Silent Scanning – Compromise Assessment with THOR Lite on a Compromised Exchange 2019 Server

The following video shows a compromise assessment with our free THOR Lite scanner on a Microsoft Exchange 2019 server detecting ProxyShell and ProxyToken exploitation.

We’ve done no post-editing in this video. You can jump to all findings using the video chapters. You’ll see log entries, web shells and a modified IIS server configuration as reported by HuntressLabs in various reports. We added some Synth-wave tracks to create the right atmosphere. Enjoy.

By the way, we compiled a blog article regarding compromise assessments of Exchange servers with THOR Lite to detect ProxyLogon exploitation with some recommendations that still apply. You can find that blog post here

Use YARA math Module Extension in THOR TechPreview and THOR Lite

Not long ago, we’ve created a pull request for the official YARA repository on Github, that would introduce new functions in the `math` module to improve the flexibility in cases in which a sample is heavily scrambled or obfuscated. These cases require further statistical evaluations that go beyond the currently available “entropy”, “mean” or “deviation” functions.

The example on the right shows a heavily obfuscated PHP web shell, as used by a Chinese actor. 

You immediately notice the high amount of “%” characters, but since each of them is preceded and followed by different characters, it’s difficult to find atoms that are long enough to maintain an acceptable performance / stability of that rule. 

 

If you could, you would formulate a rule like this: “Detect files smaller 400 bytes, that begin with ‘<?’ and consist of at least 25 percent ‘%’ characters”. 

Well, the new module extension allows you to do exactly that.

Read the documentation provided with the pull request for details on all three new functions:

  • count(byte/string, offset, size)
  • percentage(byte, offset, size)
  • mode(offset, size)

While the first two functions are self-explanatory, the “mode” function isn’t. It is is a term used in statistics for the most common value.

For your convenience, we’ve already patched our versions of THOR TechPreview and THOR Lite to support these extensions of the “math” module. You need at least v10.6.6 to use the new function in your rules. 

We wish you good hunting. 

THOR 10.6.8 TechPreview with ETW Watcher to Detect CobaltStrike Beacons

THOR TechPreview version 10.6.8 will introduce a completely new module named ETW Watcher, which runs in a separate thread and monitors the systems during a scan run. As its name indicates, the ETW Watcher module makes use of Event Tracing for Windows (ETW). 

So, whenever you start a scan run on an end system, one thread performs all the usual checks while another thread analyses certain event channels and correlates events to detect malicious activity.

Consider it an agent-less, portable, short term EDR. 

The first task of this new module is the detection of C2 beaconing activity as e.g. used in attacks that utilize Cobalt Strike.

The following screenshots show messages created by the ETW Watcher module. Since all of our modules apply a so called message enrichment, you’ll also notice further messages before and after the highlighted events. These additional messages are generated during the enrichment of the original event.

“Enrichment” means that we add additional information to the original event – e.g. if a file path is given in the original message, THOR tries to find that file, scans it using the “FileScan” module and adds information to the original event. The same is true for process ID values. This adds as much meta data as possible and helps analysts to assess the event as quickly and easily as possible.  

The example above shows a beacon detection that mentions a process named “fnord.exe” frequently calling out to 10.0.2.15 via HTTP and TCP. Message enrichment shows the result of a file scan above (red alert message; appears before the actual event because the enrichment happens before the message composition). 

The next example shows the result of a “hashdump” command sent to the beacon. It causes the beacon to open a handle to the LSASS process memory, which THOR detects and reports as a Warning level message.

The next example shows a privilege escalation attempt performed by that same beacon. 

The ETW Watcher module will be integrated in THOR TechPreview version 10.6.8 and only is available on Windows. It will not be available in THOR Lite and THOR 10.5. 

New Detection Rules for Exchange Exploitation Activity

Last week, we’ve released a blog post on how to detect HAFNIUM activity with the use of THOR Lite. Since our first set of rules, we’ve added several important new rules from fellow researchers and moved even more rules from our commercial set into the open source rule set.

