15. Debugging

Most unexpected behavior can be debugged by using the parameter --debug.

If you ever encounter a situation in which:

  • THOR doesn’t produce an alert on a known malicious element

  • THOR exits with an error

  • THOR takes a long time or unexpected short time on elements

15.1. Debugging Examples

Then try scanning that specific element with the --debug parameter set.

To run only a certain module use:

thor64.exe -a Mutex
thor64.exe -a FileScan
thor64.exe -a Eventlog

You can try to reduce the scope of a module even further by using lookbacks

thor64.exe -a Eventlog --lookback 3
thor64.exe -a FileScan -p C:\Windows\System32 --globallookback --lookback 1

To find out why a certain file couldn't be detected, use --debug with --printall and try to switch into intense mode.

thor64.exe -a Filescan -p C:\testfolder --debug --printall
thor64.exe -a Filescan -p C:\testfolder --debug --printall --intense

If it has been detected in intense mode but not in default mode, the file extension or the magic header is most likely the problem. You can adjust max_file_size in ./config/thor.yml or add a magic header in ./signatures/misc/file-type-signatures.cfg.

15.2. Finding Bottlenecks

You may get the error message "MODULE: RuntimeWatcher MESSAGE: Maximum runtime has exceeded, killing THOR" or encounter very slow or never-ending scans.

You can check the statistics table in "thor.db" on that end system after a scan to determine the last element or elements that took a long time to process.

We recommend using: https://sqlitebrowser.org/

The THOR DB is located at: C:\ProgramData\thor\thor.db

Find Bottlenecks

15.3. Most Frequent Causes of Missing Alerts

15.3.1. THOR didn’t scan file due to file size restrictions

Solution: Use --max_file_size parameter or set permanently in config file "./config/thor.yml". Also note that in lab scanning mode the default value is much bigger (--max_file_size_intense)

15.3.2. THOR didn’t scan the file due to a skipped deeper inspection

This can be caused by two reasons:

the magic header of that file is not in the list of interesting magic headers (see ./signatures/misc/file-type-signatures.cfg) AND file doesn’t have a relevant file extension (.asp, .vbs, .ps, .ps1, .rar, .tmp, .bas, .bat, .chm, .cmd, .com, .cpl, .crt, .dll, .exe, .hta, .js, .lnk, .msc, .ocx, .pcd, .pif, .pot, .pdf, .reg, .scr, .sct, .sys, .url, .vb, .vbe, .vbs, .wsc, .wsf, .wsh, .ct, .t, .input, .war, .jsp, .php, .asp, .aspx, .doc, .docx, .pdf, .xls, .xlsx, .ppt, .pptx, .tmp, .log, .dump, .pwd, .w, .txt, .conf, .cfg, .conf, .config, .psd1, .psm1, .ps1xml, .clixml, .psc1, .pssc, .pl, .www, .rdp, .jar, .docm, .ace, .job, .temp, .plg, .asm)

Solution: Use an intense scanning mode for that folder (--intense) or add the magic header to file-type-signatures.cfg (Warning: this file gets overwritten with an update; Warning: intense scanning mode threatens the scan's and system's stability).

15.3.3. THOR fails to initialize custom rules with the correct type

It happens very often that users that prepare custom IOCs or YARA rules forget to include the correct keyword in the filename of the IOC or YARA rule file.

The currect use of keywords is described in this chapter for IOCs and this chapter for YARA rules.

A wrong or missing keyword leads to situations in which a file that contains YARA rules that are meant to be applied to log files, doesn't contain a "log" keyword in it's name.

You can review a correct initialization in the command line output or log file.

Info Adding rule set from my-log-rules.yar as 'log' type

Using the keyword c2 for C2 IOCs in a filename should result in a line like the following:

Info Reading iocs from /tmp/thor10/custom-signatures/iocs/my-c2-iocs.txt as 'domains' type

15.4. Most Frequent Causes of Frozen Scans

Whenever THOR stops / pauses without any traceback or panic message and no error

Usually the following sources are responsible (descending order, by frequency):

  1. Antivirus or EDR suspends the THOR process (>90%)

  2. A frozen command line window due to Windows "Quick Edit Mode" (<10%)

  3. A constant high system load that causes THOR to stay back and wait for an idling CPU (<3%)

  4. The sensation of a stalled scan that is actually running (<3%)

15.4.1. Antivirus or EDR suspends THOR

In more than 95% and more of the cases, an Antivirus or EDR is responsible for a stalled process. Especially McAfee AV/EDR is a well-known source of issues. This is caused by the different dialogues in which exceptions have to be defined and the fact the certain kinds of blocks cannot be found in any logs.

If a THOR scans stalls in one of these modules, a Antivirus or EDR interaction is highly likely:

  • Mutex

  • Events

  • NamedPipes

  • ShimCache

  • ProcessCheck

Solution: Review all possible exclusions in your AV / EDR and add the THOR folder to the exclusion list

15.4.2. Windows Quick Edit Mode

Since Windows 10, the Windows command line window has the so-called "Quick Edit Mode" enabled by default. This mode causes a behaviour that can lead to a paused THOR scan process. Whenever a user switches the active windows from the cmd.exe to a different application, e.g. Windows Explorer, and clicks back into the command line window, the running process automatically gets suspended. A user has to press "Enter" to resume the suspended process. As the progress indicator of THOR isn't always changing, this could lead to the impression that the scan paused by itself.

See `this <https://stackoverflow.com/questions/30418886/how-and-why-does-quickedit-mode-in-command-prompt-freeze-applications`_ StackOverflow post for more details.

