Improper Neutralization of Null Byte Vulnerability in Apache Log4net by Apache
CVE-2026-105240

5.3MEDIUM

Key Information:

Vendor

Apache
Status
Apache Log4net
Vendor
CVE Published:
6 October 2026

What is CVE-2026-105240?

A vulnerability exists in the OutputDebugStringAppender of Apache Log4net, where a NUL character in logged content causes loss of all subsequent debug output within the same record. This means that any data following the NUL character—including error messages and additional log details—may be hidden, potentially compromising application debugging and security logging processes. Only applications running on Windows and utilizing the OutputDebugStringAppender are impacted. Users are encouraged to upgrade to version 3.5.0 to mitigate this issue.

Affected Version(s)

Apache log4net 1.2.9 < 3.5.0

Apache log4net 02e1e115435888485f2e28b414d267e39e799e07 < 6046fe9d7f353b49fe995361c071ec2eea8f7ef6

References

https://github.com/apache/logging-log4net/pull/315
patch
https://github.com/apache/logging-log4net/commit/6046fe9d...
patch
https://lists.apache.org/thread.html/gyw2d4ppy82t1vlpdknk...
vendor-advisory

CVSS V3.1

Score:
5.3
Severity:
MEDIUM
Confidentiality:
None
Integrity:
Low
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

The Apache Software Foundation
Claude Security
Jan Friedrich
.