# CVE-2021-41186 — fluent fluentd

> As of 2024-08-04, CVE-2021-41186 is a MEDIUM-severity vulnerability in fluent fluentd, CVSS v3.1 5.9, EPSS 2.2% (82.1th percentile). Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2021-41186, most recently “Multiple Fluentd Vulnerabilities: RCE via Tag Placeholder (CVE-2026-44024), Info Disclosure (CVE-2026-44025), Decompression Bomb DoS (CVE-2026-44160), and SSRF (CVE-2026-44161)”.

**Last updated:** 2024-08-04

## What is CVE-2021-41186?

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).

The record classifies CVE-2021-41186 under weakness class [CWE-400](https://intel.threadlinqs.com/cwe/CWE-400). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 1820 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 5.9 — MEDIUM (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **EPSS (FIRST):** 2.2% probability of exploitation in the next 30 days, higher than 82.1% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 6.2/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2021-10-29, last modified 2024-08-04

## Is CVE-2021-41186 being exploited?

It currently carries a trending score of 26 in the Threadlinqs vulnerability feed.

## Affected products and versions

- [fluent](https://intel.threadlinqs.com/vendors/fluent): fluentd

## How to fix CVE-2021-41186

No vendor patch reference has been recorded for CVE-2021-41186 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2021-41186

1 tracked threat in the Threadlinqs corpus references CVE-2021-41186, either in the campaign’s CVE list or as an indicator on the campaign record.

- [Multiple Fluentd Vulnerabilities: RCE via Tag Placeholder (CVE-2026-44024), Info Disclosure (CVE-2026-44025), Decompression Bomb DoS (CVE-2026-44160), and SSRF (CVE-2026-44161)](https://intel.threadlinqs.com/threat/TL-2026-1041) — HIGH · 2026-07-01

## Sources

Enriched from CVE.org, FIRST EPSS. Last verified by Threadlinqs on 2026-10-10.

**Vendor advisory and patch**

- [github.com](https://github.com/fluent/fluentd/security/advisories/GHSA-hwhf-64mh-r662)

**Other references**

- [github.com](https://github.com/fluent/fluentd/blob/master/CHANGELOG.md#v1142)
- [github.com (GHSL 2021 102 fluent fluentd)](https://github.com/github/securitylab-vulnerabilities/blob/52dc4a2a828c6dc24231967c2937ad92038184a9/vendor_reports/GHSL-2021-102-fluent-fluentd.md)

Canonical: https://intel.threadlinqs.com/cve/CVE-2021-41186
Full detection coverage and IOCs for threats exploiting CVE-2021-41186 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp