What’s Vulnerable › Vulnerabilities

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

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-06 · updated 2026-10-09
• Weakness: CWE-158

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.

Affected products

• Apache Software Foundation Apache log4net: from 1.2.9 before 3.5.0 — fixed in 3.5.0

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache log4net 3.5.0. Check the vendor advisory for mitigations.

References

• https://github.com/apache/logging-log4net/commit/6046fe9d7f353b49fe995361c071ec2eea8f7ef6
• https://github.com/apache/logging-log4net/pull/315
• https://lists.apache.org/thread.html/gyw2d4ppy82t1vlpdknkfd1nmzp21z17
• http://www.openwall.com/lists/oss-security/2026/10/07/18

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