Posts Tagged ‘audit’
ServerFault user ewwhite describes a rather interesting situation regarding application distribution wherein code must be compiled in production. In short he wants to keep track of changes to a specific directory path and send alerts via email.
Let’s assume that there already exists some basic form of auditd in play, so as such we’ll be building out a snippet to be inserted into your existing /etc/audit/audit.rules. Ed was sparse on some of the specifics related to the application, understandably so, so let’s make some additional assumptions. Let’s assume that the source code directory in question is “/opt/application/src” and that all binaries are installed into “/opt/application/bin“.
-w /opt/application/src -p w -k app-policy -w /opt/application/bin -p wa -k app-policyI’ve decided to process each directory, source and binaries, separately. The commonality between the two are the ‘-w’ and the ‘-k’ options. The ‘-w’ option says to watch that directory recursively. The ‘-k’ option says to add the text “app-policy” to the output log message, this is just to make log reviews easier. The ‘-p’ option is actually where the magic happens, and is the real reason to separate these two rules out.
As we discussed in the previous post, nearly 8 months ago, the option ‘-p w’ instructs the kernel to log on file writes. One would assume this is accomplished by attaching to the POSIX system call write. That syscall, however, gets called quite a lot when files are actually saved. So as to not overwhelm the logging system auditd instead attaches to the open system call. By using the (w)rite argument we look for any instance of open that uses the O_WRONLY or O_RDWR flags. It’s worth noting that this does not mean a file was actually modified, only that it was opened in such a way that would allow for modification. For example, if a user opened “/opt/application/src/app.h” in a text editor a log would be generated, however if it was written to the terminal using cat or read using less then no log would be generated. This is pretty important to remember as many people will read a file using a text editor and simply exit without saving changes (hopefully).
We also want to watch for file writes in the binary directory except here we would expect them to be more reliable. It would be rather unusual, but not out of the question, for someone to attempt to use a text editor to open an executable. In addition we added the (a)ttribute option. This will alert us if any of the ownership or access permissions change, most importantly if a file is changed to be executable or the ownership is changed. This will not catch SELinux context changes but since SELinux uses the auditd logging engine then those changes will show up in the same log file.
Now that we have the rules constructed we can move on to the alerting. Ed wanted the events to be emailed out. This is actually quite a bit more complicated. By default auditd uses it’s own built in logger. This does make some sense when you consider the type of logging this system is intended to perform. By not relying on an external logger, like syslogd or rsyslog, it can better suffer mistaken configurations. On the downside it makes alternate logging setups trickier. There does exist a subsystem called ‘audispd’ that acts as a log multiplexor. There are a number of output plugins available, such as syslog, UNIX socket, prelude IDS, etc. None of them really do what Ed wants, so I think our best bet would run reports. Auditd is, after all, an auditing tool and not an enforcement tool. So let’s look at something a little different.
Remember how we tacked on ‘-k app-policy‘ to those rules above? Now we get to the why. Let’s try running the command:
aureport -k -ts yesterday 00:00:00 -te yesterday 23:59:59We should now see a list of all of the logs that contain keys and occurred yesterday. Let’s look at a concrete example of me editing a file in that directory and the subsequent logs.
root@ node1:~> mkdir -p /opt/application/src root@ node1:~> vim /opt/application/src/app.h root@ node1:~> aureport -kThe report tells us that at 11:41:29 on September the 24th a user ran the command “/usr/bin/vim” and triggered a rule labeled app-policy. It’s all good so far, but not very detailed. The last two fields, however, are quite useful. The first, 1000, is the UID of my personal account. That is important because notice I was actually running as root. Since I had originally used “sudo -i to gain a root shell my original UID was still preserved, this is good! The last field is a unique event ID generated by auditd. Let’s look at that first event, numbered 13446.
