Fluentd Blog

fluent-package v6.0.3 has been released

Hi users!

We have released fluent-package v6.0.3 on 2026-03-27. Fluent Package is a stable distribution package of Fluentd. (successor of td-agent)

This is a maintenance release of v6.0.x LTS series.

Fluent Package v6.0.3

Fluent Package v6.0.3 includes the following improvements:

• Fixed a severe memory leak issue in the bundled cool.io v1.9.3 under specific conditions
• Updated bundled Ruby to 3.4.9
• Updated bundled Nokogiri to v1.19.2 (Windows only) for vulnerability fix

This article explains the changes in Fluent Package v6.0.3.

Changes

Fixed a severe memory leak issue in the bundled cool.io v1.9.3

In fluent-package v6.0.2, a critical issue was discovered where memory usage continuously increases over time. This release completely resolves this problem.

• Affected Environments
  • This issue occurs in environments where TCP connections are frequently connected and disconnected.
  • It is easily reproducible when using the out_forward plugin with the keepalive false setting (which is the default value).
• Root cause
  • The asynchronous I/O library cool.io (v1.9.2 and v1.9.3) bundled in fluent-package v6.0.2 had a bug in its detachment process.
  • When a TCP connection was detached, the internal watcher objects were not properly garbage collected, leaving "garbage" data accumulated in the memory.
• Resolution
  • We have released cool.io v1.9.4 which fixes this bug, and it is now bundled by default in fluent-package v6.0.3.

If operated for a long period, this memory leak will exhaust system memory resources, eventually causing the Fluentd process to be terminated unexpectedly by the OOM (Out of Memory) Killer. We strongly recommend users who are using out_forward on v6.0.2 to update to this version immediately.

Note: Users on fluent-package v6.0.1 or earlier are not affected by this specific issue.

Updated bundled Ruby to 3.4.9

Ruby 3.4.9 includes multiple bug and security fixes. Specifically, it addresses the following vulnerability:

• CVE-2026-27820

For details, please see the Ruby 3.4.9 release notes.

Updated bundled Nokogiri (Windows only) for vulnerability fix

We have updated the bundled Nokogiri from v1.18.10 to v1.19.2 for the Windows version to address the following vulnerability:

• Nokogiri does not check the return value from xmlC14NExecute

Download

Please visit the download page.

Announcement

About next LTS schedule

We plan to release the next LTS version of fluent-package v6.0.4 at June 2026. The content of updates are still TBD.

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