CVE-2026-40021: CWE-116 Improper Encoding or Escaping of Output in Apache Software Foundation Apache Log4net
Apache Log4net versions before 3. 3. 0 have a vulnerability in their XmlLayout and XmlLayoutSchemaLog4J components where forbidden XML 1. 0 characters in certain fields are not properly sanitized. This can cause exceptions during log serialization, resulting in silent loss of log events. An attacker able to influence these fields can suppress individual log records, potentially impairing audit trails and detection of malicious activity. Users are advised to upgrade to version 3. 3. 0 where this issue is fixed.
AI Analysis
Technical Summary
Apache Log4net's XmlLayout and XmlLayoutSchemaLog4J prior to version 3.3.0 do not properly sanitize characters forbidden by the XML 1.0 specification in MDC property keys, values, and the identity field. This improper encoding or escaping leads to exceptions during serialization and causes affected log events to be silently lost. The vulnerability allows an attacker who can control these fields to suppress specific log entries, undermining logging reliability and audit trail integrity. The issue is addressed in Apache Log4net 3.3.0.
Potential Impact
The vulnerability causes loss of individual log records when forbidden XML characters are present in attacker-controlled fields, which can impair audit trails and hinder detection of malicious activities. There is no indication of remote code execution or data disclosure. The impact is limited to logging reliability and forensic capabilities.
Mitigation Recommendations
Users should upgrade to Apache Log4net version 3.3.0, which fixes this issue by properly sanitizing forbidden XML characters. No other mitigation or temporary workaround is indicated. Patch status is not explicitly stated but the vendor advises upgrading to 3.3.0 as the fix.
CVE-2026-40021: CWE-116 Improper Encoding or Escaping of Output in Apache Software Foundation Apache Log4net
Description
Apache Log4net versions before 3. 3. 0 have a vulnerability in their XmlLayout and XmlLayoutSchemaLog4J components where forbidden XML 1. 0 characters in certain fields are not properly sanitized. This can cause exceptions during log serialization, resulting in silent loss of log events. An attacker able to influence these fields can suppress individual log records, potentially impairing audit trails and detection of malicious activity. Users are advised to upgrade to version 3. 3. 0 where this issue is fixed.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Apache Log4net's XmlLayout and XmlLayoutSchemaLog4J prior to version 3.3.0 do not properly sanitize characters forbidden by the XML 1.0 specification in MDC property keys, values, and the identity field. This improper encoding or escaping leads to exceptions during serialization and causes affected log events to be silently lost. The vulnerability allows an attacker who can control these fields to suppress specific log entries, undermining logging reliability and audit trail integrity. The issue is addressed in Apache Log4net 3.3.0.
Potential Impact
The vulnerability causes loss of individual log records when forbidden XML characters are present in attacker-controlled fields, which can impair audit trails and hinder detection of malicious activities. There is no indication of remote code execution or data disclosure. The impact is limited to logging reliability and forensic capabilities.
Mitigation Recommendations
Users should upgrade to Apache Log4net version 3.3.0, which fixes this issue by properly sanitizing forbidden XML characters. No other mitigation or temporary workaround is indicated. Patch status is not explicitly stated but the vendor advises upgrading to 3.3.0 as the fix.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- apache
- Date Reserved
- 2026-04-08T10:03:43.840Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 69d91fde1cc7ad14dacba2a0
Added to database: 4/10/2026, 4:05:50 PM
Last enriched: 4/10/2026, 4:20:58 PM
Last updated: 4/10/2026, 5:33:27 PM
Views: 3
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