# CVE-2026-105239: Apache log4net: NUL character truncates EventLogAppender records

> Attackers can hide log data by injecting NUL characters, causing silent truncation of Windows Event Log records in Apache log4net applications.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Oct 2026
- **Updated:** 7 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-105239, GHSA-wf3f-44xg-jxj3, UBUNTU-CVE-2026-105239

## What is CVE-2026-105239?

CVE-2026-105239 is a medium-severity vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper Neutralization of Null Byte or NUL Character vulnerability in the EventLogAppender of Apache log4net.

A NUL character in logged content ended the Windows Event Log record at that point, so everything the layout rendered after it, including exception text and trailing fields, was silently not stored. A party whose data reaches a log message could hide the rest of that record. Only applications on Windows that use EventLogAppender 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.

Weakness: CWE-158 (Improper Neutralization of Null Byte or NUL Character)

## Which versions are affected by CVE-2026-105239?

It affects 1 product.


Products: Apache Log4net.

## How do I fix CVE-2026-105239?

No fix has been published for CVE-2026-105239 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-105239 being exploited?

There is no known exploitation of CVE-2026-105239 in the wild and no public exploit code.

## Other vulnerabilities in Apache log4net

- [CVE-2018-1285](https://insights.sbomify.com/CVE-2018-1285/): XML External Entity attack in log4net (critical 9.8)
- [CVE-2026-40021](https://insights.sbomify.com/CVE-2026-40021/): Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters (medium 6.3)
- [CVE-2026-105240](https://insights.sbomify.com/CVE-2026-105240/): Apache log4net: NUL character truncates OutputDebugStringAppender records (medium 5.3)
- [CVE-2026-105241](https://insights.sbomify.com/CVE-2026-105241/): Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch (medium 5.3)
- [CVE-2026-105242](https://insights.sbomify.com/CVE-2026-105242/): Apache log4net: Request validation failure drops the event in the aspnet-request converter (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-105239 and fix it if it is affected.

What it is: CVE-2026-105239 is a medium-severity vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-105239 in the wild and no public exploit code.

Affected product: Apache log4net. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-105239/ (Markdown: https://insights.sbomify.com/CVE-2026-105239/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-105239
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-105239
- OSV: https://osv.dev/vulnerability/CVE-2026-105239

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-105239/
Summaries are generated and may contain mistakes; the official description above is the record.