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Apache Log4j2 versions ranging from 2.0 up to 2.14.1 are vulnerable to critical remote code execution flaws via the Java Naming and Directory Interface (JNDI) lookup feature. When the logging framework processes log messages containing user-supplied input, it evaluates lookup substitution strings formatted as prefix expressions like ${jndi:protocol://address}. An unauthenticated attacker can inject a malicious payload into application parameters, HTTP headers like User-Agent, or form inputs that subsequently get logged by the application. Upon encountering this injection pattern, the Log4j library parses the lookup directive and attempts to resolve the external resource using JNDI. JNDI supports multiple directory services including Lightweight Directory Access Protocol (LDAP), Domain Name Service (DNS), and Remote Method Invocation (RMI). By directing the lookup query to a malicious server controlled by the attacker, the vulnerable application initiates a network connection to retrieve directory data. The malicious LDAP server replies with a Java reference object containing a codebase pointer to an external malicious class file. Log4j automatically downloads and loads this untrusted Java class file into the runtime environment of the hosting JVM process. Execution of the class constructor or static initializers triggers arbitrary Java bytecode execution on the host machine. Because logging operations are pervasive across enterprise software components, identifying every injection vector presents an immense challenge. The vulnerability bypasses standard authentication layers because logging routines execute asynchronously or implicitly during normal request handling. Automated attack tools weaponized this flaw immediately upon disclosure, leading to widespread exploitation across global infrastructure. Remediation requires immediate library upgrades or strict configuration modifications to disable message lookups entirely.
DailyCVE Form:
Platform: Apache Log4j
Version: Below 2.15.0
Vulnerability: Remote code execution
Severity: Critical severity level
date: December 10 2021
Prediction: Already patched version
What Undercode Say:
Check vulnerable log4j dependency version
mvn dependency:tree | grep log4j
Test payload injection via curl header
curl -H 'X-Api-Version: ${jndi:ldap://127.0.0.1:1389/a}' http://target-server:8080/login
Fix configuration property via command line
export LOG4J_FORMAT_MSG_NO_LOOKUPS=true
Exploit: (Educational Purposes!)
The exploit mechanics rely on forcing the logging subsystem to interpret untrusted string data as active lookup commands. Attackers transmit specially crafted payloads inside common logging vectors such as HTTP headers (User-Agent, X-Forwarded-For), authorization tokens, or query strings. When the application logs the incoming request string using statements like logger.error("Login failed for user: " + username), Log4j inspects the string for `${…}` syntax. Finding the JNDI prefix, it queries an external directory server specified in the URL. The attacker’s server responds with a directory entry pointing to a compiled malicious Java payload hosted on an HTTP server. The vulnerable application fetches the remote class file, loads it into memory, and instantiates the object, executing embedded malicious commands directly on the underlying operating system.
Protection: from this CVE
Upgrade the Apache Log4j library to version 2.15.0 or higher where message lookups are disabled by default. For systems utilizing versions between 2.10 and 2.14.1, mitigate the risk by setting the system property `log4j2.formatMsgNoLookups` to `true` via JVM flags. For older versions ranging from 2.0-beta9 to 2.10.1, remove the `JndiLookup` class directly from the classpath by executing commands such as zip -q -d log4j-core-.jar org/apache/logging/log4j/core/lookup/JndiLookup.class. Implement strict egress filtering on firewalls to block unauthorized outbound LDAP and RMI connections from internal application servers.
Impact:
Successful exploitation results in full remote code execution with the privileges of the application process running the Java Virtual Machine. Attackers can compromise system confidentiality, integrity, and availability by installing persistent backdoors, extracting sensitive environment variables, deploying ransomware, and moving laterally across internal network segments.
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Sources:
Reported By: github.com
Extra Source Hub:
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