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FluentdApplication

CVE-2021-41186

HIGH · 7.5 CVSS v3.1 Published 2021-10-29
Fix available
A fix is available. Upgrade to after 1.14.1 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Fluentd collects events from various data sources and writes them to files to help unify logging infrastructure. The parser_apache2 plugin in Fluentd v0.14.14 to v1.14.1 suffers from a regular expression denial of service (ReDoS) vulnerability. A broken apache log with a certain pattern of string can spend too much time in a regular expression, resulting in the potential for a DoS attack. This issue is patched in version 1.14.2 There are two workarounds available. Either don't use parser_apache2 for parsing logs (which cannot guarantee generated by Apache), or put patched version of parser_apache2.rb into /etc/fluent/plugin directory (or any other directories specified by the environment variable `FLUENT_PLUGIN` or `--plugin` option of fluentd).

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The parser_apache2 plugin in Fluentd versions v0.14.14 through v1.14.1 contains a regular expression denial of service (ReDoS) vulnerability. Specially crafted Apache log entries with certain string patterns cause the regex engine to consume excessive CPU time, potentially leading to service disruption.

MitigationUpgrade Fluentd to version 1.14.2 or later to apply the patch. Alternatively, avoid using parser_apache2 for non-Apache-generated logs or manually deploy the patched parser_apache2.rb to the plugin directory.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FluentdApplication
Affected:>= 0.14.14, <= 1.14.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Fluentd version
    Run 'fluentd --version' or check the gem version with 'gem list fluentd'
    Affected if Version is between v0.14.14 and v1.14.1 inclusive (run 'gem list fluentd' and look for the installed version number)
  2. Locate Fluentd configuration file
    Common paths are /etc/fluent/fluent.conf, /etc/fluent.conf, or check for FLUENT_CONF environment variable
    Affected if Configuration file exists and is readable for inspection
  3. Check if parser_apache2 plugin is enabled
    In the configuration file, search for '@parser apache2' or 'parser apache2' directives within <parse> sections
    Affected if The parser_apache2 parser is explicitly configured in any <parse> block
  4. Verify plugin file exists
    Locate parser_apache2.rb in the Fluentd plugin directory - typically in the Ruby gems path under fluent/plugin/parser_apache2.rb
    Affected if The parser_apache2.rb file exists in the Fluentd plugin directory

If Fluentd version is 0.14.14 through 1.14.1 and the parser_apache2 plugin is configured in use, the environment is vulnerable to ReDoS via crafted Apache log entries.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 1.14.1
Interim mitigation

Upgrade Fluentd to version 1.14.2 or later to apply the patch. Alternatively, avoid using parser_apache2 for non-Apache-generated logs or manually deploy the patched parser_apache2.rb to the plugin directory.

Recommended fix High confidence

1.14.2

  1. Identify your current Fluentd version using `fluentd --version` or your package manager
  2. Stop the Fluentd service to prevent disruption during upgrade
  3. Upgrade Fluentd to version 1.14.2 using your installation method: for gem use `gem install fluentd -v 1.14.2`, for package managers use your distro's upgrade command (e.g., apt-get upgrade fluentd, yum update fluentd)
  4. Verify the upgrade was successful by checking `fluentd --version` shows 1.14.2
  5. Start the Fluentd service
  6. Monitor logs to ensure Fluentd is running correctly after upgrade

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Fluentd Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,040
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-41186 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data