What is CVE-2021-41186?

CVE-2021-41186 is a medium-severity uncontrolled resource consumption vulnerability in fluentd (rubygems), affecting versions >= 0.14.14, < 1.14.2. It is fixed in 1.14.2. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.

How severe is CVE-2021-41186?

CVE-2021-41186 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which versions of fluentd are affected by CVE-2021-41186?

fluentd (rubygems) versions >= 0.14.14, < 1.14.2 is affected.

Is there a fix for CVE-2021-41186?

Yes. CVE-2021-41186 is fixed in 1.14.2. Upgrade to this version or later.

Is CVE-2021-41186 exploitable, and should I be worried?

Whether CVE-2021-41186 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2021-41186 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

How do I fix CVE-2021-41186?

Upgrade fluentd to 1.14.2 or later.

CVE-2021-41186 in fluentd

Q: How severe is CVE-2021-41186?

CVE-2021-41186 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Q: Which versions of fluentd are affected by CVE-2021-41186?

fluentd (rubygems) versions >= 0.14.14, < 1.14.2 is affected.

Q: Is there a fix for CVE-2021-41186?

Yes. CVE-2021-41186 is fixed in 1.14.2. Upgrade to this version or later.

Q: Is CVE-2021-41186 exploitable, and should I be worried?

Whether CVE-2021-41186 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2021-41186 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2021-41186?

Upgrade fluentd to 1.14.2 or later.

Summary

ReDoS vulnerability in parser_apache2

Workarounds

Either of the following:

Don't use parser_apache2 for parsing logs which cannot guarantee generated by Apache.
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).

References

Impact

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.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2021-41186 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.14.2); upgrading removes the vulnerable code path.

Affected versions

fluentd (>= 0.14.14, < 1.14.2)

Security releases

fluentd → 1.14.2 (rubygems)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

v1.14.2

Frequently Asked Questions

What is CVE-2021-41186? CVE-2021-41186 is a medium-severity uncontrolled resource consumption vulnerability in fluentd (rubygems), affecting versions >= 0.14.14, < 1.14.2. It is fixed in 1.14.2. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
How severe is CVE-2021-41186? CVE-2021-41186 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
Which versions of fluentd are affected by CVE-2021-41186? fluentd (rubygems) versions >= 0.14.14, < 1.14.2 is affected.
Is there a fix for CVE-2021-41186? Yes. CVE-2021-41186 is fixed in 1.14.2. Upgrade to this version or later.
Is CVE-2021-41186 exploitable, and should I be worried? Whether CVE-2021-41186 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2021-41186 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2021-41186? Upgrade fluentd to 1.14.2 or later.

Other vulnerabilities in fluentd

CVE-IDSeverityPackage summary

CVE-2026-44161Highfluentd: Fluentd is Vulnerable to Server-Side Request Forgery (SSRF) via Placeholder Expansion in `out_http`CVE-2026-44160Highfluentd: Fluentd is Vulnerable to Denial of Service (DoS) via Gzip Decompression Bomb in `in_http` and `in_forward`CVE-2026-44025Highfluentd: Fluentd is Vulnerable to Exposure of Sensitive Information via Monitor Agent API CVE-2026-44024Criticalfluentd: Fluentd is Vulnerable to Remote Code Execution (RCE) via Arbitrary File Write in `${tag}` Placeholder CVE-2022-39379Lowfluentd: fluentd vulnerable to remote code execution due to insecure deserialization (in non-default configuration)