Key Report \=============================================== # date time key success exe auid event \=============================================== 1. 09/24/2013 11:41:29 app-policy yes /usr/bin/vim 1000 13446 2. 09/24/2013 11:41:29 app-policy yes /usr/bin/vim 1000 13445 3. 09/24/2013 11:41:29 app-policy yes /usr/bin/vim 1000 13447 4. 09/24/2013 11:41:29 app-policy yes /usr/bin/vim 1000 13448 5. 09/24/2013 11:41:29 app-policy yes /usr/bin/vim 1000 13449 6. 09/24/2013 11:41:35 app-policy yes /usr/bin/vim 1000 13451 7. 09/24/2013 11:41:35 app-policy yes /usr/bin/vim 1000 13450
root@ node1:~> grep :13446 /var/log/audit/audit.log type=SYSCALL msg=audit(1380037289.364:13446): arch=c000003e syscall=2 success=yes exit=4 a0=bffa20 a1=c2 a2=180 a3=0 items=2 ppid=21950 pid=22277 auid=1000 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 ses=1226 tty=pts0 comm="vim" exe="/usr/bin/vim" subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key="app-policy" type=CWD msg=audit(1380037289.364:13446): cwd="/root" type=PATH msg=audit(1380037289.364:13446): item=0 name="/opt/application/src/" inode=2747242 dev=fd:01 mode=040755 ouid=0 ogid=0 rdev=00:00 obj=unconfined_u:object_r:usr_t:s0 type=PATH msg=audit(1380037289.364:13446): item=1 name="/opt/application/src/.app.h.swx" inode=2747244 dev=fd:01 mode=0100600 ouid=0 ogid=0 rdev=00:00 obj=unconfined_u:object_r:usr_t:s0This is what we mean when we say audit logs are verbose. In the introductory blog post we discussed some of those fields so I’ll save us the pain of going over it again. What we can see, however, is that the user with uid 1000 (see auid=1000) ran the command vim as root (see euid=0) and that command resulted in a change to both “/opt/application/src/” and “/opt/application/src/.app.h.swx“.
What we should be able to see here is that the report generated by aureport doesn’t contain everything we need to see what happened, but it does tell us something happened and gives us the information necessary to find the information. In an ideal world you would have some kind of log aggregation system, like Splunk or a SIEM, and send the raw logs there. That system would then have all the alerting functionality built in to alert an admin to to the potential policy violation. However, we don’t live in a perfect world and Ed’s request for email alerts indicate he doesn’t have access to such a system. What I would do is set up a daily cron job to run that report for the previous day. Every morning the log reviewer can check their mailbox and see if any of those files changed when they weren’t supposed to. If daily isn’t reactive enough then we can simply change the values passed to ‘-ts’ and ‘-te’ and run the job more frequently.
Pulling it all together we should have something that looks like this.
#/etc/audit/audit.rules # This file contains the auditctl rules that are loaded # whenever the audit daemon is started via the initscripts. # The rules are simply the parameters that would be passed # to auditctl.
# First rule - delete all -D
# Increase the buffers to survive stress events. # Make this bigger for busy systems -b 320
# Feel free to add below this line. See auditctl man page -w /opt/application/src -p w -k app-policy -w /opt/application/bin -p wa -k app-policy #/etc/cron.d/audit-report MAILTOemail@example.com
1 0 * * * root /sbin/aureport -k -ts yesterday 00:00:00 -te yesterday 23:59:59
The auditd subsystem is an access monitoring and accounting for Linux developed and maintained by RedHat. It was designed to integrate pretty tightly with the kernel and watch for interesting system calls. Additionally, likely because of this level of integration and detailed logging, it is used as the logger for SELinux.
All in all, it is a pretty fantastic tool for monitoring what’s happening on your system. Since it operates at the kernel level this gives us a hook into any system operation we want. We have the option to write a log any time a particular system call happens, whether that be unlink or getpid. We can monitor access to any file, all network traffic, really anything we want. The level of detail is pretty phenomenal and, since it operates at such a low level, the granularity of information is incredibly useful.
