List Of Applications Blocked By Communication And Information – This post provides a set of recommendations based on audit data collected over the past two years by Palantir’s Infosec team through Windows Defender Attack Surface Reduction (ASR) security controls. We hope this helps other security teams considering implementation. We will try to highlight considerations for each setting based on experiences with production deployments.
For those of you unfamiliar with the topic, Windows Defender Attack Surface Reduction (ASR) is the name Microsoft has given to a set of controls that limit common malware and exploit technologies on Windows endpoints.
List Of Applications Blocked By Communication And Information
Unlike Windows Defender Exploit Guard, ASR controls are simple on/off switches that administrators can implement in a very short time using Group Policy or Intune, especially if they plan to use audit mode only.
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This behavior sometimes occurs in legitimate applications; However, they are considered risky because they are often exploited by malware. Surface attack mitigation policies can limit this type of risky behavior and keep your organization safe.
In this post, we wanted to be a little confident in providing value to readers while recognizing that all environments have their own environmental nuances. See the descriptions at the bottom of this summary section to find out why we made the recommendations. It’s very likely that you’ll face a different set of restrictions on the networks you protect.
For those who want to dig deeper into the logs in their environment, information is recorded in two different events:
Also record the event. The obvious challenge is that Windows will limit potentially legitimate use cases in some environments, and we hope to avoid that with this release. The standalone audit mode provides an excellent data source for defenders, and the fleet can be configured at scale in less than an hour in most enterprise environments using Group Policy.
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Note about ASR implementation and event log forwarding: At Palantir, we make extensive use of Windows Event Forwarding (WEF) to collect events 1121 and 1122 for analysis. You can find our World Economic Forum guide here and configuration templates here. Our ASR implementation is configured by setting the group policy settings described here. You can apply one policy for all ASR rules; For each setting you configure, you just have to choose between blocking mode and audit mode. You can also create multiple policies to allow exceptions to the blocking mode rules and configure the exceptions as a higher priority Group Policy, limited by Apply Group Policy access control items in the Group Policy object. Collapse rule Block untrusted and unsigned processes running from USB
Configuring “Block untrusted and unsigned processes running from USB” in Block mode did not cause any problems. We configured this rule in block mode on day one and have not had a single event related to this ASR rule in the last 18 months. We believe that implementing this rule in block mode will be safe in almost any business environment.
Adobe’s application update engine tries to start a subprocess that does the work. This is a problem for the ASR rule.
Rather than break the standard, we decided to run Adobe updates through a central software patching and maintenance service so that Adobe has no legitimate need to run subprocesses. We have decided to accept a small operational cost to provide additional protection against PDF malware.
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We have not recorded any events related to this audit in the data collected. Many of our users benefit from our targeted email and webmail clients. Our theory is that other controls in the email delivery path reasonably protect users from useful email content. Of course, we still recommend leaving this check as a last resort to protect endpoints.
However, the Windows environment this post is based on contains approximately 1000 endpoints and we have not seen any sign of this behavior. Therefore, we think it is probably safe for most corporate networks, but administrators should be aware of the warning above.
In over 18 months of data collection, we have not recorded any events related to this audit. We actually started in audit mode because we didn’t notice it was just audit mode (the default audit mode is block).
This persistent behavior has been implemented in many C2 frameworks to make it easier for opponents to win. Therefore, we have already developed additional warnings regarding this behavior, but we appreciate the simplicity of this control. We configured the cluster and encountered no problems.
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The lsass guard rule is one of the most common ASR audit mode events we have encountered. In our environment it generates about 12 million events every six months.
Many secure operations will generate ASR warnings for the lsass.exe line, and from a defender’s perspective it is fairly difficult to distinguish between legitimate use cases and an adversary’s business skills.
After thinking about it for a while and digging into some of the use cases and caveats, we felt like we mostly covered the events described by Microsoft in this note:
In some applications, the code lists all running processes and attempts to open them with global permissions. This line rejects the application opening and records the details in the security event log. This base can make a lot of noise. If you have an application that simply lists LSASS but has no real impact on functionality, you don’t need to add it to the exception list. This event log entry in itself does not necessarily indicate the presence of a malicious threat.
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We should also mention that we already have Credential Guard installed in the environment. This gave us more comfort in believing that it was unlikely that there would be a legitimate, critical use for accessing lsass through processes.
For these reasons we moved the lsass rule to blocking mode and it paid off. The rule has been in place for three months as of this data release and we have not seen any impact on users. We will be sure to update this post if any new concerns arise.
In this post, Microsoft also provides some advice on a statistical approach to distinguishing signal from noise.
Tip: You will see many audit mode events for this setting. Don’t let this be the end of your attempt to switch to blocking mode.
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We log approximately 100 audit events every six months for a small subset of users. After careful investigation, we determined that the file causing the ASR rule “Block Office applications from creating executable content” issue is the following:
This file appears to be part of Office’s Smart Search feature, which Microsoft describes here.
The question we ask ourselves as defenders is: can an attacker exploit this .js file? While unlikely, it seems like this file could at least provide a continuation option. For this reason, we have not excluded the file from ASR’s investigation at this stage. We’ve put the rule in blocking mode, with some exceptions for users with a business case, and updated our internal wiki to make it easier for people to request exceptions based on business needs.
It was surprising and disappointing to learn that we have legitimate use cases that prevent us from immediately moving forward with a blocking method for this rule. It seemed easy.
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This rule blocks attempts to inject code from Office applications into other processes. Attackers can try to use Office applications to move malicious code into other processes by injecting code so that the code can masquerade as a clean process. There are no known legitimate commercial purposes for using code injection.
Most incidents involve a private/commercial, non-Microsoft application that we have identified in an environment deployed to a very small number of users. To give you an idea of how erratic the injection process can be:
At this time we cannot continue to enforce this rule in block mode for most of the fleet and have had to create a separate group policy to accommodate a small subset of users.
Tip: This is a check you need to carefully evaluate your audit data. If you are not fortunate enough to have a business-critical application with this behavior and only a few people need the add-on, we recommend using a separate exception group policy for a small group of users.
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If you’ve read this far, thanks, but you’re probably thinking, “This should be an easy decision to ban.” We had the same feeling and were surprised by the data.
All events refer to the use of the older style of Office macro, and the events refer to the versions:
“The vbaProject.bin file is a binary OLE COM container. This was the format used in older xls versions of Excel before Excel 2007. Unlike all other components of an xlsx/xlsm file, the data is not saved in XML format. Instead, the data is saved in XML format, macros work and it is also saved
