What’s Vulnerable › Vulnerabilities

CVE-2021-41186: ReDoS vulnerability in parser_apache2

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-10-29 · updated 2026-06-17
• Weakness: CWE-400

Description

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).

Affected products

• fluentd: from 0.14.14 through 1.14.1

References

• https://github.com/fluent/fluentd/blob/master/CHANGELOG.md#v1142
• https://github.com/fluent/fluentd/security/advisories/GHSA-hwhf-64mh-r662
• https://github.com/github/securitylab-vulnerabilities/blob/52dc4a2a828c6dc24231967c2937ad92038184a9/vendor_reports/GHSL-2021-102-fluent-fluentd.md
• https://github.com/fluent/fluentd/blob/master/CHANGELOG.md#v1142
• https://github.com/fluent/fluentd/security/advisories/GHSA-hwhf-64mh-r662
• https://github.com/github/securitylab-vulnerabilities/blob/52dc4a2a828c6dc24231967c2937ad92038184a9/vendor_reports/GHSL-2021-102-fluent-fluentd.md

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