The biggest downfall is actually a result of the design that makes it so handy. This is itself a logging system and as a result does not use syslog. The good thing here is that it doesn’t have to rely on anything external to operate, so a typo in your (syslog|rsyslog|syslog-ng).conf file won’t result in losing your system audit logs. As a result you’ll have to manage all the audit logging using the auditd suite of tools. This means any kind of log collection, organization, or archiving may not work with these files, including remote logging. As an aside, auditd does have provisions for remote logging, however they are not as trivial as we’ve come to expect from syslog.
Thanks to the level of integration that it provides your auditd configurations can be quite complex, but I’ve found that there are primarily only two options you need to know.
- -a exit,always -S <syscall>
- -w <filename>
The first of these generates a log whenever the listed syslog exits, and whenever the listed file is modified. Seems pretty easy right? It certainly can be, but it does require some investigation into what system calls interest you, particularly if you’re not familiar with OS programming or POSIX. Fortunately for us there are some standards that give us some guidance on what to look out for. Let’s take, for example, the Center for Internet Security Red Hat Enterprise Linux 6 Benchmark. The relevant section is “5.2 Configure System Account (auditd)” starting on page 99. There is a large number of interesting examples listed, but for our purposes we’ll whittle those down to a more minimal and assume your /etc/audit/audit.rules looks like this.
# This file contains the auditctl rules that are loaded # whenever the audit daemon is started via the initscripts. # The rules are simply the parameters that would be passed # to auditctl. # First rule - delete all -DBased on our earlier discussion we should be able to see that we generate a log message every time any of the following system calls exit: adjtimex, settimeofday, stime, clock_settime, sethostname, setdomainname. This will let us know whenever the time gets changed or if the host or domain name of the system get changed.
# Increase the buffers to survive stress events. # Make this bigger for busy systems -b 1024 -a always,exit -S adjtimex -S settimeofday -S stime -k time-change -a always,exit -S clock_settime -k time-change -a always,exit -S sethostname -S setdomainname -k system-locale -w /etc/group -p wa -k identity -w /etc/passwd -p wa -k identity -w /etc/shadow -p wa -k identity -w /etc/sudoers -p wa -k identity -w /var/run/utmp -p wa -k session -w /var/log/wtmp -p wa -k session -w /var/log/btmp -p wa -k session -w /etc/selinux/ -p wa -k MAC-policy # Disable adding any additional rules - note that adding new rules will require a reboot -e 2
We’re also watching a few files. The first four (group, passwd, shadow, sudo) will let us know whenever users get added, modified, or privileges changed. The next three files (utmp, wtmp, btmp) store the current login state of each user, login/logout history, and failed login attempts respectively. So monitoring these will let us know any time an account is used, or failed login attempt, or more specifically whenever these files get changed which will include malicious covering of tracks. Lastly, we’re watching the directory ‘/etc/selinux/’. Directories are a special case in that this will cause the system to recursively monitor the files in that directory. There is a special caveat that you cannot watch ‘/’.
When watching files we also added the option ‘-p wa’. This tells auditd to only watch for (w)rites or (a)ttribute changes. It should be noted that for write (and read for that matter) we aren’t actually logging on those system calls. Instead we’re logging on ‘open’ if the appropriate flags are set.
It should also be said that the logs are also rather…complete. As an example I added the system call rule for sethostname to a Fedora 17 system, with audit version 2.2.1. This is the resultant log from running “hostname audit-test.home.private” as root.
type=SYSCALL msg=audit(1358306046.744:260): arch=c000003e syscall=170 success=yes exit=0 a0=2025010 a1=17 a2=7 a3=18 items=0 ppid=23922 pid=26742 auid=1000 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts4 ses=16 comm="hostname" exe="/usr/bin/hostname" subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key="system-locale"There are gobs of fields listed, however the ones that interest me the most are the various field names containing the letters “id”, “exe” and that ugly string of numbers in the first parens. The first bit, 1358306046.744, is the timestamp of the event in epoch time. The exe field contains the full path tot he binary that was executed. Useful, since we know what was run, but it does not contain the full command line including arguments. Not ideal.
