CVE-2026-44161 in Fluentd
Summary
by MITRE • 07/09/2026
Fluentd collects events from various data sources and writes them to files, RDBMS, NoSQL, IaaS, SaaS, Hadoop and so on. Prior to 1.19.3, the Fluentd out_http output plugin allows placeholders such as ${tag} in the endpoint configuration parameter, and if a placeholder value is derived from untrusted input an attacker can control the destination hostname of outbound HTTP requests and force requests to arbitrary internal services. This issue is fixed in version 1.19.3.
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Analysis
by VulDB Data Team • 07/09/2026
The vulnerability in Fluentd's out_http output plugin represents a critical server-side request forgery (SSRF) flaw that enables remote attackers to manipulate outbound HTTP requests through insecure placeholder handling. This issue affects versions prior to 1.19.3 and specifically targets the plugin's ability to process placeholders like ${tag} within endpoint configuration parameters, creating an avenue for unauthorized network access and potential internal system compromise.
The technical flaw stems from insufficient input validation and sanitization within the out_http plugin's configuration parsing mechanism. When placeholders such as ${tag} are used in endpoint URLs, the system processes these values without proper validation of their source or content. Since these placeholders can be populated with data derived from untrusted inputs such as incoming log messages or network requests, attackers can craft malicious payloads that inject arbitrary hostnames into the HTTP destination configuration. This vulnerability directly maps to CWE-918, Server-Side Request Forgery, and falls under the ATT&CK technique T1071.1004 for application layer protocol manipulation.
The operational impact of this vulnerability extends beyond simple network reconnaissance, as it allows attackers to force Fluentd instances to make HTTP requests to arbitrary internal services that might otherwise be protected by firewalls or network segmentation. An attacker could potentially access internal databases, service endpoints, or other sensitive systems that are not directly exposed to the internet but are reachable from the Fluentd server. This creates a significant risk for organizations using Fluentd in environments where internal network boundaries are important for security controls.
Mitigation strategies should focus on immediate version upgrades to 1.19.3 or later, which includes proper input validation and sanitization for placeholder values in HTTP endpoint configurations. Organizations should also implement network segmentation to limit access to internal services from the Fluentd server, utilize firewall rules to restrict outbound HTTP traffic, and consider implementing additional monitoring for unusual outbound requests. The fix addresses the root cause by ensuring that placeholder values are properly validated before being used to construct destination URLs, preventing malicious inputs from influencing the final HTTP request target. Security teams should also review all other Fluentd plugins that might handle similar placeholder mechanisms and ensure comprehensive input validation across the entire configuration system.