CVE-2026-40021: Apache Log4net log suppression in XmlLayout

Quick answers

Is CVE-2026-40021 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-40021 being exploited in the wild.

How severe is CVE-2026-40021?

Medium severity, with a CVSS score of 5.3, published 10 April 2026.

Which products and versions are affected?

log4net (NuGet) ECOSYSTEM:introduced=0,fixed=3.3.0.

Is there a fix for CVE-2026-40021?

Yes. log4net (NuGet): upgrade to 3.3.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

Am I affected by CVE-2026-40021?

Check whether you run Apache Log4net or log4net (NuGet) 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

• log4net (NuGet): upgrade to 3.3.0 or later

  dotnet add package log4net --version 3.3.0

Affected products

• Apache Log4net
• log4net (NuGet) ECOSYSTEM:introduced=0,fixed=3.3.0

Identifiers

• CVE-2026-40021
• GHSA-4f7c-pmjv-c25w

References

• https://api.github.com/users/FreeAndNil
• https://github.com/FreeAndNil
• https://api.github.com/users/FreeAndNil/gists%7B/gist_id%7D
• https://api.github.com/users/FreeAndNil/repos
• https://avatars.githubusercontent.com/u/43271715?v=4
• https://api.github.com/users/FreeAndNil/events%7B/privacy%7D