Next we see that the command was run by root, since the euid is 0. Interestingly, the field auid (called audit uid) contains 1000, which is the uid of my regular user account on that host. The auid field actually contains the user id of the original logged in user for this login session. This means, that even though I used “su -” to gain a root shell the auditing subsystem still knows who I am. Using su to gain a root shell has always been the bane of account auditing, but the auditd system records information to usefully identify a user. It does not forgive the lack of command line options, but certainly makes me feel better about it.
These examples, while handy, are also only the tip of the iceberg. One would be hard pressed to find a way to get more detailed audit logging than is available here. To help make our way down the rabbit hole of auditd let’s turn this into a series. We’ll collect ideas for use cases and work up an audit config to meet the requirements, much like what I ended up doing on this security.stackexchange.com answer.
If this sounds like fun let me know in the comments and I’ll work up a way to collect the information. Until then…Happy Auditing!
There will be future auditd posts, so check back regularly on the auditd tag.
My workplace recently, for some definitions of recent, switched the company we use for certificate signing to InCommon. There were quite a few technical/administrative advantages, and since we’re educational, price was a big factor. Everyone has been really happy with the results. Well, except for this one thing. InCommon is not a top level trusted CA, they chain through AddTrust. This isn’t actually all that big a deal, really, as AddTrust is a common CA to have in your trusted bundle, and all we had to do was configure the InCommon chain certificate on our web servers. Other than the occasional chain breakage on some mobile browsers everything seemed peachy. Except, that is, when we ran a vulnerability scan.
Shortly after we switched we started noticing some odd alerts coming out of our vulnerability scans. At first one or two were reporting that the SSL certificate could not be validated. We manually verified the certificates, declared them as false positives, and moved on. Over time more and more systems started reporting this error. Eventually the problem had propagated out far enough that I started digging into it. For reference, the PluginID we’re looking at here is 51192.
I learned two very important, and relevant, pieces of information that day:
- Nessus was not properly validating the chain.
- Chain Certificate files are a little stranger than expected.
Instead of using a system default CA bundle, Nessus ships with its own. You can find the bundle, called known_CA.inc, in the plugin directory. So on Linux systems you should be looking at /opt/nessus/lib/nessus/plugins/known_CA.inc. If you are using a Windows scanner, well, you’re on your own. This is a fairly standard looking CA bundle, and I found that AddTrust was, in fact, included. I did not, however, find any reference to InCommon. Since they are somehow related to Internet2 I looked for them, also no luck.
This isn’t really that big a deal, though. Nessus will also look for, but will not update, a secondary bundle called custom_CA.inc. In most cases, this file would be used to include a local CA, for instance in a closed corporate network where one generates self-signed certificates as a matter of course. However, since you can use it to include arbitrary CA certs we can use it to fix our problem.
It’s easy enough for me to get the intermediate cert, what with it being public and all. This is where things started to get a little weird, though. In order to stay consistent with the known_CA.inc I included the certificate as a decoded X.509+PEM. Placing only the intermediate cert in this file resulted in, again, the certificate chain failing to validate. Next, what follows is a Nessus debugging tip that was roughly an hour’s worth of swearing in the discovering:
If you don’t think the web interface is showing you sufficient information, look at the plugin output in the raw XML.
You can get this by either exporting the report, or by finding it in the user’s reports folder on the scanner. What I discovered was that all of the various and sundry certificates were being read and validated. The chain, however, was being checked in the wrong order, in this case: webserver->AddTrust->InCommon.
After a little more trial and error I learned that, not only, did I need to have both the InCommon intermediate, but also the AddTrust certificates in my custom_CA.inc file, but that the order of the certs in the file also mattered. As it happens, AddTrust had to be entered first, followed by InCommon. This does make some amount of sense, when I adjusted my thought process to an actual chain where AddTrust was the “top-level”.
