Vulnerability · Published Oct 6, 2026 · Assigned by apache
CVE-2026-105240Apache log4net: NUL character truncates OutputDebugStringAppender records
Medium 5.3
In plain English
A medium-severity security flaw in Apache log4net from Apache Software Foundation. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.8% (higher than 57% of all known vulnerabilities).
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:13 | log4net | no fix listed | DEBIAN-CVE-2026-105240 |
| Debian:14 | log4net | no fix listed | DEBIAN-CVE-2026-105240 |
| Ubuntu:22.04:LTS | log4net | no fix listed | UBUNTU-CVE-2026-105240 |
| Ubuntu:24.04:LTS | log4net | no fix listed | UBUNTU-CVE-2026-105240 |
| Ubuntu:26.04:LTS | log4net | no fix listed | UBUNTU-CVE-2026-105240 |
| Ubuntu:26.10 | log4net | no fix listed | UBUNTU-CVE-2026-105240 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Apache Software Foundation | Apache log4net | 1.2.9 – before 3.5.0 |
| Apache Software Foundation | Apache log4net | 02e1e115435888485f2e28b414d267e39e799e07 – before 6046fe9d7f353b49fe995361c071ec2eea8f7ef6 |
Weakness: CWE-158 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N