Learn how to manage the Log4Shell vulnerability in Microsoft Defender for Endpoint

The Log4Shell vulnerability is a remote code execution (RCE) vulnerability found in the Apache Log4j 2 logging library. As Apache Log4j 2 is commonly used by many software applications and online services, it represents a complex and high-risk situation for companies across the globe. Referred to as "Log4Shell" (CVE-2021-44228, CVE-2021-45046) it introduces a new attack vector that attackers can exploit to extract data and deploy ransomware in an organization.

Overview of discovery, monitoring, and mitigation capabilities

Defender Vulnerability Management provides you with the following capabilities to help you identify, monitor, and mitigate your organizational exposure to the Log4Shell vulnerability:

• Discovery: Detection of exposed devices, both Microsoft Defender for Endpoint onboarded devices and devices that have been discovered but aren't yet onboarded, is based on vulnerable software and vulnerable files detected on disk.
• Threat awareness: A consolidated view to assess your organizational exposure. This view shows your exposure at the device level and software level, and provides access to details on vulnerable files like, the last time it was seen, the last time it was executed and the last time it was executed with open ports. You can use this information to prioritize your remediation actions. It can take up to 24 hours for data related to exposed devices to appear on the dashboard.
• Mitigation options: Apply mitigation options to help lower your exposure risk.
• Advanced hunting: Use advanced hunting to return details for vulnerable log4j files identified on disk.

Exposed devices discovery

Embedded Defender Vulnerability Management capabilities, along with enabling Log4j detection, in the Microsoft Defender portal, will help you discover devices exposed to the Log4Shell vulnerability.

Onboarded devices are assessed using existing embedded Defender Vulnerability Management capabilities that can discover vulnerable software and files.

For detection on discovered but not yet onboarded devices, Log4j detection must be enabled. This initiates probes in the same way device discovery actively probes your network. Probing includes multiple onboarded endpoints (Windows 10+ and Windows Server 2019+ devices) and only probing within subnets to detect devices that are vulnerable and remotely exposed to CVE-2021-44228.

To enable Log4 detection:

1. Go to Settings > Device discovery > Discovery setup.

2. Select Enable Log4j2 detection (CVE-2021-44228).

3. Select Save.

Running these probes triggers the standard Log4j flow without causing any harmful impact on either the device being probed or the probing device. The probing itself is done by sending multiple HTTP requests to discovered devices, targeting common web application ports (for example - 80,8000,8080,443,8443) and URLs. The request contains HTTP headers with a JNDI payload that triggers a DNS request from the probed machine.

For example, User-Agent: ${jndi:dns://192.168.1.3:5353/MDEDiscoveryUser-Agent} where 192.168.1.3 is the IP of the probing machine.

Vulnerable software and files detection

Defender Vulnerability Management provides layers of detection to help you discover:

• Vulnerable software: Discovery is based on installed application Common Platform Enumerations (CPE) that are known to be vulnerable to Log4j remote code execution.

• Vulnerable files: Both files in memory and files in the file system are assessed. These files can be Log4j-core jar files with the known vulnerable version or an Uber-JAR that contains either a vulnerable jndi lookup class or a vulnerable log4j-core file. Specifically, it:

  • determines if a JAR file contains a vulnerable Log4j file by examining JAR files and searching for the following file: \META-INF\maven\org.apache.logging.log4j\log4j-core\pom.properties - if this file exists, the Log4j version is read and extracted.
  • searches for the JndiLookup.class file inside the JAR file by looking for paths that contain the string "/log4j/core/lookup/JndiLookup.class" - if the JndiLookup.class file exists, Defender Vulnerability Management determines if this JAR contains a Log4j file with the version defined in pom.properties.
  • searches for any vulnerable Log4j-core JAR files embedded within a nested-JAR by searching for paths that contain any of these strings:
    • lib/log4j-core-
    • WEB-INF/lib/log4j-core-
    • App-INF/lib/log4j-core-

This table describes the search capabilities supported platforms and versions:

(1) Capabilities are available when KB5005292 is installed on Windows Server 2012 R2 and 2016.

Learn about your Log4Shell exposure and mitigation options

1. In the Microsoft Defender portal, go to Vulnerability management > Weaknesses.

2. Select CVE-2021-44228.

3. Select Open vulnerability page.

Log4Shell vulnerability mitigation

The log4Shell vulnerability can be mitigated by preventing JNDI lookups on Log4j versions 2.10 - 2.14.1 with default configurations. To create this mitigation action, from the Threat awareness dashboard:

1. Select View vulnerability details.

2. Select Mitigation options.

You can choose to apply the mitigation to all exposed devices or select specific onboarded devices. To complete the process and apply the mitigation on devices, select Create mitigation action.

Mitigation status

The mitigation status indicates whether the workaround mitigation to disable JDNI lookups was applied to the device. You can view the mitigation status for each affected device in the Exposed devices tabs. This can help prioritize mitigation and/or patching of devices based on their mitigation status.

The following table lists the potential mitigation statuses:

Revert mitigations applied for the Log4Shell vulnerability

In cases where the mitigation needs to be reverted, follow these steps:

For Windows:

1. Open an elevated PowerShell window.

2. Run the following command:

   [Environment]::SetEnvironmentVariable("LOG4J\_FORMAT\_MSG\_NO\_LOOKUPS", $null,[EnvironmentVariableTarget]::Machine)
   

The change will take effect after the device restarts.

For Linux:

1. Open the file /etc/environment and delete the line LOG4J_FORMAT_MSG_NO_LOOKUPS=true

2. Delete the file /etc/systemd/system.conf.d/log4j_disable_jndi_lookups.conf

3. Delete the file /etc/systemd/user.conf.d/log4j_disable_jndi_lookups.conf

The change will take effect after the device restarts.

For macOS:

Remove the file setenv.LOG4J_FORMAT_MSG_NO_LOOKUPS.plist from the following folders:

• /Library/LaunchDaemons/
• /Library/LaunchAgents/
• /Users/[username]/Library/LaunchAgents/ - for all users

The change will take effect after the device restarts.

Apache Log4j security recommendations

To see active security recommendation related to Apache log4j, select the Security recommendations tab from the vulnerability details page. In this example, if you select Update Apache Log4j you see another flyout with more information:

Select Request remediation to create a remediation request.

Explore the vulnerability in the Microsoft Defender portal

Once exposed devices, files and software are found, relevant information is conveyed through the following experiences in the Microsoft Defender portal:

Software inventory

On the software inventory page, search for CVE-2021-44228 to see details about the Log4j software installations and exposure:

Weaknesses

On the weaknesses page, search for CVE-2021-44228 to see information about the Log4Shell vulnerability:

Use advanced hunting

You can use the following advanced hunting query to identify vulnerabilities in installed software on devices:

   DeviceTvmSoftwareVulnerabilities
   | where CveId in ("CVE-2021-44228", "CVE-2021-45046")

You can use the following advanced hunting query to identify vulnerabilities in installed software on devices to surface file-level findings from the disk:

   DeviceTvmSoftwareEvidenceBeta
   | mv-expand DiskPaths
   | where DiskPaths contains "log4j"
   | project DeviceId, SoftwareName, SoftwareVendor, SoftwareVersion, DiskPaths

Related articles

• What is Defender Vulnerability Management
• Security recommendations