For completeness, I copied the newly complete custom_CA.inc file to my test webserver and included it as a chain cert using the SSLCertificateChainFile option. This is Apache httpd on Linux, you nginx or IIS folks are on your own. After removing the custom_CA.inc on the Nessus scanner and re-running the scan resulted in the certificate properly validating.
This left me in a good place in two ways:
- I now had a properly formatted custom_CA.inc file that I could put into puppet for all the scanners.
- I now also had a properly formatted chain cert file for inclusion on the web servers.
This fixes the problem from both sides, the server presenting all the correct information, as well as the scanner for cleaning up a false positive. For reference, included below is the chain cert file that was generated. As mentioned previously, it is the same format as a CA bundle. For each certificate you’ll find the ASCII text decoded certificate information, followed by the Base64 encoded PEM version of the same certificate. In my testing, Nessus would accept only the PEM versions, however I wanted to include both outputs since it appears to be the standard.
Certificate: Data: Version: 3 (0x2) Serial Number: 7f:71:c1:d3:a2:26:b0:d2:b1:13:f3:e6:81:67:64:3e Signature Algorithm: sha1WithRSAEncryption Issuer: C=SE, O=AddTrust AB, OU=AddTrust External TTP Network, CN=AddTrust External CA Root Validity Not Before: Dec 7 00:00:00 2010 GMT Not After : May 30 10:48:38 2020 GMT Subject: C=US, O=Internet2, OU=InCommon, CN=InCommon Server CA Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (2048 bit) Modulus: 00:97:7c:c7:c8:fe:b3:e9:20:6a:a3:a4:4f:8e:8e: 34:56:06:b3:7a:6c:aa:10:9b:48:61:2b:36:90:69: e3:34:0a:47:a7:bb:7b:de:aa:6a:fb:eb:82:95:8f: ca:1d:7f:af:75:a6:a8:4c:da:20:67:61:1a:0d:86: c1:ca:c1:87:af:ac:4e:e4:de:62:1b:2f:9d:b1:98: af:c6:01:fb:17:70:db:ac:14:59:ec:6f:3f:33:7f: a6:98:0b:e4:e2:38:af:f5:7f:85:6d:0e:74:04:9d: f6:27:86:c7:9b:8f:e7:71:2a:08:f4:03:02:40:63: 24:7d:40:57:8f:54:e0:54:7e:b6:13:48:61:f1:de: ce:0e:bd:b6:fa:4d:98:b2:d9:0d:8d:79:a6:e0:aa: cd:0c:91:9a:a5:df:ab:73:bb:ca:14:78:5c:47:29: a1:ca:c5:ba:9f:c7:da:60:f7:ff:e7:7f:f2:d9:da: a1:2d:0f:49:16:a7:d3:00:92:cf:8a:47:d9:4d:f8: d5:95:66:d3:74:f9:80:63:00:4f:4c:84:16:1f:b3: f5:24:1f:a1:4e:de:e8:95:d6:b2:0b:09:8b:2c:6b: c7:5c:2f:8c:63:c9:99:cb:52:b1:62:7b:73:01:62: 7f:63:6c:d8:68:a0:ee:6a:a8:8d:1f:29:f3:d0:18: ac:ad Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Authority Key Identifier: keyid:AD:BD:98:7A:34:B4:26:F7:FA:C4:26:54:EF:03:BD:E0:24:CB:54:1A
X509v3 Subject Key Identifier: 48:4F:5A:FA:2F:4A:9A:5E:E0:50:F3:6B:7B:55:A5:DE:F5:BE:34:5D X509v3 Key Usage: critical Certificate Sign, CRL Sign X509v3 Basic Constraints: critical CA:TRUE, pathlen:0 X509v3 Certificate Policies: Policy: X509v3 Any Policy
X509v3 CRL Distribution Points:
Full Name: URI:http://crl.usertrust.com/AddTrustExternalCARoot.crl
Authority Information Access: CA Issuers - URI:http://crt.usertrust.com/AddTrustExternalCARoot.p7c CA Issuers - URI:http://crt.usertrust.com/AddTrustUTNSGCCA.crt OCSP - URI:http://ocsp.usertrust.com
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This week, a newly-minted master’s degree holder asked on our site, “[How do I go about] Carrying out a professional IT audit procedure?” That’s a really big question in one sentence. In trying to break that down to some parts we can address, let’s look at the perspective people involved in an audit might see.
