(10/02/2015 15:26:16.299:306512) : item=0 name=/etc/
shadow inode=394070 dev=fd:00 mode=file,000 ouid=root ogid=root rdev=00:00
obj=system_u:object_r:shadow_t:s0 nametype=NORMAL!
type=CWD msg=audit(10/02/2015 15:26:16.299:306512) : cwd=/home/dr-horrible!
type=SYSCALL msg=audit(10/02/2015 15:26:16.299:306512) : arch=x86_64
syscall=open success=no exit=-13(Permission denied) a0=0x7ffde2dd8739
a1=O_WRONLY|O_TRUNC a2=0x0 a3=0x7ffde2dd6bd0 items=1 ppid=27491 pid=5630
auid=dr-horrible uid=dr-horrible gid=users euid=dr-horrible suid=dr-horrible
fsuid=dr-horrible egid=users sgid=users fsgid=users tty=pts0 ses=16292
comm=cp exe=/bin/cp subj=unconfined_u:unconfined_r:unconfined_t:s0-
s0:c0.c1023 key=identity!
33

Advanced Rule Writing: The Evil Sysadmin
! You suspect a sysadmin acting as “root” is trawling thru
the /home file system looking for intellectual property to
sell to competitors. Write an audit rule(s) that will record
root trawling.
! The way to do this is use the –C option to build an inter-
field comparison rule:
!
-a exit,always -F dir=/home/ -F uid=0 -C auid!=obj_uid -k admin-access!
!
!
34

Advanced Rule Writing: The Evil Sysadmin
(1)
! Since there was no explicit syscall referenced in the rule,
all syscalls are tested against this rule.
! This might could be a significant performance hit.
! A better way of doing this would be:
-a always,exit -F arch=b32 -S open -S openat -S open_by_handle_at -S truncate -S
ftruncate -F dir=/home/ -F uid=0 -C auid!=obj_uid -F auid>=500 -F auid!=4294967295 -k
admin-access!
-a always,exit -F arch=b64 -S open -S openat -S open_by_handle_at -S truncate -S
ftruncate -F dir=/home/ -F uid=0 -C auid!=obj_uid -F auid>=500 -F auid!=4294967295 -k
admin-access!
35

Advanced Rule Writing: Setuid Programs
! Recall the script that writes audit rules to find setuid
executables. How do you record the execution of a setuid
program that is put someplace you don’t expect it?
! Write a audit rule(s) to catch the execution of a setuid
program regardless of its location.
-a always,exit -F arch=b64 -S execve -F euid=0 -C uid!=euid!
36

Visualizing Audit Data
! Recall the listing from aureport of the failed logins:
Login Report!
============================================!
# date time auid host term exe success event!
============================================!
! We can extract fields from the report with any of the text
manipulation tools in Linux to produce a column of
usernames, IP addresses, etc. that can be feed into a
word cloud generator to create graphs of the data.
37

Resources
! The Audit Manual Pages
! There are several man pages installed along with the audit tools
that provide valuable and very detailed information:
! auditd(8) The Linux Audit daemon
! auditd.conf(5) The Linux Audit daemon configuration file
! auditctl(8) A utility to assist controlling the kernel's audit
system
! autrace(8) A program similar to strace
! ausearch(8) A tool to query audit daemon logs
! aureport(8) A tool that produces summary reports of audit
daemon logs
! audispd.conf(5) The audit event dispatcher configuration file
! audispd(8) The audit event dispatcher daemon talking to
plugin programs.
! augenrules(8) – A script that merges component audit rule
files
40

Resources (1)
! http://people.redhat.com/sgrubb/audit/index.html The
home page of the Linux audit project. This site contains
several specifications relating to different aspects of Linux
audit, as well as a short FAQ.
! /usr/share/doc/audit The audit package itself contains a
README with basic design information and sample .rules
files for different scenarios:
! capp.rules: Controlled Access Protection Profile (CAPP)
! lspp.rules: Labeled Security Protection Profile (LSPP)
! nispom.rules: National Industrial Security Program Operating
Manual Chapter 8(NISPOM)
! stig.rules: Secure Technical Implementation Guide (STIG)
! Word Cloud Generator: http://www.wordclouds.com/
41

T-t-t-t-that’s all, folks!
42
Gary Smith
Information System Security Officer, Molecular Science
Computing, EMSL, Pacific Northwest National
Laboratory
Richland, WA
gary.smith@pnnl.gov

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