13. Other Topics

13.1. License Retrieval

THOR allows for a more flexible way to fetch licenses, besides the classic way of placing a license file in the program folder. In this chapter we will show both available options for license retrieval.

It is important to know that those two options also work with THOR Remote. In this case, all licenses will be downloaded to the host which is running the initial THOR Remote command (the host running THOR Remote does not need a license).

Important

If you have already a valid THOR license for your host placed within THOR's program folder, no (new) license will be downloaded/issued from the remote locations.

13.1.1. ASGARD License Retrieval

If you are having a local instance of the ASGARD Management Center installed and using its license pool for your THOR scans, you can use the --asgard flag to download a valid license. This flag also needs the --asgard-token flag to work. The Token can be found in the Download section of your ASGARD Management Center.

Example:

nextron@unix:~/thor$ ./thor-linux-64 --asgard "my-asgard.local" --asgard-token "download-token"
[...SNIP...]
Info License file found LICENSE: my-asgard.local OWNER: John Doe TYPE: Server STARTS: 2023/08/30 EXPIRES: 2023/11/01 SCANNER: THOR VALID: true REASON:

The retrieved license will be placed in the program folder of THOR, so you can run THOR the next time without all the extra flags. The name of the license is <hostname>.lic. Rerunning the command will not issue a new license, but rather download the already valid license again from your ASGARD Management Center.

13.1.2. Nextron Portal License Retrieval

If you are using standalone packages of THOR, you can speed up the process of deploying THOR by using its --portal-key flag. This allows you to download a THOR license straight from the Licensing Portal, without the need to generate all the licenses and downloading them manually first. This is a good way to automate scanning.

The argument --portal-key is expecting one argument, which can be one of:

The API key of your Portal user

A download token for one of your contracts

When using an API key, THOR will grab the first available license from the contract with the lowest ID and issue one to your host. No new license will be issued if a valid license was found for the host. If no valid license was found, a new one will be issued. This also means that you should provide the --portal-contracts flag if you want to limit THOR to a specific contract(s) for issuing/downloading licenses.

Example:

nextron@unix:~/thor$ ./thor-linux-64 --portal-key "my-api-key" --portal-contracts "3,5,12,13"
...SNIP...
Info License file found LICENSE: portal.nextron-systems.com OWNER: Jane Doe TYPE: Server STARTS: 2023/03/10 EXPIRES: 2023/09/29 SCANNER: THOR VALID: true REASON:

The retrieved license will be placed in the program folder of THOR, so you can run THOR the next time without all the extra flags. The name of the license is <hostname>.lic. Rerunning the command will not issue a new license, but rather download the already valid license again from the portal.

Attention

If no valid license is found, a new one will be issued. This can be prevented with the --portal-nonewlic flag. If THOR can't find a valid license within the account/contracts, it will simply exit. This is a useful feature if you want to prevent over-issuing of licenses within your contracts.

13.2. Evidence Collection

13.2.1. Process Memory Dumps (--dump-procs)

Since THOR version 10.5 it supports process dumping to backup volatile malware information.

THOR on Windows creates a process dump of any process that is considered malicious. Maliciousness is determined as anything that triggers a warning or an alert.

Activate process memory dumping with --dump-procs.

This process dump can then be analyzed with standard tools later on to examine the found malware.

To prevent flooding the disk fully in case many dumps are created, old dumps of a process are overwritten if a new dump is generated. Also, THOR will not generate dumps by default if less than 5 GB disk space is available. This can be overwritten to always or never dump malicious processes.

Also note that THOR will never dump lsass.exe to prevent these dumps from potentially being used to extract passwords by any attackers.

13.2.2. File Collection (Bifrost)

13.2.2.1. Bifrost v1 with Script-Based Server

The ./tools folder in the program directory contains a simple Python based file collection server named Bifrost. The script is named bifrost-server.py.

You can run that script on any internal system with a Python script interpreter installed. By default, it uses port 1400/tcp for incoming connections but you can use any port you like.

Usage is:

nextron@unix:~$ python ./bifrost-server.py -h
usage: bifrost-server.py [-h] [-d out-dir] [-i ip] [-p port]

Bifrost optional arguments:

-h, --help  show this help message and exit
-d out-dir  Quarantine directory
-i ip       IP address to bind to
-p port     Port to bind to (tcp, default 1400)

You can run the server script with:

nextron@unix:~$ python ./bifrost-server.py

In order to send suspicious file to that server, you have to set some command line flags when running THOR, e.g.

