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 outhttp 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.

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, Fluentd's inhttp and inforward plugins support gzip-compressed data but enforce limits only on compressed payloads through settings such as bodysizelimit and chunksizelimit, allowing crafted compressed payloads to decompress in memory to an excessive size and cause denial of service through memory exhaustion. This issue is fixed in version 1.19.3.

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, Fluentd's Monitor Agent plugin inmonitoragent exposes internal metrics and plugin information via a REST API, and responses from /api/plugins.json and related endpoints unintentionally include internal instance variables that may contain database passwords, API keys, or cloud credentials. This issue is fixed in version 1.19.3.

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 outfile 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:

1. Restrict Network Access Ensure that Fluentd input ports (such as inforward on default port 24224) are deployed within a closed, trusted network. Use firewall rules (e.g., iptables, AWS Security Groups) to block access from untrusted networks or instances. 2. 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. 3. Revise configurations Do not use the ${tag} placeholder in the path parameter of output plugins (like outfile) if the tag originates from an untrusted source. 4. 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.

Escape sequence injection vulnerability in Fluentd versions 0.12.29 through 0.12.40 may allow an attacker to change the terminal UI or execute arbitrary commands on the device via unspecified vectors.