Staff at an audit client interact with auditors who ask a lot of questions off of a script that is usually referred to as a work plan. Many times, they ask you to open the appropriate settings dialogs or configuration files and show them the configuration details that they are interested in. Management at an audit client will discuss what systems beforehand and the managers for the auditors will select or create appropriate work plans.
The quality, nature, and scope of work plans vary widely, and a single audit will often involve the use of many different plans. Work plans might be written by the audit company or available from regulatory bodies, standards organizations, or software vendors. One example of a readily-available plan is the PCI DSS 2.0 standard. That plan displays both high-level overviews and mid-level configuration requirements across a broad spectrum. Low-level details would be operating system or application specific. Many audit plans related to banking applications have granular detail about core system configuration settings. Also have a look at this question about audit standards for law firms for an example from a different industry showing similarities and differences.
While some plans are regulatory-compliance related, most are best-practices focused. All plans are written with best practices in mind, and for those who are new to the world of IT security, that’s the most challenging part about them. There is no universal standard; many plans greatly overlap, but still differ. If the auditor is appropriately considering their client’s needs, they’ll almost certainly end up marking parts of plans as not applicable or not compliant yet okay because of mitigating circumstances.
Another challenging point for those new to auditing is the sometimes hard-to-grasp concept is the separation between objectives and controls. An objective might be to ensure that each authenticates to their own account. Controls listed in a work plan might include discussion about password expiry requirements to help prevent shared account passwords (among other things). Don’t get crossed-up focusing on the control if the objective is met through some other means – perhaps everybody is using biometric authentication instead. There are too many instances of this in the audit world, and it’s a common mistake among newer auditors.
From a good auditor’s perspective, meeting the goals of the work plan might be considered 60% of the goal of the work. An auditor’s job isn’t complete unless they’re looking at the whole organization. Some examples: to fulfill a question about password change requirements, the auditor should ask an administrator, see the configuration and ask users about it (“When was the last time the system made you change your password?”). To review a system setting, the auditor should ask to see settings in a general manner, adding detail only as needed: “Can you show me the net effect of policies on this user’s account?” as opposed to “Start->run->rsop.msc”. Users reporting different experiences about password resets than the system configuration shows or a system administrator who fumbles their way around for everything won’t ever be in the work plan, but it will be a concern.
With that background in mind, here are some general steps to performing an IT audit procedure:
- Meet with management and determine their needs. You should understand many of the possible accepted risks before you begin the engagement. For example, high-speed traders may stand to lose more money by implementing a firewall than not.
- Select appropriate audit plans based on available resources and your own relevant work.
- Properly review the controls with the objectives they meet in mind. Use multiple references when possible, and always try to confirm any settings directly.
- Pay attention to the big picture. Things should “feel right.”
- Review your findings with management and consider their thoughts on them. Many times the apparent severity of something needs to be adjusted.
- At the end of the day, sometimes a business unit may accept the risk from a weak control, despite it looking severe to you as an auditor. That is their prerogative, as long as you have correctly articulated the risks
The last part of that, auditors reviewing findings with client management, takes the most finesse and unexplainable skill. Does your finding really matter? How can you smooth things over and still deliver over 100 findings? At the end of the day, experience and repetition is the biggest part of delivering professional work, and that’s regardless of the kind of work.
Some further starting points for more detail can be found at http://en.wikipedia.org/wiki/Information_technology_audit_process and http://ithandbook.ffiec.gov/.