CVE-2026-105239 Apache log4net: NUL character truncates EventLogAppender records

Medium 5.3 Published 6 Oct 2026 Updated 7 Oct 2026

Attackers can hide log data by injecting NUL characters, causing silent truncation of Windows Event Log records in Apache log4net applications.

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-2026-105239?

CVE-2026-105239 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 EventLogAppender of Apache log4net.

A NUL character in logged content ended the Windows Event Log record at that point, so everything the layout rendered after it, including exception text and trailing fields, was silently not stored. A party whose data reaches a log message could hide the rest of that record. Only applications on Windows that use EventLogAppender 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-2026-105239?

It affects 1 product.

Products include Apache Log4net.

How do I fix CVE-2026-105239?

No fix has been published for CVE-2026-105239 yet. Limit exposure of the affected software until one is available.

Is CVE-2026-105239 being exploited?

There is no known exploitation of CVE-2026-105239 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-105239, GHSA-wf3f-44xg-jxj3, UBUNTU-CVE-2026-105239.