C:\nextron\thor>thor64.exe --bifrostServer myserver

A more complex statement setting a minimum score and custom port would look like this:

C:\nextron\thor>thor64.exe --bifrostServer myserver --bifrost-port 8080 --bifrostLevel 80

THOR will then try to submit all samples with score equal or higher than 80 to a Bifrost service running on myserver port 8080/tcp.

13.2.2.2. Bifrost v2 with ASGARD

Bifrost v2 cannot be used standalone yet. The required API Key is set by ASGARD v2 during initialization and is unknown to a THOR user.

You can activate the quarantine function via Bifrost v2 when creating a single or group scan via the ASGARD management interface.

Configure Quarantine via Bifrost in New Scan Dialogue

13.3. Resource Control

THOR's internal resource control feature puts the system's stability and the responsiveness of running services first.

Resource control is active by default. You can deactivate it using --norescontrol.

Be advised that due to Resource Control, the THOR scan may terminate its completion. The scan gets terminated under the following conditions:

If the available physical memory drops below 60MB
If more than 60 MB of log data have been written (disk / syslog)

In this case, THOR switches in the "reduced-logging" mode in which it only transmits "Notices, Warnings and Alerts" and after another 4 MB of log data THOR terminates itself in order to prevent log flooding due to a high number of false positives

If the scan constantly terminates you should check what causes the performance issues or choose times with less workload (e.g. weekends, night). To debug such states, you can check the last warning that THOR generates before exiting the scan. It includes the top memory consumers that could have caused the memory exhaustion.

Warning

Deactivating Resource Control on systems with exhausted resources can put the system's stability at risk.

13.3.1. Automatic Soft Mode

Soft mode is automatically activated on systems with low hardware resources.

One of the following conditions activates soft mode:

Less than 2 CPU cores
Less than 1024 MB of RAM

In Soft mode several checks and features that could risk system's stability or could provoke an Antivirus or HIDS to intervene with the scanner are disabled. See Scan Modes for a complete overview.

13.4. Scoring System

The scoring system is one of THOR's most prominent features. Both YARA signatures and filename IOCs contain a score field. The score is an integer value that can be negative to reduce the score on elements that are prone to false positives.

Only YARA rules and Filename IOCs support a user defined score. But since you are able to write YARA rules for almost every module, the scoring system is very flexible.

The total score of an element determines the level/severity of the resulting log message.

THOR <= 10.6
Score	Level	Condition
40	Notice
60	Warning
100	Alert	At least 1 sub score more than 75

THOR >= 10.7
Score	Level	Condition
>= 40	Notice
>= 60	Warning
> 80	Alert	At least 1 sub score more than 75

Note

As of THOR version 10.7, we reworked the scoring system to only use scores between 0 and 100. The score is a metric that expresses a combination of confidence and severity in percent. This means a finding with a score of 95 can be seen as a severe finding with a high confidence. Exceptions might be - as always - obvious false positives like unencrypted or in-memory AV signatures.

13.4.1. Scoring per Signature Type Match

Type	Score
YARA match	Defined in the meta data of the YARA rule as integer value (e.g. "score = 50")
Filename IOC match	Defined in the 2^nd field of the CSV (e.g. `\\evil.exe;80`)
Keyword IOC match	"warning" level messages, see Default Scores
C2 IOC match	"warning" and "alert" level massages, see Default Scores

13.4.2. Accumulated Scores

If an element has multiple sub-scores, all sub-scores will be accumulated and calculated into one final score. The following chapters show you how those scores are calculated.

13.4.2.1. THOR <= 10.6

Module

Cumulated Scoring

Score

Filescan
Archive Scan
DeepDive
Prefetch
WER

Yes

Score is a sum of the scores of all "REASONs" (YARA matches, filename IOCs, other anomalies)

Note 1: Only positive scores are shown by default

Note 2: Only the top 2 reasons are shown by default (use --allreasons to show all positive scores)

All Other Modules

No

Individual score of each signature match (YARA, filename IOC, keywords, C2)

Note 1: This means that multiple matches for a single element are possible

13.4.2.2. THOR >= 10.7

Most modules and features summarize via reasons. Please keep in mind that only positive scores and the top two reasons are shown by default. You can use --allreasons to show all positive scores.

