I’ll show you why APT detection is difficult – for the big players and spirited newcomers like us.
Metadata is the Key
Only recently I recognized and named the methods that we apply since we introduced the scoring system in our scanner product. Instead of looking at a file only by its content we collect numerous attributes and evaluate a score based on certain rules that indicate conspicuous features or anomalies.
What I recognized was that Metadata is the key to successful APT detection. Let me give you some examples.
The “Sticky Keys” Backdoor
During our investigations we found that the attackers used a simple backdoor that allowed them to avoid AV detection and use tools that were already available on the target systems. What they did was to copy a valid “cmd.exe” over the “sethc.exe” in the System32 folder in order to establish a backdoor that waits for the user pressing five times shift consecutively on a RDP logon screen and pops up a Windows command line running as LOCAL_SYSTEM. Another method sets the Windows command line as debugger for the stickykeys binary.
"C:\Windows\system32\reg.exe add "HKLM\SOFTWARE\Microsoft\Windows
NT\CurrentVersion\Image File Execution Options\sethc.exe"
/v "Debugger" /t REG_SZ /d "cmd.exe" /f"
With the necessary rights it is easy to install and difficult to detect.
As I already said, an Antivirus engine won’t detect this backdoor as the content of the file is a valid Windows executable with an intact signature. Windows 7+ users won’t stumble over it as they have Network Level Authentication (NLA) enabled by default, which prompts the user for username and password before fully establishing a Terminal Services connection. Attackers modify their local “default.rdp” file and add “enablecredSSPsupport:i:0:1” in order to disable this behaviour.
APT detection therefore means the following:
- Check system files like the stickykey binary for modifications – not by comparing MD5 hashes from whitelist databases like the NSRL, but by comparing the expected content for a certain file name with the actual content of the file. I described this method in a blog article and Chad Tilbury from Crowdstrike described how to apply this method using their CrowdResponse tool.
- Identify “default.rdp” files on server systems that have NLA disabled. (Administrators shouldn’t do that)
- Check if the Windows command line (cmd.exe) is registered as a debugger for any program
PsExec’s Evil Clone
I often use the example of the well-known Sysinternals tool “PsExec”, which is likewise used by administrators and APT groups. It doesn’t make much sense adding it to the indicators of compromise (IOCs) of your triage sweep although it may have played a substantial role in lateral attacker movement.
The human eye is able to distinguish between a PsExec that has been used for administration and a PsExec that has been used by the attackers. The essential difference, which enables us to distinguish between both versions does not lie in the content of the file but the Metadata.
Look at the following table and tell me which of both files is valid and which has been placed on the system by the attackers. Remember, the file content is the same – a MD5 hash of both files is equal.
|File 1||File 2|
|Modified Time Stamp||2013-02-10 09:22:04||1970-01-01 00:00:00|
It is not that difficult, isn’t it?
APT detection therefore means the following:
- Imitating the human point of view by pulling together all Metadata connected with an element, be it a file, a process or eventlog message and evaluate the legitimacy of the element based on all available Metadata attributes
The Simplest Webshell
We often encounter so-called webshells that were placed in web server directories to establish a simple backdoor. Webshells can be very specific and therefore easy to detect. The C99 webshell is a good example for a PHP webshell, JspSpy is a well-known JSP webshell. Both are easily detected, even by Antivirus engines (see: C99 on VT, JSPSpy on VT).
However, APT groups tend to use two different types of webshells:
- Tiny webshells
- Code snippets with certain functions copied from legitimate software
There are a lot of well-known tiny webshells. The following one is my favorite. Add a space or change the request parameter “abc” to something else and the detection ratio is alarmingly low (Example). It allows an attacker to evaluate (execute) an arbitrary command on the web server. There are numerous blog posts and other articles describing what can be done with a webshell like this. However, the protection level provided by AV engines, firewalls and NIDS is almost zero.
Another method we discovered was the use code snippets copied from blog entries or tutorial pages that allowed them to use only certain functions like “file upload” or “directory listing”.
They often use a weakness in web applications to upload and run their own scripts or even whole application containers (.war). By placing a known webshell like the JspSpy webshell into that web server folder, they would run the risk of being detected. What they really need is a distribution point for their toolset or a simple tool to execute code on the server (like a tiny webshell). We’ve seen simple upload scripts that provide nothing more than a upload function, which they use to store their toolsets for lateral movement. A google search for “upload jsp” revealed various scripts they used in their attacks. It’s obvious that AV engines won’t detect this type of threat. How could they? The attackers abuse benign pieces of code to establish malicious backdoors.
