CVE-2025-12977 in FluentBit
Summary
by MITRE • 11/24/2025
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, impacting data integrity and log routing.
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Analysis
by VulDB Data Team • 12/22/2025
The vulnerability identified as CVE-2025-12977 affects Fluent Bit's input plugins including in_http, in_splunk, and in_elasticsearch which are commonly used for collecting and forwarding log data across distributed systems. This issue stems from insufficient sanitization of tag_key inputs within these plugins, creating a potential attack vector that can be exploited by malicious actors with network access or the ability to inject records into Splunk or Elasticsearch systems. The flaw specifically manifests when these plugins process tag_key values containing special characters such as newlines or directory traversal sequences like ../ which are normally rejected by standard input validation mechanisms. The absence of proper sanitization allows these characters to be interpreted as valid tag components, enabling attackers to manipulate the routing behavior of log data within the Fluent Bit pipeline.
The technical implementation of this vulnerability involves the improper handling of user-supplied tag_key values in the input processing logic of Fluent Bit's plugins. When these plugins encounter tag_key values containing newline characters or path traversal sequences, they fail to properly validate or sanitize the input before using it to construct routing decisions or file paths. This processing flaw creates opportunities for attackers to inject malicious content that can be interpreted by downstream components, particularly when these tags are used to determine output destinations or file naming conventions. The vulnerability specifically impacts the tag_key parameter which is used to define how log records are tagged and subsequently routed through the system, making it a critical component in the data flow architecture.
The operational impact of this vulnerability extends beyond simple data corruption, potentially enabling several serious security consequences that affect data integrity and system availability. Attackers can exploit this weakness to perform newline injection attacks that may disrupt log processing pipelines or cause unexpected behavior in downstream systems that parse log files. Path traversal attacks using ../ sequences can allow unauthorized access to files outside of intended directories, potentially exposing sensitive system information or enabling arbitrary file read/write operations. Additionally, forged record injection techniques can manipulate the routing of log data, causing records to be misdirected to incorrect destinations or even to be dropped entirely from the logging pipeline. These capabilities can significantly compromise the integrity of log data and make it difficult to maintain accurate audit trails or perform effective security monitoring operations.
The vulnerability aligns with CWE-77 and CWE-20 categories from the Common Weakness Enumeration catalog, specifically addressing issues related to improper input validation and command injection. It also maps to several ATT&CK techniques including T1070.004 for Indicator Removal on Host and T1566.001 for Phishing for Information, as attackers may use this vulnerability to manipulate log data to hide malicious activities or gain unauthorized access to system information. Organizations using Fluent Bit with these input plugins are particularly vulnerable to attacks that exploit the lack of proper input sanitization, especially in environments where external systems can submit data or where network access is not properly restricted. The impact is amplified in multi-tenant environments or systems where different users or applications share the same logging infrastructure, as the vulnerability can enable cross-tenant data manipulation or information leakage.
Effective mitigation strategies should focus on implementing comprehensive input validation and sanitization mechanisms within the affected Fluent Bit plugins. Organizations should immediately update to patched versions of Fluent Bit that address this vulnerability, while also implementing network segmentation and access controls to limit the ability of unauthorized parties to submit data to affected systems. Additional protective measures include monitoring for unusual tag patterns in log data, implementing strict input validation at the network boundary, and using automated tools to detect and alert on potential exploitation attempts. System administrators should also review existing tag_key configurations to ensure that they do not inadvertently allow potentially malicious inputs to be processed by the affected plugins, and consider implementing additional logging and monitoring specifically focused on tag processing activities to detect potential exploitation attempts.