Summary

Note: CISA will continue to update this webpage as well as our community-sourced GitHub repository as we have further guidance to impart and additional vendor information to provide.

CISA and its partners, through the Joint Cyber Defense Collaborative, are responding to active, widespread exploitation of a critical remote code execution (RCE) vulnerability (CVE-2021-44228) in Apache’s Log4j software library, versions 2.0-beta9 to 2.14.1, known as "Log4Shell." Log4j is very broadly used in a variety of consumer and enterprise services, websites, and applications—as well as in operational technology products—to log security and performance information. An unauthenticated remote actor could exploit this vulnerability to take control of an affected system.

• On December 10, 2021, Apache released Log4j version 2.15.0 in a security update to address the CVE-2021-44228 vulnerability.
• (Updated December 15, 2021) On December 13, 2021, Apache released Log4j version 2.16.0 in a security update to address the CVE-2021-45046 vulnerability. A remote attacker can exploit this second Log4j vulnerability to cause a denial-of-service (DOS) condition in certain non-default configurations.
  • Note: affected organizations that have already upgraded to Log4j 2.15.0 will need to upgrade to Log4j 2.16.0 to be protected against both CVE-2021-44228 and CVE-2021-45046.

In order for these vulnerabilities to be remediated in products and services that use affected versions of Log4j, the maintainers of those products and services must implement these security updates. Users of such products and services should refer to the vendors of these products/services for security updates. Given the severity of the vulnerabilities and the likelihood of an increase in exploitation by sophisticated cyber threat actors, CISA urges vendors and users to take the following actions. 

• Vendors
  • (Updated December 15, 2021) Immediately identify, mitigate, and update affected products using Log4j to version 2.16.0. Note: as stated above, affected organizations will need to upgrade to Log4j 2.16.0 to be protected against both CVE-2021-44228 and CVE-2021-45046.
  • Inform your end users of products that contain these vulnerabilities and strongly urge them to prioritize software updates.
• Affected Organizations
  • In addition to the immediate actions detailed in the box above, review CISA's GitHub repository for a list of affected vendor information and apply software updates as soon as they are available. See Actions for Organizations Running Products with Log4j below for additional guidance. Note: CISA has added CVE-2021-44228 to the Known Exploited Vulnerabilities Catalog, which was created according to Binding Operational Directive (BOD) 22-01: Reducing the Significant Risk of Known Exploited Vulnerabilities. In accordance with BOD 22-01, federal civilian executive branch agencies must mitigate CVE-2021-44228 by December 24, 2021. 

Technical Details

The CVE-2021-44228 RCE vulnerability—affecting Apache’s Log4j library, versions 2.0-beta9 to 2.14.1—exists in the action the Java Naming and Directory Interface (JNDI) takes to resolve variables. According to the CVE-2021-44228 listing, affected versions of Log4j contain JNDI features—such as message lookup substitution—that "do not protect against adversary-controlled LDAP [Lightweight Directory Access Protocol] and other JNDI related endpoints."

• (Updated December 15, 2021) Note: the Apache Log4j version 2.16.0 security update that address the CVE-2021-45046 vulnerability disables JDNI.

An adversary can exploit this CVE-2021-44228 by submitting a specially crafted request to a vulnerable system that causes that system to execute arbitrary code. The request allows the adversary to take full control over the system. The adversary can then steal information, launch ransomware, or conduct other malicious activity. 

Actions for Organizations Running Products with Log4j

CISA recommends affected entities:

• Determine whether your organization's products with Log4j are vulnerable by following the chart below, using both verification methods:
  • [1] CISA's GitHub repository and
  • [2] CERT/CC's CVE-2021-44228_scanner.
    
  • https://www.cisa.gov/sites/default/files/publications/log4jchart.png

• Review Apache’s Log4j Security Vulnerabilities page for additional information and, if appropriate, apply the provided workaround:
  • In releases >=2.10, this behavior can be mitigated by setting either the system property log4j2.formatMsgNoLookups or the environment variable LOG4J_FORMAT_MSG_NO_LOOKUPS to true.
  • For releases from 2.7 through 2.14.1 all PatternLayout patterns can be modified to specify the message converter as %m{nolookups} instead of just %m.
  • For releases from 2.0-beta9 to 2.7, the only mitigation is to remove the JndiLookup class from the classpath: zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class.
• Apply available patches immediately.
  • Prioritize patching, starting with mission critical systems, internet-facing systems, and networked servers. Then prioritize patching other affected information technology and operational technology assets. 
  • Until patches are applied, set log4j2.formatMsgNoLookups to true by adding -Dlog4j2.formatMsgNoLookups=True to the Java Virtual Machine command for starting your application. Note: this may impact the behavior of a system’s logging if it relies on Lookups for message formatting. Additionally, this mitigation will only work for versions 2.10 and above. 
  • As stated above, BOD 22-01 directs federal civilian agencies to mitigate CVE-2021-44228 by December 24, 2021, as part of the Known Exploited Vulnerabilities Catalog.
• Conduct a security review to determine if there is a security concern or compromise. The log files for any services using affected Log4j versions will contain user-controlled strings. 
• Consider reporting compromises immediately to CISA and the FBI.

