Fluentd is Vulnerable to Remote Code Execution (RCE) via Arbitrary File Write in `${tag}` PlaceholderGHSA-44hj-4m45-frj3
CRITICALGHSA-44hj-4m45-frj3 is a critical-severity (CVSS 9.8) Path Traversal vulnerability in fluentd. A fix is available for fluentd — see the affected versions and patch details below.
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-44hj-4m45-frj3.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
GHSA-44hj-4m45-frj3 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 384,189 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
fluentdReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Fluentd allows dynamically constructing file paths using the ${tag} placeholder.
It was discovered that validation for this placeholder was insufficient.
If a Fluentd instance is configured to receive logs from untrusted sources and uses the ${tag} placeholder in file configurations (such as the path parameter in the out_file plugin), an attacker can inject path traversal characters (e.g., ../).
When combined with certain formatting options, this vulnerability allows an attacker to write arbitrary files or overwrite existing files on the system with attacker-controlled content, bypassing intended directory restrictions.
Impact
This vulnerability allows for Arbitrary File Write, which can be directly escalated to full Remote Code Execution (RCE). An attacker could achieve RCE by overwriting critical system files, injecting executable plugins, or modifying configuration files. The impact is Critical as it can lead to full system compromise without any authentication, depending on the Fluentd configuration and the privileges of the Fluentd process.
Patches
v1.19.3
Workarounds
If an immediate upgrade is not possible, users are strongly advised to apply the following mitigations:
- Restrict Network Access
- Ensure that Fluentd input ports (such as
in_forwardon default port24224) are deployed within a closed, trusted network. Use firewall rules (e.g., iptables, AWS Security Groups) to block access from untrusted networks or instances.
- Ensure that Fluentd input ports (such as
- Run Fluentd as a non-root user
- Dropping privileges prevents Fluentd from writing to sensitive system directories (e.g.,
/etc/), significantly mitigating the risk of system-wide RCE.
- Dropping privileges prevents Fluentd from writing to sensitive system directories (e.g.,
- Revise configurations
- Do not use the
${tag}placeholder in thepathparameter of output plugins (likeout_file) if the tag originates from an untrusted source.
- Do not use the
- Filter incoming tags
- Strictly validate and filter incoming tags at the input layer (e.g., using
fluent-plugin-rewrite-tag-filter) to drop any tags containing.or/characters.
- Strictly validate and filter incoming tags at the input layer (e.g., using
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | fluentd | all versions | 1.19.3bundle update fluentd --conservative |
Affected Products
fluentdfluentdDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fluentd, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fluentd to 1.19.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-44hj-4m45-frj3 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Red Hat has determined that no Red Hat product is currently affected by this vulnerability. Exploitation requires a Fluentd deployment configured with an output plugin, such as out_file, that constructs file paths dynamically from the ${tag} placeholder, combined with the ability for an attacker to submit untrusted,…
Upgrade to Fluentd 1.19.3 or later, which validates the ${tag} placeholder in output plugin path configuration and rejects path traversal characters. Where an immediate upgrade is not possible: avoid using the ${tag} placeholder in file-path-based output plugin configuration (such as the path parameter of out_file); if dynamic tag-based paths are required, restrict allowed tag values with an explicit allowlist/regex filter (for example via a <filter> directive) before they reach the output plugin; and ensure only trusted sources can submit tags to the Fluentd instance by not exposing its forward or HTTP input directly to untrusted networks.Source: Red Hat security advisory for GHSA-44hj-4m45-frj3 (CC BY 4.0)
Frequently Asked Questions
Is GHSA-44hj-4m45-frj3 in your dependencies?
Find it across RubyGems, including transitive dependencies.