CVE-2021-41186Network attack vector

Availability loss via catastrophic regex backtracking

Published Oct 29, 2021 · Updated Aug 4, 2024

Regular expression denial of service in Fluentd 0.14.14 through 1.14.1 allows remote attackers to interrupt log processing via crafted Apache log input. The parser_apache2 plugin applies a regular expression that can catastrophically backtrack on a broken Apache log pattern, monopolizing processing time. No authentication or user interaction is required, but the malformed record must reach a Fluentd instance configured to use parser_apache2, and the bounded consequence is a service interruption rather than data disclosure or modification.

CVSS severity5.9
Medium
EPSS probability2.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Regular expression denial of service in Fluentd 0.14.14 through 1.14.1 allows remote attackers to interrupt log processing via crafted Apache log input. The parser_apache2 plugin applies a regular expression that can catastrophically backtrack on a broken Apache log pattern, monopolizing processing time. No authentication or user interaction is required, but the malformed record must reach a Fluentd instance configured to use parser_apache2, and the bounded consequence is a service interruption rather than data disclosure or modification.

The record

CVE
CVE-2021-41186
Published
Oct 29, 2021
Updated
Aug 4, 2024
Vendor
Fluentd
Product
Fluentd
Classifications
CWE-400, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 29, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 0.14.14, < 1.14.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Fluentd · Fluentdversion=>= 0.14.14, < 1.14.2

Published CVSS scores

5.9NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5GitLab Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo