CVE-2025-12978 in FluentBit
Summary
by MITRE • 11/24/2025
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.
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Analysis
by VulDB Data Team • 12/22/2025
The vulnerability identified as CVE-2025-12978 affects Fluent Bit's input plugins including in_http, in_splunk, and in_elasticsearch which are commonly used for log collection and forwarding in distributed systems. This flaw resides in the tag_key validation mechanism that is responsible for ensuring proper routing of log records based on predefined tag structures. The issue stems from insufficient validation logic that fails to enforce strict exact matching of key lengths, creating a path for malicious input manipulation.
The technical implementation of this vulnerability allows attackers to exploit a logic error in how tag keys are validated during input processing. When a tag_key is configured with specific length requirements, the validation process does not properly enforce these constraints, enabling crafted inputs to bypass normal tag matching procedures. This occurs because the system treats a prefix match as equivalent to a full match when it should require exact length correspondence between the incoming tag and the configured tag_key. The flaw essentially creates a tag injection vector where malicious actors can manipulate the routing behavior of log records.
From an operational impact perspective, this vulnerability enables remote attackers with authenticated access or those who can reach exposed input endpoints to manipulate the entire log ingestion pipeline. The compromised authenticity of ingested logs directly affects security monitoring and incident response capabilities, as attackers can forge data that appears legitimate to the system. This manipulation can result in alert flooding when records are incorrectly routed to multiple destinations, or more subtly when data is directed to unauthorized systems. The vulnerability essentially undermines the integrity of log collection processes and can be leveraged for data exfiltration or system compromise through crafted log entries.
The security implications extend beyond simple data manipulation as this vulnerability can be exploited to bypass security controls that rely on proper tag routing. Attackers can redirect records to unintended destinations, potentially causing confusion in security operations centers or enabling them to inject malicious data into specific monitoring systems. This behavior aligns with attack patterns documented in the ATT&CK framework under data injection and credential access techniques, where adversaries manipulate system inputs to achieve their objectives. The vulnerability also relates to CWE-20, which addresses improper input validation, and CWE-345, which covers insufficient verification of data authenticity.
Organizations should implement immediate mitigations including updating to patched versions of Fluent Bit that properly enforce tag_key length validation, implementing network segmentation to limit access to input endpoints, and deploying additional monitoring for unusual tag routing patterns. Configuration reviews should ensure that tag_key validation is properly enforced and that input endpoints are not exposed to untrusted networks. Additional defensive measures include implementing input validation at multiple layers, using authentication and authorization controls, and establishing anomaly detection for log routing behavior to identify potential exploitation attempts. The vulnerability highlights the critical importance of proper input validation in security-critical components and demonstrates how seemingly minor validation logic flaws can have significant operational security implications.