Apache Log4cxx up to 1.6.x escape output

Summaryinfo

A vulnerability identified as problematic has been detected in Apache Log4cxx up to 1.6.x. Affected by this issue is some unknown functionality. Performing a manipulation results in escape output. This vulnerability is identified as CVE-2026-40023. The attack can be initiated remotely. There is not any exploit available. You should upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic has been found in Apache Log4cxx up to 1.6.x. This affects some unknown functionality. The manipulation with an unknown input leads to a escape output vulnerability. CWE is classifying the issue as 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. This is going to have an impact on integrity. The summary by CVE is:

Apache Log4cxx's XMLLayout https://logging.apache.org/log4cxx/1.7.0/classlog4cxx_1_1xml_1_1XMLLayout.html , in versions before 1.7.0, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in log messages, NDC, and MDC property keys and values, producing invalid XML output. Conforming XML parsers must reject such documents with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records. An attacker who can influence logged data can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4cxx 1.7.0, which fixes this issue.

The weakness was released by Olawale Titiloye. The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-40023 since 04/08/2026. The exploitability is told to be difficult. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Neither technical details nor an exploit are publicly available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 04/11/2026).

The vulnerability scanner Nessus provides a plugin with the ID 306018 (Linux Distros Unpatched Vulnerability : CVE-2026-40023), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.7.0 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (306018). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• Apache

Name

• Log4cxx

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6

License

• open-source

Website

• Vendor: https://www.apache.org/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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