Multiple vulnerabilities reported in Fluent Bit expose risks to cloud infrastructure

Oligo Security is reporting a chain of five vulnerabilities in Fluent Bit, an open-source log collection and forwarding tool that is embedded in billions of containers and has been deployed over 15 billion times.

 Fluent Bit is used across major cloud providers including AWS, Google Cloud, and Microsoft Azure, as well as in AI labs, financial institutions, automotive manufacturers, and countless other organizations. 

The flaws allow attackers to bypass authentication, perform path traversal attacks, achieve remote code execution, cause denial-of-service conditions, and manipulate logging tags. 

Vulnerabilities summary:

• CVE-2025-12977 (CVSS score 9.1) - An improper input validation flaw affecting HTTP, Splunk, and Elasticsearch inputs that allows tags derived from user-controlled fields to bypass sanitization, enabling attackers to inject newlines, traversal sequences, and control characters that corrupt downstream logs or enable broader output-based attacks.
• CVE-2025-12970 (CVSS score 8.8) - A stack buffer overflow in the Docker input plugin that enables attackers to trigger crashes or execute arbitrary code by creating containers with excessively long names, potentially giving them full control over the Fluent Bit agent on the host.
• CVE-2025-12969 (CVSS score 7.5) - An authentication bypass vulnerability where Fluent Bit forwarders configured with Security.Users but without a Shared_Key silently disable authentication, allowing remote attackers to send logs, inject false telemetry, or flood detection systems despite appearing secured.
• CVE-2025-12978 (CVSS score 5.4) - A partial string comparison flaw in tag-matching logic affecting HTTP, Splunk, and Elasticsearch inputs that allows attackers to spoof trusted tags by guessing only the first character of a Tag_Key, enabling them to reroute logs, bypass filters, and inject malicious or misleading records.
• CVE-2025-12972 (CVSS score 5.3) - A path traversal vulnerability in the file output plugin where unsanitized tag values are used to generate output filenames, allowing attackers to inject path-traversal sequences like "../" to write or overwrite arbitrary files on disk, enabling log tampering and, in many configurations, full remote code execution.

An attacker exploiting this chain of flaws could compromise the logging service itself, gain control over which events are recorded, erase or rewrite incriminating entries to hide their tracks, and inject fake telemetry to mislead security responders.

Affected versions are all Fluent Bit installations prior to version 4.1.1 and 4.0.12. Organizations using configurations with HTTP, Splunk, or Elasticsearch inputs combined with Tag_Key settings are especially vulnerable, as well as deployments using the Docker input plugin or forward input configurations with Security.Users but no Shared_Key. 

AWS has confirmed that it secured all internal systems relying on Fluent Bit and released version 4.1.1 to address these vulnerabilities.

Organizations are strongly urged to immediately update Fluent Bit to version 4.1.1 or 4.0.12 as the primary mitigation step. Additional recommended measures include: 

• avoid dynamic tags for routing by preferring static,
• use predefined tags in configurations,
• explicitly set fixed Path or File parameters in file outputs to prevent tag-based path expansion or traversal,
• mount configuration directories as read-only to prevent runtime tampering,
• run Fluent Bit as non-root users with restricted filesystem access.