This alone would be reason enough to recommend another scan. But during the last three days, we’ve added a special group of rules (see below) and fixed some bugs in the code base of THOR that could have lead to false negative on some of the relevant log files (exclusion under certain conditions).

We therefore recommend a signature update, an upgrade to THOR v10.5.12 (THOR TechPreview v10.6.4) and a new scan run to uncover traces of hacking activity using the newest detection rules.

The following sections explain the extended coverage.

Compiled ASPX Files

We’ve added rules for the compiled ASPX files that often remain on a system even in cases in which an attacker has removed the original web shell.

These are perfect rules to uncover actual post-exploitation attacker activity and not “just an exploitation” and a webshell drop.

You can find more information on the creation and meaning of these forensic artefacts in this Trustwave blog post.

(Source: Trustwave)

Improved Generic Webshell Coverage

Arnim Rupp provided many improvements to its public rule set that detect all kinds of webshells based on generic characteristcs. 

Frequent updates improved these rules and extended the coverage to include the newest unknown webshells mentioned in the most recent reports. 

More Filename IOCs

Over the last few days we’ve added many new filename IOCs mentioned in reports by ESET and others. 

The ESET report mentions and lists IOCs of 10 different APT groups exploiting the Exchange vulnerbility and leaving traces on compromised systems.

Rule Improvements

We’ve improved several rules to extend their coverage.

E.g. the rule that looked for POST requests to a single letter JavaScript file now looks for a certain pattern that includes exploitation attempts with the new Metasploit module.

Due to all the mentioned improvements and bugfixes, we recommend another scan run on your Exchange servers. The following commands upgrade THOR and its signature set.

THOR

thor-util.exe upgrade

THOR Lite

thor-lite-util.exe upgrade

Remember these recommendations from the initial blog post:

  • If you’ve installed Exchange on a drive other than C: use `–allhds`
  • Use `–sigma` feature when scanning with THOR (not available in THOR Lite)
  • Add the following exclusion to the file `./config/directory-excludes.cfg` to skip all mailbox directories:

\\(MDBDATA|Mailbox|Mailbox Database)\\

Which extra value provides THOR in Exchange ProxyLogon related assessments?

Since we’ve decided to migrate many of the HAFNIUM / Exchange vulnerability related signatures into the open source signature database of our free scanner THOR Lite, both users of the free and the commercial version started asking questions of coverage and if a scan of the respective other version is still recommended.

This blog post tries to shed some light on the issue by pointing out the differences between both scanners regarding coverage, scan intensity and availability of signatures.

The obvious advantage of THOR Lite – which is usually one of the disadvantages – is the immediate availability of untested new YARA signatures. While users usually prefer tested signatures that won’t cause hundreds or thousands of false positives, in case of the ProxyLogon vulnerability, new releases of rules cannot be fast enough.

So the obvious and only advantage of THOR Lite is that it receives rule updates multiple times a day, while THOR currently gets new signatures every 1-2 days.

The signature release schedule is as follows: 

  • THOR Lite (untested): on every commit in the repository
  • VALHALLA (goodware tested): once per day
  • THOR (goodware tested, full CI tests on 20+ operating systems): currently every 1-2 days, normally 1 per week

A good example of a rule that caused several false positives and, as a consequence, some trouble is an experimental rule named APT_fnv1a_plus_extra_XOR_in_x64_experimental, which even triggered on files from the Microsoft software catalogue.

It has never been quality tested and has only been in the community signature set used in THOR Lite.

Since we just extend our coverage with every new signature, users who use the ruleset released on Monday the 8th should at least see all different types of exploitation attempts, successful or unsuccessful. They also see many types of web shells, old and new, tools like PowerCat and Nishangs PowerShell one-liner as well as LSASS process memory dumps and other more generic indicators.

So both scanners provide a reasonable coverage and should indicate a successful attack.

THOR may not have the newest signatures, but it provides the bigger rule set with many generic signatures for all kinds of malicious activity, including post-exploitation activity. The following list tries to cover the advantages of a THOR scan in contrast to a THOR Lite scan.

