CVE-2021-45105

A flaw was found in the Apache Log4j logging library 2.x. when the logging configuration uses a non-default Pattern Layout with a Context Lookup. Attackers with control over Thread Context Map (MDC) input data can craft malicious input data that contains a recursive lookup and can cause Denial of Service.

Red Hat Product Security has performed an analysis of this flaw and has classified the Attack Complexity(AC) as High because there are multiple factors involved which are beyond attacker's control:

• The application has to use the logging configuration using a Context Map Lookup (for example, $${ctx:loginId}) which is a non-default Pattern Layout.
• The application developer has to use the map org.apache.logging.log4j.ThreadContext in the application code and save at-least one key (for example, ThreadContext.put("loginId", "myId");) in the ThreadContext map object.
• Attackers must also know this saved key name in order to exploit this flaw.

Note that saving keys in this map is a non-essential usage of log4j and just an optional feature provided. Refer to https://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup to know more about the Context Map Lookup feature of Log4j.

Log4j 1.x is not impacted by this vulnerability. Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using ONLY the log4j-api JAR file without the log4j-core JAR file are NOT impacted by this vulnerability.

Despite including a vulnerable version of Log4j 2.x, this vulnerability is not exploitable in Elasticsearch[0], as shipped in OpenShift Container Platform and OpenShift Logging. OpenShift 3.11 specifically does not contain any context lookups:

https://github.com/openshift/openshift-ansible/blob/release-3.11/roles/openshift_logging_elasticsearch/templates/log4j2.properties.j2

This vulnerability is therefore rated Low for Elasticsearch in OpenShift Container Platform and OpenShift Logging.

[0] https://discuss.elastic.co/t/apache-log4j2-remote-code-execution-rce-vulnerability-cve-2021-44228-esa-2021-31/291476#update-december-18-4

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

For Log4j 2 versions up to and including 2.16.0, this flaw can be mitigated by:

• In PatternLayout in the Log4j logging configuration, replace Context Lookups like ${ctx:loginId} or $${ctx:loginId} with Thread Context Map patterns (%X, %mdc, or %MDC) like %X{loginId}.
• Otherwise, in the Log4j logging configuration, remove references to Context Lookups like ${ctx:loginId} or $${ctx:loginId} where they originate from sources external to the application such as HTTP headers or user input.