# CVE-2026-105243: Apache log4net insufficient logging in EventLogAppender

Source: https://junglewise.ai/threats/cve-2026-105243-insufficient-logging-vulnerability-in-the-eventlogappender-of  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-06. Updated: 2026-10-06  
License: CC BY 4.0 with a link back

CVE-2026-105243 · Severity: medium · CVSS 5.3 · Published 2026-10-06

Apache log4net is a logging library used by .NET applications to record system events. When using the Windows Event Log appender, long log messages are incorrectly truncated, causing entire log entries to be dropped without notification. An attacker who can inject long messages into logs could suppress audit records, hiding their activity from detection.

Technologies: [Apache Log4net](https://junglewise.ai/threats/technologies/log4net). Vendors: [Apache](https://junglewise.ai/threats/vendors/apache).

## Quick answers

**Is CVE-2026-105243 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-105243 being exploited in the wild.

**How severe is CVE-2026-105243?** 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 (before 3.5.0).

**Is there a fix for CVE-2026-105243?** 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-2026-105243 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-105243?** 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 component fails to properly calculate the Windows Event Log size limit before writing messages, resulting in messages that exceed the maximum size being silently dropped. This affects applications running on Windows using the EventLogAppender from log4net 1.2.9 through 3.4.x. An attacker with the ability to generate or control log message content can exploit this to erase audit trails.

## Affected products

- Apache log4net 1.2.9 through 3.4.x (before 3.5.0)

## Timeline

- 2026-10-06: disclosed
- 2026-10-06: patched: Version 3.5.0 fixes the issue

## References

- [https://nuget.org/packages](https://nuget.org/packages)
- [https://github.com/apache/logging-log4net/commit/28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed](https://github.com/apache/logging-log4net/commit/28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed)

## Related threats

- [Apache log4net improper character escaping in RemoteSyslogAppender](https://junglewise.ai/threats/cve-2026-105244-improper-encoding-or-escaping-of-output-vulnerability-in-the)
- [Apache log4net improper handling of exceptions in aspnet-request pattern converter](https://junglewise.ai/threats/cve-2026-105242-improper-handling-of-exceptional-conditions-vulnerability-in-the)
- [Apache log4net improper Unicode encoding in SmtpPickupDirAppender](https://junglewise.ai/threats/cve-2026-105241-improper-handling-of-unicode-encoding-vulnerability-in-the)
- [Apache log4net null byte handling in OutputDebugStringAppender](https://junglewise.ai/threats/cve-2026-105240-improper-neutralization-of-null-byte-or-nul-character)
- [Apache log4net null byte injection in EventLogAppender](https://junglewise.ai/threats/cve-2026-105239-improper-neutralization-of-null-byte-or-nul-character)