Undisclosed Signatures

We have included many rules in the open source signature set that we use for LOKI and THOR Lite, but not all of them. As stated in a previous post, we have kept some of the more elaborate ones secret to avoid attackers evading the detection in future attacks. 

These rules include detection for specific forensic evidence that is often still present on a once compromised system even when the attackers have already removed the previously dropped web shells. 

This rule e.g. looks for compiled DLLs that we believe are generated once a dropped web shell gets executed at least once and often resides on a compromised system after the attackers removed their tools, data and web shells.  

They are usually not detected by Antivirus software and proved to be a good indicator for a successful compromise and actual malicious activity. 

More Modules, Better Coverage

As you can see in the scanner comparison table, the full THOR version provides many different modules in which it scans different elements of an operating system to discover traces of hacking activity. 

We apply many different IOCs like filename patterns, hash values and keywords in these modules to provide the best possible coverage. Find more information on THOR’s IOC scanning in this blog post. 

In regards to the HAFNIUM and ProxyLogon activity, we’ve seen enterprise customers with additional findings in

  • the Eventlog (Sigma scanning) and
  • Scheduled Task module

Other modules that could reveal HAFNIUM activity and are not available in THOR Lite are: MFT, ShimCache, Registry

Better Overall Coverage

The following graph aims to visualise the coverage differences of both scanners only in relation to the HAFNIUM / ProxyLogon activity. In all other cases, the coverage provided by THOR is much higher, since it uses a signature database with more than 14,000 YARA rules and applies these signatures in more than 20 different modules. 

As you can see, especially payloads/evidence used the “delivery” and “exploitation” phase are covered very well by both scanners, but THOR is much better when it comes to detecting post exploitation activity and backdoors or activity other than the described HAFNIUM group activity.

ESET has just recently published a report in which it mentions activity of more than 10 different APT groups.

As this vulnerability attracts more and more threat groups, it gets more and more important to cover as many shells, tools and techniques as possible and widen the view for other actors.  

We continue to provide IOCs and signatures regarding that threat in both scanners and also merge rules provided by community members as quickly as possible. 

THOR Seed v0.18 Improves Integration with Microsoft Defender ATP

A new version of THOR Seed improves the integration with Microsoft Defender ATP by handling the script termination caused by exceeded timeouts. Due to a runtime limit for all scripts in the Live Response library we had to configure previous versions of THOR Seed to perform a reduced scan that tried to finish within that runtime limit.

This lead to two major issues:

  • Only a reduced set of modules could be activate and a limited set of elements could be scanned
  • Some script runs were terminated before completion

THOR Seed version 0.18 is now able to handle this situation and provides guidance on how to proceed. 

While resolving this issue we noticed that only the script run gets terminated but not the sub process, which is the actual THOR scan. So, the execution of “thor-seed.ps” gets interrupted but the sub process “thor64.exe” keeps on running in the background. 

After a terminated script run, you can now simply “run thor-seed.ps1” a second time and get the info that the THOR process in the background is still running. 

It includes the location of the log file and shows the last 3 lines of that file so that you can review the scan progress. 

After the scan has been completed, THOR Seed shows a message that it cannot start a new scan until the log files and HTML reports have been reviewed and removed from the system. 

It includes all necessary commands for you to just copy, paste and execute them.

A new guide explains all the steps and describes the integration in more detail. 

The release version can be found here.

Please contact us for a current version of that document in case you encounter any issues due to outdated information. 

THOR Process Memory Matches with Surrounding Strings

Following THOR’s approach of showing suspicious elements, it is not feasible to completely avoid false positives. Therefore we always try to provide as much information as possible for an analyst to assess such a suspicious element as quickly as possible.

Users liked the DeeDive feature in which a string match on a chunk of data does not only include the matching string but also the surrounding strings, which help enormously to evaluate the criticality of a matching YARA signature. 

The TechPreview version of THOR 10.6 now introduces this extra information in many other modules. 

