Overview
CVE-2025-12977 identifies a vulnerability affecting the in_http, in_splunk, and in_elasticsearch input plugins in Fluent Bit. The issue stems from a failure to properly sanitize tag_key inputs. This allows an attacker with network access or the ability to write records into Splunk or Elasticsearch to inject malicious tag_key values. These malicious values, containing special characters like newlines or path traversal sequences (../), are then treated as valid tags, leading to a variety of security concerns.
Technical Details
The vulnerability resides in how Fluent Bit handles the tag_key parameter within the affected input plugins. This parameter is intended to specify a key within the incoming data that will be used to derive the tag for the log record. Due to insufficient input validation, an attacker can inject arbitrary characters into the tag_key. For example, by sending a specially crafted HTTP request to the in_http plugin, or by injecting data into Splunk or Elasticsearch that is then consumed by the corresponding Fluent Bit input plugin.
These injected characters can have unintended consequences, especially when tags are used for routing decisions or to construct filenames or content in output plugins.
Consider the following example of a malicious tag_key value:
malicious_tag_key = "important\n../sensitive/data"CVSS Analysis
Due to the potentially diverse impact of this vulnerability, including data integrity compromise and log misrouting, further analysis and a CVSS score will be provided when available. As of the published date, a CVSS score and severity rating have not been officially assigned.
Possible Impact
The impact of CVE-2025-12977 can be significant and may include:
- Newline Injection: Injecting newline characters into logs, potentially disrupting log parsing and analysis tools.
- Path Traversal: Using
../sequences to manipulate filenames or paths in output plugins, potentially leading to unauthorized file access or modification. - Forged Record Injection: Crafting malicious log records with modified tags that bypass intended routing logic.
- Log Misrouting: Directing logs to incorrect destinations, potentially hiding malicious activity or exposing sensitive information.
- Data Integrity Issues: Inaccurate routing and processing of logs resulting in data loss or corruption.
Mitigation or Patch Steps
The recommended mitigation is to upgrade to Fluent Bit version 4.1.0 or later. This version contains a fix that properly sanitizes tag_key inputs, preventing the injection of malicious characters. You can download the latest version of Fluent Bit from the official website.
If upgrading is not immediately possible, consider implementing input validation and sanitization measures at the source where the data is being ingested into Fluent Bit (e.g., web application, Splunk configuration). This can provide an additional layer of defense until the upgrade can be performed.