Reason scores are not added up for the total score. Instead, given a number of scores (s_0, s_1, ...) that are ordered descending. The total score is calculated with the following formula:

100 * (1 - (1 - s_0 / 100 / 2^0) * (1 - s_1 / 100 / 2^1)  * (1 - s_2 / 100 / 2^2) * ...)

This means, scores are "capped" at a maximum of 100, and multiple lower scores are weighted far less.

You can use python to calculate the score and try the formula. Please note that we use an example with five sub-scores and no sub-score higher than the threshold of 75 to turn classify this as an alert:

subscore0 = 1 - 70 / 100 / pow(2, 0)
subscore1 = 1 - 70 / 100 / pow(2, 1)
subscore2 = 1 - 50 / 100 / pow(2, 2)
subscore3 = 1 - 40 / 100 / pow(2, 3)
subscore4 = 1 - 40 / 100 / pow(2, 4)
score = 100 * (1 - (subscore0 * subscore1 * subscore2 * subscore3 * subscore4))
print(score)
84.195859375

13.4.3. Default Scores

If no score is set in an "alert" or "warning" message, THOR automatically appends a score that corresponds to the message level:

THOR <= 10.6
Level	Score
Warning	70
Alert	100

THOR >= 10.7
Level	Score
Warning	60
Alert	80

13.4.4. Exception: High total score with low sub scores

"Alerts" on file system elements are only generated if one of the sub scores is more than 75.

Before that change, multiple low scoring reasons had led to a score higher 100 and caused an "Alert" level message although not a single hard match was included in the "Reasons". A wrong extension, e.g. .txt for an executable, which is often used by employees to hand executables through tight mail filters, and a suspicious location, e.g. C:\Temp\funprog.txt caused an "Alert" level message.

Since version 8.27.2, one of the sub scores that pushes the total score over 100 has to be more than 75. (internally calculated as "alert_level - 25" because the user can adjust the alert level via the --alert parameter)

13.4.5. Exception: Filename IOC Matches

The "Filename IOC Check" is a sub check of the "String Check", which is applied to many elements, like Eventlog messages or Registry keys.

The function checkString() receives a string as input and returns possible matches.

The string is checked in multiple sub-checks against different signature lists. The most important sub-checks are checkKeyword() and checkFilename().

While the checkKeyword() sub-check returns each individual match, the checkFilename() sub check accumulates the score of all matches and returns a single total score. It is possible that many different filename signatures have matched on that string but only one match with a total score is reported. This is an exception to the usual behavior where only the "FileScan" module accumulates scores.

13.4.5.1. Filename IOC Matching in String Check Example

Imagine the following filename IOC signatures:

\\nmap.exe;70
\\bin\\nmap.exe;-30

and the following Keyword signature:

nmap.exe

The checkString() function receives the following string from the Eventlog scan module (here: a Sysmon Eventlog entry):

Process Create:
UtcTime: 20180110 10:22:25.277
ProcessGuid: {c1b49677e9615a5500000010bbc80702}
ProcessId: 3912
Image: C:\\Program Files\\Nmap\\bin\\nmap.exe
CommandLine: nmap.exe
CurrentDirectory: C:\\Windows\\system32\\
User: PROMETHEUS\\user1
LogonGuid: {c1b496771d725a5300000020d4232500}
LogonId: 0x2523d4
TerminalSessionId: 1
IntegrityLevel: High
Hashes: SHA1=F5DC12D658402900A2B01AF2F018D113619B96B8, MD5=9FEA051A9585F2A303D55745B4BF63AA
ParentProcessGuid: {c1b496771d745a530000001057452500}
ParentProcessId: 1036
ParentImage: C:\\Windows\\explorer.exe
ParentCommandLine: C:\\Windows\\Explorer.EXE

The checkString() function would create two messages: 1 "warning" for the keyword signature and 1 "notice" of the filename IOC signatures.

The keyword IOC matches in the checkKeyword() sub-check and checkString() returns a match, that generates a "Warning" level message that automatically receives a score of 75 (see Default Scores).

The filename IOCs would both match on the string in the checkFilename() sub-check and both scores would be summed up to a total score of 40 (70 + (-30) = 40), which would generate a "Notice".

13.5. Action on Match

The action command allows you define a command that runs whenever THOR encounters a file during "Filescan" that has a certain total score or higher. The default score that triggers the action command (if set) is 40.

