Quick answers
- Is CVE-2026-105239 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-105239 being exploited in the wild.
- How severe is CVE-2026-105239?
- Medium severity, with a CVSS score of 5.3, published 6 October 2026.
- Which products and versions are affected?
- Apache Log4net 1.2.9 through 3.4.x.
- Is there a fix for CVE-2026-105239?
- Yes: fixed in 3.5.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-105239 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-105239?
- Check whether you run Apache Log4net at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
How to fix
- Fixed in: 3.5.0
Technical details
The EventLogAppender on Windows passes logged content to ReportEventW, which treats null bytes as string terminators. A null byte in log data causes the Windows Event Log to store only the portion before it, dropping layout-rendered content including exceptions and trailing fields. The vulnerability affects versions 1.2.9 through 3.4.x; version 3.5.0 escapes null bytes before writing to the event log.
Affected products
- Apache log4net 1.2.9 through 3.4.x
Timeline
- 2026-10-06: disclosed