VU#761751: fluentbit contains stack buffer overflow, authentication bypass, and path traversal flaws

Overview

Fluent Bit is a logging and metrics processor and forwarder that is used in a variety of cloud and container networking environments. Several vulnerabilities in Fluent Bit have been discovered that could allow for authentication bypass, remote code execution (RCE) and denial of service (DoS) largely enabled by various Fluent Bit plugins and by how Fluent Bit processes tags. Many of these vulnerabilities require an attacker to have network access to a Fluent Bit instance. The vulnerabilities have been patched in version 4.1.0 and onward of Fluent Bit.

Description

Fluent Bit is a logging and metrics processor and forwarder, intended for usage in various cloud and container environments. It is commonly used to forward traffic to a Security Information and Event Management (SIEM) service, such as Splunk, for further analysis. Fluent Bit uses a tagging system to process and manage traffic that it moves. Multiple vulnerabilities have been discovered within Fluent Bit, largely facilitated by various plugins that manipulate or support tags.

Each individual vulnerability is listed below:

CVE-2025-12972
The Fluent Bit out_file plugin does not properly sanitize tag values when deriving output file names. When the File option is omitted, the plugin uses untrusted tag input to construct file paths. This allows attackers with network access to craft tags containing path traversal sequences that cause Fluent Bit to write files outside the intended output directory.

CVE-2025-12970
The extract_name() function in the Fluent Bit in_docker input plugin copies container names into a fixed size stack buffer without validating length. An attacker who can create containers or control container names, can supply a long name that overflows the buffer, leading to process crash or arbitrary code execution.

CVE-2025-12969
The Fluent Bit in_forward input plugin does not properly enforce the security.users authentication mechanism under certain configuration conditions. This allows remote attackers with network access to the Fluent Bit instance exposing the forward input to send unauthenticated data. By bypassing authentication controls, attackers can inject forged log records, flood alerting systems, or manipulate routing decisions, compromising the authenticity and integrity of ingested logs.

CVE-2025-12977
The Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins fail to sanitize tag_key inputs. An attacker with network access or the ability to write records into Splunk or Elasticsearch can supply tag_key values containing special characters such as newlines or ../that are treated as valid tags. Because tags influence routing and some outputs derive filenames or contents from tags, this can allow newline injection, path traversal, forged record injection, or log misrouting, thus impacting data integrity and log routing.

CVE-2025-12978
Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins contain a flaw in the tag_key validation logic that fails to enforce exact key-length matching. This allows crafted inputs where a tag prefix is incorrectly treated as a full match. A remote attacker with authenticated or exposed access to these input endpoints can exploit this behavior to manipulate tags and redirect records to unintended destinations. This compromises the authenticity of ingested logs and can allow injection of forged data, alert flooding and routing manipulation.

Impact

The vulnerabilities could be used for authentication bypass, RCE, DoS, and tag manipulation leading to improper function of Fluent Bit.

Solution

The vulnerabilities are all fixed in Fluent Bit version 4.1.0 and onward. Users should download and install the latest version of Fluent Bit as soon as possible. The latest version of Fluent Bit is available at https://fluentbit.io/announcements/

Acknowledgements

Thanks to the reporter, Uri Katz of Oligo Security. This document was written by Christopher Cullen.