Apache log4net up to 3.4.x OutputDebugStringAppender neutralization

Summaryinfo

A vulnerability marked as problematic has been reported in Apache log4net up to 3.4.x. This issue affects some unknown processing of the component OutputDebugStringAppender. Performing a manipulation results in neutralization. This vulnerability is known as CVE-2026-105240. Remote exploitation of the attack is possible. No exploit is available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic has been found in Apache log4net up to 3.4.x. This affects an unknown code of the component OutputDebugStringAppender. The manipulation with an unknown input leads to a neutralization vulnerability. CWE is classifying the issue as CWE-707. The product does not ensure or incorrectly ensures that structured messages or data are well-formed and that certain security properties are met before being read from an upstream component or sent to a downstream component. This is going to have an impact on integrity. The summary by CVE is:

Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache log4net. A NUL character in logged content ended the debug output record at that point, so everything the layout rendered after it, including exception text and trailing fields, was silently lost. A party whose data reaches a log message could hide the rest of that record. Only applications on Windows that use OutputDebugStringAppender are affected. This issue affects Apache log4net: from 1.2.9 before 3.5.0. Users are recommended to upgrade to version 3.5.0, which fixes the issue.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-105240 since 10/04/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 10/06/2026).

Upgrading to version 3.5.0 eliminates this vulnerability.

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Productinfo

Vendor

• Apache

Name

• log4net

Version

• 3.0
• 3.1
• 3.2
• 3.3
• 3.4

License

• open-source

Website

• Vendor: https://www.apache.org/

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