Executive Summary

The vulnerability in Apache Log4cxx's XMLLayout, before version 1.7.0, is due to the failure to sanitize characters that are forbidden by the XML 1.0 specification in log messages, NDC, and MDC property keys and values. This results in invalid XML output.

Because conforming XML parsers must reject such invalid XML documents with a fatal error, downstream log processing systems may drop or fail to index affected log records.

An attacker who can influence the logged data can exploit this vulnerability to suppress individual log records, which impairs audit trails and the detection of malicious activity.

The issue was fixed in Apache Log4cxx version 1.7.0 by sanitizing illegal characters to ensure valid XML output.

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Impact Analysis

This vulnerability can impact you by causing your logging system to produce invalid XML output, which conforming XML parsers will reject with fatal errors.

As a result, downstream log processing systems may drop or fail to index affected log records, leading to incomplete or missing logs.

If an attacker can influence the logged data, they can exploit this to suppress specific log entries, impairing audit trails and making it harder to detect malicious activity.

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Detection Guidance

This vulnerability involves Apache Log4cxx versions before 1.7.0 producing invalid XML output due to unsanitized forbidden XML characters in log messages, NDC, and MDC property keys and values.

To detect this vulnerability on your system, you can inspect log files generated by Log4cxx for malformed XML entries or fatal XML parsing errors in downstream log processing systems.

Commands to help detect the issue might include searching logs for invalid XML characters or parsing errors. For example, you could use grep or xmllint to identify problematic log entries:

• Use grep to find control or forbidden XML characters in log files: grep -P '[\x00-\x08\x0B\x0C\x0E-\x1F]' /path/to/logfile
• Use xmllint to validate XML log files and detect fatal errors: xmllint --noout /path/to/logfile.xml

Note that the logs may not be complete XML files but fragments embedded in larger XML documents, so validation might require extracting or reconstructing the full XML context.

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Mitigation Strategies

The primary mitigation step is to upgrade Apache Log4cxx to version 1.7.0 or later, where this issue has been fixed by sanitizing illegal XML characters in log output.

The fix involves replacing control characters in XML and HTML output with appropriate character references, ensuring well-formed XML logs and preventing parsing errors.

Until you can upgrade, consider implementing input validation or sanitization on data that is logged to prevent forbidden XML characters from being included.

Also, monitor your log processing systems for errors or dropped records that may indicate exploitation of this vulnerability.

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Compliance Impact

This vulnerability allows an attacker who can influence logged data to suppress individual log records by causing invalid XML output that conforming XML parsers reject with fatal errors. This suppression can impair audit trails and hinder detection of malicious activity.

Since audit trails are critical for compliance with common standards and regulations such as GDPR and HIPAA, which require accurate and complete logging for security monitoring and forensic analysis, this vulnerability can negatively impact compliance by enabling loss or omission of log records.

Therefore, failure to sanitize forbidden XML characters in logs may lead to incomplete or missing logs, undermining the integrity and reliability of audit logs required by these regulations.

Users are advised to upgrade to Apache Log4cxx 1.7.0, which fixes this issue by properly escaping forbidden XML characters, thereby helping maintain compliant and reliable logging.

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