The most popular use case for the action command is sample collection.

13.5.1. Action Flags

Parameter	Description
--action_command string	Run this command for each file that has a score greater than the score from `--action_level`
--action_args strings	Arguments to pass to the command specified via `--action_command`. The placeholders `%filename%`, `%filepath%`, `%file%`, `%ext%`, `%md5%`, `%score%` and `%date%` are replaced at execution time
--action_level int	Only run the command from `--action_command` for files with at least this score (default `40`)

13.5.2. Command Line Use

A typical use would be e.g. to copy a sample to a network share:

C:\Users\nextron>copy %filepath% \\\\server\\share1

To instruct THOR to run this command, you need

C:\nextron\thor>thor64.exe --action_command copy --action_args %filepath% --action_args \\server\share1

13.5.3. Use in a Config File

The ./config folder contains a template for a config file that uses the action commands.

Content of 'tmpl-action.yml':

# Action to perform if file has been detected with a score more than the defined 'action_level'
# You may use all environment variables that are available on the system, i.e. %COMPUTERNAME%.
# Further available meta vars are:
# %score% = Score
# %file% = Filename without extension
# %filename% = Basename
# %filepath% = Full path
# %ext% = Extension without dot
# %md5% = MD5 value
# %date% = Detection time stamp

action_level: 35
action_command: "copy"
action_args:
- "%filepath%"
- "\\\\VBOXSVR\\Downloads\\restore_files\\%COMPUTERNAME%_%md5%_%file%_%ext%_%date%"

13.6. THOR DB

This simple SQLite database is created by default in the "%ProgramData%\thor" (Linux, macOS: /var/lib/thor/) directory as "thor10.db". You can deactivate THOR DB and all its features by using the --nothordb flag.

It stores persistent information over several scan runs:

Scan State Information
- This information is used to resume scan runs where they were stopped
Delta Comparison
- This detection feature allows to compare the result of a former module check with the current results and indicate suspicious changes between scan runs

The THOR DB related command line options are:

Parameter	Description
--nothordb	Disables THOR DB completely. All related features will be disabled as well.
--dbfile [string]	Allows to define a location of the THOR database file. File names or path names are allowed. If a path is given, the database file `thor10.db` will be created in the directory. Environment variables are expanded.
--resume	Resumes a previous scan (if scan state information is still available and the exact same command line arguments are used)
--resumeonly	Only resume a scan if a scan state is available. Do not run a full scan if no scan state can be found.

13.6.1. Resume a Scan

THOR tries to resume a scan when you set the --resume parameter. Since THOR version 10.5 the resume state doesn't get tracked by default due to its significant performance implications. If you want to be able to resume a scan, you have to start scans with the --resume flag. If you start a scan and a previous resume state is present, then THOR is going to resume the interrupted scan.

It will only resume the previous scan if

You have started the scan with --resume
The argument list is exactly the same as in the first scan attempt
You haven't used the flag --nothordb
Scan state information is still available (could have been cleared by running THOR a second time without the --resume parameter)

You can always clear the resume state and discard an old state by running thor.exe once without using the --resume parameter.

13.6.2. Delta Comparison

The delta comparison feature allows comparing former scan results on a system with the current results, indicating changes in system configurations and system components.

Currently, the following scan modules feature the delta comparison check:

Autoruns
- THOR compares the output of the Autoruns module with the output of the last scan run. The Autoruns module does not only check "Autorun" locations but also elements like browser plugins, drivers, LSA providers, WMI objects and scheduled tasks.
Services
- The comparison detects new service entries and reports them.
Hosts
- New or changed entries in the "hosts" file could indicate system manipulations by attackers to block certain security functions or intercept connections.

13.7. Archive Scan

The Archive Scan feature supports the following archive types:

ZIP
RAR
TAR
TAR + GZIP (.tar.gz)
TAR + BZIP2 (.tar.bz2)
GZIP (THOR 10.7+)
7ZIP (THOR 10.7+)
CAB (THOR 10.7+)

When scanning a file within any of these file types, THOR will append the path within the archive to the archive's own path for scan purposes (like filename IOCs or YARA rules). For example, an archive C:\temp\test.zip containing a file path/in/zip.txt will cause the simulated path to be C:\temp\test.zip\path\in\zip.txt.