Resources

This information is provided “as-is” for informational purposes only. CISA does not endorse any company, product, or service referenced below.

Ongoing List of Impacted Products and Devices

CISA is maintaining a community-sourced GitHub repository that provides a list of publicly available information and vendor-supplied advisories regarding the Log4j vulnerability.

Ongoing Sources for Detection Rules 

CISA will update sources for detection rules as we obtain them.

For detection rules, see Florian Roth's GitHub page, log4j RCE Exploitation Detection. Note: due to the urgency to share this information, CISA has not yet validated this content.

For a list of hashes to help determine if a Java application is running a vulnerable version of Log4j, see Rob Fuller's GitHub page, CVE-2021-44228-Log4Shell-Hashes. Note: due to the urgency to share this information, CISA has not yet validated this content. 

Mitigation Guidance from JCDC Partners 

• Broadcom's Symantec Enterprise blog: Apache Log4j Zero-Day Being Exploited in the Wild content
• Cisco Talos Intelligence Group - Comprehensive Threat Intelligence: Threat Advisory: Critical Apache Log4j vulnerability being exploited in the wild
• Cloudflare Blog: CVE-2021-44228 - Log4j RCE 0-day mitigation
• CrowdStrike blog: Log4j2 Vulnerability Analysis and Mitigation Recommendations
• IBM Security Intelligence blog: How Log4j Vulnerability Could Impact You
• Investigating CVE-2021-44228 Log4Shell Vulnerability: VMWare Threat Research
• Mandiant blog: Log4Shell Initial Exploitation and Mitigation Recommendations
• Microsoft blog: Guidance for Preventing, Detecting, and Hunting for CVE-2021-44228 Log4j 2 Exploitation
• Palo Alto Networks blog: Apache log4j Vulnerability CVE-2021-4428: Analysis and Mitigations
• Splunk's blog: Log4Shell - Detecting Log4j Vulnerability (CVE-2021-44228) Continued
• Tenable blog: CVE-2021-44228: Proof-of-Concept for Critical Apache Log4j Remote Code Execution Vulnerability Available (Log4Shell)
• VMware Blog: Log4j Vulnerability Security Advisory: What You Need to Know

Additional Resources

• Joint Cybersecurity Advisory – Technical Approaches to Uncovering and Remediating Malicious Activity provides general incident response guidance.
• NIST Special Publication 800-40 Revision 3, Guide to Enterprise Patch Management Technologies offers more information on the basics of enterprise patch management technologies.
• CISA’s Cyber Essentials serve as a guide for leaders of small businesses as well as leaders of small and local government agencies to develop an actionable understanding of where to start implementing organizational cybersecurity practices.
• CISA offers a range of no-cost cyber hygiene services—including vulnerability scanning and ransomware readiness assessments—to help critical infrastructure organizations assess, identify, and reduce their exposure to cyber threats.
• CERT/CC: Apache Log4j allows insecure JNDI lookups
• New Zealand Computer Emergency Response Team’s Advisory: Log4j RCE 0-Day Actively Exploited
• Canadian Centre for Cyber Security Alert: Active Exploitation of Apache Log4j Vulnerability
• United Kingdom National Cyber Security Centre Alert: Apache Log4j vulnerability (CVE-2021-44228)
• Australian Cyber Security Centre Alert: Critical remote code execution vulnerability found in Apache Log4j2 library
• Australian Cyber Security Centre Advisory: 2021-007: Apache Log4j2 vulnerability – advice and mitigations
• F5 Security Update: Apache Log4j2 Remote Code Execution vulnerability CVE-2021-44228
• Qualys blog: CVE-2021-44228: Apache Log4j2 Zero-Day Exploited in the Wild (Log4Shell)
• Royce Williams’s Tech Solvency blog: Log4Shell log4j vulnerability (CVE-2021-44228 / CVE-2021-45046) - cheat-sheet reference guide
• Center for Internet Security: Log4j Zero-Day Vulnerability Response