CVE-2026-105240 Apache log4net: NUL character truncates OutputDebugStringAppender records

Medium 5.3 Published 6 Oct 2026 Updated 7 Oct 2026

An attacker can truncate log records by injecting NUL characters, hiding subsequent data in Windows applications using OutputDebugStringAppender.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-105240?

CVE-2026-105240 is a medium-severity vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-158

Which versions are affected by CVE-2026-105240?

It affects 1 product.

Products include Apache Log4net.

How do I fix CVE-2026-105240?

No fix has been published for CVE-2026-105240 yet. Limit exposure of the affected software until one is available.

Is CVE-2026-105240 being exploited?

There is no known exploitation of CVE-2026-105240 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-105240, GHSA-q2xj-998m-58gf, UBUNTU-CVE-2026-105240.