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How the mentioned vulnerability works:
The vulnerability exists in the Socket Appender component of Apache Log4j Core. Affected versions range from 2.0-beta9 to 2.25.2. When configured to use TLS for secure log transmission, the Socket Appender fails to verify the hostname on the peer’s SSL/TLS certificate. This occurs even when the `verifyHostName` configuration attribute or the `log4j2.sslVerifyHostName` system property is explicitly set to true. The missing verification step breaks the trust chain in TLS handshakes. During connection establishment, the appender accepts any certificate that is signed by a trusted Certificate Authority (CA) without matching the certificate’s subject alternative name (SAN) or common name (CN) to the intended hostname. This flaw allows an attacker positioned as a man-in-the-middle (MITM) to present a valid certificate for a different domain. By intercepting network traffic, the attacker can reroute log data to a server under their control. The interception is possible if the attacker can compromise the network path, such as through ARP spoofing or DNS poisoning. The log traffic, which may contain sensitive application data, is then decrypted and readable by the attacker. Alternatively, the attacker could modify log entries in transit. The vulnerability specifically impacts deployments where Log4j is configured to send logs over TLS-encrypted sockets to remote servers like Logstash or Splunk. The absence of hostname verification makes the encryption effectively useless for authentication. It undermines the confidentiality and integrity of log data. Upgrading to version 2.25.3 fixes this by enforcing hostname verification as configured.
DailyCVE Form:
Platform: Apache Log4j Core
Version: 2.0-beta9 to 2.25.2
Vulnerability: Missing TLS hostname verification
Severity: Moderate
Date: Dec 18 2025
Prediction: Patched Dec 19 2025
What Undercode Say:
Analytics:
Check Log4j Core version in use find / -name "log4j-core.jar" -type f 2>/dev/null | xargs basename -a Example configuration snippet with vulnerable setting (log4j2.xml) <Socket name="SocketAppender" host="logs.example.com" port="6514" protocol="ssl"> <Property name="verifyHostName">true</Property> <!-- Ineffectual in affected versions --> </Socket> System property attempt to enable verification java -Dlog4j2.sslVerifyHostName=true -jar application.jar Test connectivity to log server with hostname verification (using openssl) openssl s_client -connect logs.example.com:6514 -servername logs.example.com | openssl x509 -noout -subject -issuer
How Exploit:
An attacker conducts a man-in-the-middle attack on the network between the application and the log server. They use a tool like `sslstrip` or `mitmproxy` to intercept TLS connections. The attacker provides a valid certificate from a trusted CA that does not match the log server’s hostname. The vulnerable Log4j Socket Appender accepts this certificate, allowing the attacker to decrypt and capture all log traffic. Logs can be redirected to a malicious server.
Protection from this CVE
Upgrade to Log4j Core version 2.25.3 immediately. If upgrade is not possible, configure the Socket Appender to use a private, restricted trust root (custom truststore) containing only the specific certificate for the intended log server. Ensure network segmentation and use VPNs for log traffic. Monitor for unusual outbound connections.
Impact:
Potential exposure of sensitive log data. Loss of log integrity. Possible log data manipulation. Information disclosure risk.
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Sources:
Reported By: github.com
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