Solution: Press "Enter" in the command line window

15.4.3. Constant High System Load

Since THOR automatically sets a low process priority a scan can slow down to a level that appears to be paused or suspended on systems that are under a contant high load.

Solution: You can avoid this behaviour by using the --nolowprio flag. Be aware that scans on a system with a contant high CPU load take longer than on other systems and could slow down the processes that would otherwise take all the CPU capacity.

15.4.4. The Sensation of a Stalled Scan

Under certain circumstances the scan may appear stalled but is still running. You can always interrupt a scan using CTRL+C that brings THOR into the interrupt menu in which you can see the currently scanned element. In case of the "FileScan" module, this is a file or folder. In case of the "EventLog" module, this is an event with an ID. If you resume the scan by pressing "C" and interrupt it again a few minutes later, you should see another element in the interrupt menu.

If THOR still processes the same element for several hours, we recommend checking that element (size, format, access rights, location).

Solution: Check progress using the interrupt menu (CTRL+C)

15.5. Most Frequent Causes of Failed Scans

15.5.1. External Processes Terminating THOR

Whenever THOR dies without any traceback or panic message and no error message in the log file, an external process terminated the THOR process.

Usually the four following sources are responsible (descending order, by frequency):

  1. Antivirus or EDR killed the THOR process

  2. A user killed the THOR process

  3. A management solution that noticed a high CPU load caused by the THOR process killed it

  4. Attackers killed the THOR process

Note: A process termination that always happens at the same element is a sign for an Antivirus or EDR detection.

15.5.2. Insufficient Free Memory

fatal error: out of memory

Probable causes:

  1. Other processes consume a lot of memory

  2. THOR's scanning of certain elements requires a lot of memory

  3. You are using the wrong THOR version for your architecture

  4. You've actvated a feature that consumes a lot of memory (e.g. --mft or --intense)

Whenever THOR recognizes a low amount of free memory, THOR checks the top 3 memory consumers on the system and includes them in the log message that it writes before exiting.

You could try running THOR in Soft Mode (--soft) in which modules and features that require a lot of memory are deactivated.

Using the 32bit binary of thor named thor.exe on a 64bit system can lead to interrupted scans with this error message. The 32bit binary isn't able to address as much memory as the 64bit version. Always make sure to use the correct THOR version for the respective architecture.

15.6. Help Us With The Debugging

If you cannot find the source of a problem, please contact us using the support@nextron-systems.com email address.

You can help us find and debug the problem as quickly as possible by providing the following information.

15.6.1. Which THOR version do you use?

Tell us which THOR version you are using:

  1. for which operating system (Windows, Linux, macOS, AIX)

  2. for which architecure (32bit, 64bit)

Run thor --version and copy the resulting text into the email.

On Windows:

thor64.exe --version

On Linux:

./thor-linux-64 --version

On Mac OS:

./thor-macosx --version

This should produce a message like this:

THOR 10.6.6
Build bea8066 (2021-04-27 14:32:40)
YARA 4.0.5
PE-Sieve 0.2.8.5
OpenSSL 1.1.1j
Signature Database 2021/05/03-150936
Sigma Database 0.19.1-1749-g2f12c5c5

15.6.2. What is the target platform that THOR fails on?

Please provide the output of the following commands.

On Windows:

systeminfo > systeminfo.txt

On Linux:

uname -a

On Mac OS:

system_profiler -detailLevel mini > system_profile.txt

15.6.3. Which command line arguments do you use?

Please provide a complete list of command line arguments that you've used when the error occurred.

thor64.exe --quick -e D:\logs -p C:\Windows\System32

15.6.4. Provide the log of a scan with --debug flag

The most important element is a scan log of a scan with set --debug flag.

The easiest way is to run the scan exactly as you've run it when the problem occured adding the --debug command line flag.

e.g.

thor64.exe --quick -e D:\logs -p C:\Windows\System32 --debug

If you're able to pinpoint the error to a certain module, you could limit the scan to that module to get to the problematic element more quickly, e.g.

thor64.exe -a Rootkit --debug

After the scan you will find the normal text log (*.txt) in the program folder. It is okay to replace confidential information like the hostname or IP addresses.

Note: The debug log files can be pretty big, so please compress the file before submitting it to us. Normal log files have a size between 1 and 4 MB. Scans started with the --debug flag typically have sizes of 30-200 MB. The compression ratio is typically between 2-4%, so a compressed file shouldn't be larger than 10 MB.

15.6.5. Provide a Screenshot (Optional)

Sometimes errors lead to panics of the executables, which causes the situation that the relevant log lines don't appear in the log file. In these cases, please also create a screenshot of a panic shown in the command line window.

15.6.6. Provide the THOR database (Optional)

The THOR DB helps us debug situations in which the THOR scan timed out or didn't complete at all. It contains statistics on the run time of all used modules and the durations of all folders up to the second folder level from the root of a partition. (e.g. C:\Windows\SysWow64).

The default location of that file is:

  • Windows: C:\ProgramData\thor\thor.db

  • Linux/macOS: /var/lib/thor/thor.db

Please provide that file in situations in which:

  • THOR exceeded its maximum run time

  • THOR froze and didn't complete a scan for days

  • THOR scans take too long for the selected scan targets

15.6.7. Further Notes

  • If the files are too big to send, even after compression, please contact us and you'll receive a file upload link that you can use

  • If a certain file or element (eventlog, registry hive) caused the issue, please check if you can provide that file or element for our analysis