CVE-2026-40021: Apache Log4net Information Disclosure

CVE-2026-40021 Overview

Apache Log4net versions prior to 3.3.0 contain an improper encoding vulnerability in the XmlLayout and XmlLayoutSchemaLog4J components. These layout classes fail to properly sanitize characters that are forbidden by the XML 1.0 specification in MDC (Mapped Diagnostic Context) property keys and values, as well as the identity field. When attacker-influenced data containing invalid XML characters is logged, serialization fails with an exception, causing the affected log event to be silently dropped.

This vulnerability enables attackers to selectively suppress log entries by injecting forbidden XML characters into fields that are subsequently logged. The silent loss of audit data impairs an organization's ability to detect and investigate malicious activity.

Critical Impact

Attackers can exploit this vulnerability to suppress individual log records, undermining audit trails and potentially hiding evidence of malicious activity from security monitoring systems.

Affected Products

• Apache Log4net versions prior to 3.3.0
• Applications using XmlLayout or XmlLayoutSchemaLog4J log formatting
• .NET applications with user-influenced MDC properties or identity fields

Discovery Timeline

• April 10, 2026 - CVE-2026-40021 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-40021

Vulnerability Analysis

This vulnerability stems from improper encoding of output (CWE-116) within Apache Log4net's XML layout components. The XmlLayout and XmlLayoutSchemaLog4J classes are responsible for serializing log events into XML format for structured logging output. However, these components do not validate or sanitize input data against the XML 1.0 character set requirements before serialization.

The XML 1.0 specification prohibits certain character ranges that cannot be represented in XML documents. When log data containing these forbidden characters reaches the XML layout serializers, the serialization process throws an exception. Rather than propagating this error or falling back to alternative handling, the affected log event is silently discarded.

This creates a scenario where an attacker who can influence logged data—such as through user input fields, HTTP headers, or other external sources—can craft payloads containing forbidden XML characters to suppress specific log entries.

Root Cause

The root cause is insufficient input validation and character encoding in the XML serialization path. The XmlLayout and XmlLayoutSchemaLog4J components trust that all incoming data can be safely represented in XML format without performing the necessary character validation or sanitization. The XML 1.0 specification defines specific character ranges that are valid, and characters outside these ranges must be escaped, replaced, or rejected—none of which occurs in vulnerable versions.

Attack Vector

An attacker exploiting this vulnerability must be able to influence data that is subsequently logged through the affected XML layouts. Common attack vectors include:

• User-supplied input fields that are logged for debugging or audit purposes
• HTTP request headers or parameters captured in application logs
• MDC context values derived from user sessions or external identifiers
• Identity fields populated from authentication systems accepting external input

When the attacker-controlled data contains characters such as control characters (0x00-0x08, 0x0B-0x0C, 0x0E-0x1F) that violate XML 1.0 requirements, the serialization fails and the log entry is silently dropped. This allows an attacker to perform actions while ensuring those specific actions leave no audit trail.

The network-based attack vector requires high complexity as the attacker must understand the application's logging behavior and identify which input fields flow through to the affected XML layouts.

Detection Methods for CVE-2026-40021

Indicators of Compromise

• Unexpected gaps or discontinuities in XML-formatted log sequences
• Log serialization exceptions in application error logs referencing XML character encoding failures
• Missing audit entries for user actions that should have been logged
• Discrepancies between database transaction records and corresponding log entries

Detection Strategies

• Implement log completeness monitoring by comparing expected log events against actual logged events using transaction IDs or correlation identifiers
• Configure alerting on serialization exceptions within the Log4net logging pipeline
• Cross-reference critical audit events with secondary logging systems that do not use XML layouts
• Analyze application logs for patterns of XML encoding errors correlated with specific user input

Monitoring Recommendations

• Deploy integrity monitoring for audit-critical log streams to detect unexpected event suppression
• Enable verbose error logging for the Log4net serialization pipeline to capture failed log attempts
• Implement compensating controls such as duplicate logging to non-XML formats for sensitive operations
• Consider using the Apache CycloneDX VDR to monitor for vulnerability disclosures affecting logging components

How to Mitigate CVE-2026-40021

Immediate Actions Required

• Upgrade Apache Log4net to version 3.3.0 or later, which includes the fix for this vulnerability
• Review application logging configurations to identify usage of XmlLayout or XmlLayoutSchemaLog4J
• Assess risk exposure by identifying where user-influenced data may flow into MDC properties, identity fields, or other logged values
• Consider temporarily switching to alternative layout formats (such as PatternLayout) for critical audit logs until patching is complete

Patch Information

Apache has released Log4net version 3.3.0 which addresses this vulnerability by properly sanitizing forbidden XML characters before serialization. The fix ensures that characters outside the valid XML 1.0 character set are handled appropriately without causing serialization failures.

For technical details on the fix implementation, refer to the GitHub Log4net Pull Request. Additional information is available in the Apache Security Advisory for CVE-2026-40021 and the Apache Mailing List Thread.

Workarounds

• Implement input validation to filter forbidden XML characters before data reaches the logging subsystem
• Use alternative layout classes such as PatternLayout or JsonLayout that do not have XML character restrictions
• Deploy wrapper functions around logging calls that sanitize MDC values and identity fields prior to logging
• Consider implementing a custom ILayout implementation that performs character validation before XML serialization

# NuGet package update command for .NET projects
dotnet add package log4net --version 3.3.0

# Or update packages.config / PackageReference in project files
# <PackageReference Include="log4net" Version="3.3.0" />