Apache Log4net up to 3.2.x XmlLayout/XmlLayoutSchemaLog4J escape output

Summaryinfo

A vulnerability categorized as problematic has been discovered in Apache Log4net up to 3.2.x. Affected by this vulnerability is the function XmlLayout/XmlLayoutSchemaLog4J. Such manipulation leads to escape output. This vulnerability is referenced as CVE-2026-40021. It is possible to launch the attack remotely. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in Apache Log4net up to 3.2.x. It has been rated as problematic. Affected by this issue is the function XmlLayout/XmlLayoutSchemaLog4J. The manipulation with an unknown input leads to a escape output vulnerability. Using CWE to declare the problem leads to CWE-116. The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved. Impacted is integrity. CVE summarizes:

Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.3.0, fail to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in MDC property keys and values, as well as the identity field that may carry attacker-influenced data. This causes an exception during serialization and the silent loss of the affected log event. An attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue.

The weakness was published by f00dat. The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-40021 since 04/08/2026. The exploitation is known to be difficult. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 04/11/2026).

The vulnerability scanner Nessus provides a plugin with the ID 306027 (Linux Distros Unpatched Vulnerability : CVE-2026-40021), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.3.0 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (306027). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Vendor

• Apache

Name

• Log4net

Version

• 3.0
• 3.1
• 3.2

License

• open-source

Website

• Vendor: https://www.apache.org/

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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