APT detection therefore means the following:
- Use the Metadata like file size, creation timestamp, file extension in combination with generic content detection rules – e.g. check for the string “eval” in a file smaller 40 byte and a script extension like “.jsp”.
- Check the content of upload directories for the expected file types (and don’t use the extension to determine the file type)
- Check web server processes for executables running in the web server directories – e.g. curl.exe in D:\Inetpub\wwwroot
- Generate and send frequent reports on modified files within the web server directories
The Heavy Burden of Definite Detection
One could be tempted to believe that I wrote this article in order to degrade Antivirus engines, but this isn’t the case. Antivirus solutions are still play a key role and carry the heavy burden of definite detection. Their scan result has to be “thumbs up” or “thumbs down” as there is no middle ground.
Years ago they introduced signatures to detect “Potentially Unwanted Applications” (PUA). Users or administrators decide on what to do if one of these “dual use” tools has been found on a system. Handling thousands of the events generated by the Antivirus agents is a difficult task, even with a central console or SIEM integrated log files. It is easy to understand why PUA events do not play an important role in view of dozens of Trojan detections per day.
APT detection is the art of suspicion. A missing “stickykeys” string in the “sethc.exe” indicates a manipulation, a replaced system file. It is not a definite detection but the certainty that something is wrong.
Considering the given examples an attentive reader may be inclined to believe that Antivirus and simple IOC scanning (Triage) is not enough to defend against Advanced Persistent Threats. After the experiences of the last 3 years I have to confirm that assumption.
Who would recognize and report the execution of a “sethc.exe” on a server system, the “PUA/PsExec” message generate by the Antivirus or another JSP file on the web server?
I even doubt that so-called “APT solutions” are able to detect
- a “.war”-file upload to a Tomcat server by the use of “tomcat/tomcat” as credentials,
- encrypted file uploads,
- lateral movement using PsExec, Powershell or WMIC and
- “StickyKey” backdoor access via RDP.
An extensive security monitoring in form of a SIEM system allows you to detect a needle in the haystack but only if you are able to distinguish between straw and needles.
The question is: How can I define such soft indicators to detect the described anomalies? The OpenIOC framework already contains options to combine certain characteristics like filename and filesize, but rather than using it as a tool to describe anomalies it is often used to tighten the detection to the level of a hash value. I prefer hash values over “Name:PsExec.exe” combined with “Filesize:381816” because it doesn’t make you believe that you’re looking at a clever rule.
I therefore recommend the following:
- Assume compromise and start from there
Ask yourself: How would I detect a breach? What if attackers already took control of the Windows Domain and worked with domain admin accounts? What if they worked with tools that my Antivirus is unable to detect?
- Use all Metadata you can get to determine the legitimacy of an element
This does no only apply to files or processes in APT and IOC scanning, but also to the discipline of security monitoring.E.g. Select interesting Antivirus events based on various characteristics and not only the status that indicates if the malware has be removed or not. Consider the location of the malware (Temporary Internet files or System32 folder), the user account (Restricted user, Administrator or LOCAL_SYSTEM), the malware type (JS/Redirector or PUA/PasswordDump), system type (server or client workstation), detection time (02:00am with noone working in the night shift), detected form (in a RAR archive or extracted). Develop similar schemes for other log types. The most interesting ones are Antivirus, Proxy and Windows logs.
SysForensics published an article about process anomaly detection, which was adapted to other OS versions and included in our THOR APT Scanner and my LOKI IOC Scanner.
Use SysInternals Sysmon to enrich you Windows log data.
- Don’t create detection rules that are too tight but concentrate on filtering the false positives
If you regard filtering false positives as a pain in the neck you’re probably using the wrong SIEM system. (Warning – product placement: I prefer Splunk over all others, especially with the Enterprise Security App installed)
- Use IOCs from published APT reports to enrich your detection rules
We use the APT reports to create new rules in our customer’s SIEM systems and as input for our APT scanner THOR.Useful Links:
APT Notes is a IOC repository with hundreds of reports from the last years. You can download the github repository from here.