CVE-2025-12969: Fluent Bit Auth Bypass Vulnerability

CVE-2025-12969 Overview

CVE-2025-12969 is an authentication bypass vulnerability in the Fluent Bit in_forward input plugin. The plugin fails to properly enforce the security.users authentication mechanism under specific configuration conditions. Remote attackers with network access to an exposed Fluent Bit forward input can send unauthenticated data into the logging pipeline. The flaw maps to CWE-306: Missing Authentication for Critical Function and affects Fluent Bit version 4.1.0. Attackers exploiting this issue can inject forged log records, flood downstream alerting systems, and manipulate routing decisions that depend on log content.

Critical Impact

Unauthenticated attackers can inject arbitrary log records into Fluent Bit pipelines, compromising the authenticity and integrity of ingested telemetry used for security monitoring and alerting.

Affected Products

• Treasure Data Fluent Bit 4.1.0
• Fluent Bit deployments using the in_forward input plugin with security.users authentication
• Cloud and container environments that expose the forward input port to untrusted networks

Discovery Timeline

• 2025-11-24 - CVE-2025-12969 published to NVD
• 2025-11-28 - Last updated in NVD database

Technical Details for CVE-2025-12969

Vulnerability Analysis

Fluent Bit is a widely deployed open-source log processor and forwarder used in Kubernetes, cloud, and observability stacks. The in_forward input plugin accepts log events over the Fluent Forward protocol and supports a security.users directive that defines username and password pairs for client authentication. Under certain configuration conditions, the plugin does not enforce these credentials before accepting incoming events.

The consequence is a break in the trust boundary between log producers and the log aggregation tier. Attackers do not need credentials, user interaction, or elevated privileges. They only need network reachability to the listening forward port. Because Fluent Bit pipelines often feed Security Information and Event Management (SIEM) systems, intrusion detection alerts, and billing or auditing systems, forged events can poison downstream decisions.

Root Cause

The root cause is a missing authentication check in the in_forward plugin's connection handling path. The plugin parses the security.users configuration but fails to require a valid handshake under specific conditions, allowing unauthenticated TCP clients to submit Forward, PackedForward, or Message mode events. This is a classic instance of CWE-306, where a critical function is exposed without enforcing the configured authentication control.

Attack Vector

Exploitation requires only network access to the Fluent Bit forward listener, which defaults to TCP port 24224. An attacker connects to the exposed port and transmits MessagePack-encoded Forward protocol frames containing arbitrary tags, timestamps, and record fields. Without authentication enforcement, the daemon accepts the events and routes them according to its configured output pipeline. See the Fluent Bit Security Blog Post and the Oligo Security Vulnerability Analysis for protocol-level details.

Detection Methods for CVE-2025-12969

Indicators of Compromise

• Unexpected log records arriving with tags or source identifiers that do not match deployed producers
• Connections to TCP port 24224 (or the configured forward port) from IP addresses outside the documented producer fleet
• Sudden spikes in log volume, duplicate events, or records with timestamps inconsistent with their stated origin
• Downstream alert storms triggered by suspicious but unverifiable log content

Detection Strategies

• Inspect Fluent Bit configurations to identify in_forward listeners that bind to non-loopback interfaces without TLS and security.users enforcement
• Correlate log records with authenticated source inventories to flag events that do not map to a known producer
• Monitor process and container telemetry for the Fluent Bit binary accepting connections from unexpected network segments

Monitoring Recommendations

• Capture NetFlow or VPC flow records for the Fluent Bit forward port and baseline expected client IPs
• Forward Fluent Bit's own diagnostic logs to a separate, trusted pipeline so authentication anomalies are not lost in the affected stream
• Alert on configuration drift in Fluent Bit manifests, Helm charts, or DaemonSets that introduce or modify in_forward blocks

How to Mitigate CVE-2025-12969

Immediate Actions Required

• Upgrade Fluent Bit to a fixed release as published in the Fluent Bit Security Blog Post covering v4.1 and the v4.0 backport
• Restrict the forward input listener to trusted networks using firewall rules, security groups, or Kubernetes NetworkPolicies
• Audit all Fluent Bit configurations for in_forward blocks and confirm that security.users is correctly applied and enforced after upgrading

Patch Information

The Fluent Bit maintainers addressed CVE-2025-12969 in Fluent Bit v4.1 with backports to the v4.0 branch. Operators running v4.1.0 should upgrade to the patched build referenced in the vendor security blog. Container image consumers should rebuild downstream images that pin vulnerable Fluent Bit versions and redeploy DaemonSets, sidecars, and standalone forwarders.

Workarounds

• Bind the in_forward listener to 127.0.0.1 where possible and aggregate events from local producers only
• Place Fluent Bit forward endpoints behind mutual TLS using the tls and tls.verify options to enforce client certificate validation
• Block ingress to TCP port 24224 from untrusted networks at the host firewall, cloud security group, or service mesh layer until the patch is deployed

# Configuration example: harden in_forward until patched
[INPUT]
    Name           forward
    Listen         127.0.0.1
    Port           24224
    tls            on
    tls.verify     on
    tls.ca_file    /etc/fluent-bit/ca.crt
    tls.crt_file   /etc/fluent-bit/server.crt
    tls.key_file   /etc/fluent-bit/server.key
    security.users /etc/fluent-bit/users.conf