The following example shows a false positive in which the string ‘ -p 0x53A4C60B’ matched on the process memory of the ‘svchost.exe’ process with the full command line as ‘svchost.exe -k ClipboardSvcGroup -p’.   

In previous versions THOR you would only see the matching string, but the new versions will also show the 40 bytes before and after the string match. (in the example it has been set to 100 bytes by using `–string-context 100`)

This helps analysts to assess the match more easily without having a process memory dump. In the example above, analyst can review that data block in which the string match occurred and see that it has been within HTML text that has been copied to memory. It could be an analyst system on which someone handling forensic reports copied sections from one document to another, but it’s certainly not the threat, which the YARA rule tried to detect. 

This feature will be available in the upcoming THOR TechPreview 10.6.4. 

New Features: Progress Bar and HTML Report Filter Functions

We would like to inform you about three new comfort features that will be available in the upcoming THOR versions including THOR Lite. 

Improved HTML Reports

The new HTML reports allow analysts to filter elements that turn out to be false positives and remove them from the current view. It also adds useful lookup functions for Virustotal, RiskIQ and VALHALLA. 

Filter and remove false positives in your analysis

Apply filters directly from the modules menu and reduce the events to events from module X only

Direct lookups on Virustotal, RiskIQ and VALHALLA right from the report

The new report functions will be available in the upcoming THOR v10.5.10 and THOR TechPreview v10.6.3, which will be released in January 2021. 

Smart Progress Bar

Due to ongoing demand, we’ve added a progress bar to all longer running modules and a progress indicator to all the other modules. So far, we’ve avoided adding a progress bar or any kind of command line output that works with control characters to reduce the risks of side effects caused by THOR running in non-interactive sessions, e.g. with Splunk Forwarders’ scripted input. 

But THOR version 10 is able to determine if it is running in an interactive session and enables the progress bar only in these cases.

Progress bar in “Filescan” module

Progress bar in “Eventlog” module

New Option in Interrupt Menu

Another feature to highlight is the option to skip a module that doesn’t finish or seems to be stalled. 

The interrupt menu (CTRL+C) offers another option (X) to skip the current module and continue with the next one.

Performance Refactoring in THOR v10.5.9 and THOR TechPreview v10.6.2

We are glad to announce significant performance improvements in the latest versions of THOR.

We’ve refactored several processing units to bulk scan elements that have previously been checked each at a time. These changes affect the modules “Eventlog”, “Registry”, “RegistryHive” and “Logscan”. 

The performance improvements are impressive, especially on systems with big Windows event logs or log files on disk, but also on systems that contain a lot of registry hives like domain controllers or multi-user systems. 

As these changes result in significant speed benefits, we’ve decided to exclude some elements from the “max-file-size” limit.

In the past, log files or registry hives bigger than “max-file-size” (default 12MB) have been skipped in normal scan modes. Only in intense (–intense) and lab scanning mode (–fsonly / –lab in TechPreview) these files have been included and analyzed with the respective modules.

THOR v10.5.9 and THOR v10.6.2 TechPreview now include these elements in their deeper analysis during file system scans. This could lead to longer scan times in some cases. We believe that overall scans turn out to be suprisingly faster and would be interested in feedback on the scan durations in your environments. 

THOR 10 Legacy for Windows XP and Windows 2003

We’ve been working on a legacy version of our scanner THOR 10 for a while and started our closed BETA, which is available to all current customers on special request.

The THOR legacy version does not include the following modules/features:

  • Module: Eventlog scanning
  • Feature: Deeper process analysis for injection, Doppelgaenging, hollowing etc. using PE-Sieve

THOR Legacy runs on:

  • Windows XP x86
  • Windows Server 2003 x86 / x64
  • Windows Vista x86 / x64
  • Windows Server 2008 x64

We offer only limited support for this version and don’t plan to release it for old Linux or macOS versions.

THOR 10 Legacy on Windows Server 2003

THOR 10 Legacy on Windows XP

Please contact us if you are interested in participating in the closed BETA. 

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