A critical unauthenticated data-injection flaw in Fluent Bit’s forward input plugin has been publicly cataloged as CVE-2025-12969; the bug lets an attacker who can reach a Fluent Bit forward listener send unauthenticated records by bypassing the configured security.users control, enabling forged log injection, alert flooding, and routing manipulation that can undermine detection, forensics, and trust in centralized logging pipelines.
Fluent Bit is a lightweight, high-performance log forwarder widely used inside containers, Kubernetes DaemonSets, cloud-hosted telemetry pipelines, and edge agents. It consumes logs, transforms or enriches records, and forwards them to a broad set of outputs (Elasticsearch, Splunk, cloud logging services, files, etc.. For many organizations, Fluent Bit is the first trusted gatekeeper for telemetry; that trust makes any flaw that allows forged records or dropped records consequential beyond the host that runs the agent.
On November 24, 2025 the issue recorded as CVE-2025-12969 was published: the Fluent Bit in_forward input plugin “does not properly enforce the security.users authentication mechanism under certain configuration conditions,” allowing remote attackers with network access to bypass intended authentication and submit unauthenticated payloads. The problem was disclosed as part of a larger group of vulnerabilities addressed in Fluent Bit v4.1 and backported to v4.0 releases.
The core security imperative is clear: follow the vendor patches now, lock down network exposure, and treat logging pipelines as a hardened service rather than a convenience. Injected or lost logs are not mere noise— they are the blind spots attackers rely on. Patch, verify, and monitor.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
Fluent Bit is a lightweight, high-performance log forwarder widely used inside containers, Kubernetes DaemonSets, cloud-hosted telemetry pipelines, and edge agents. It consumes logs, transforms or enriches records, and forwards them to a broad set of outputs (Elasticsearch, Splunk, cloud logging services, files, etc.. For many organizations, Fluent Bit is the first trusted gatekeeper for telemetry; that trust makes any flaw that allows forged records or dropped records consequential beyond the host that runs the agent.On November 24, 2025 the issue recorded as CVE-2025-12969 was published: the Fluent Bit in_forward input plugin “does not properly enforce the security.users authentication mechanism under certain configuration conditions,” allowing remote attackers with network access to bypass intended authentication and submit unauthenticated payloads. The problem was disclosed as part of a larger group of vulnerabilities addressed in Fluent Bit v4.1 and backported to v4.0 releases.
What the vulnerability actually is
The technical root cause (concise)
- The vulnerability affects the in_forward plugin, the component that accepts Fluentd/forward protocol traffic (commonly used by Fluentd and other forwarders).
- When operators configure authentication via the plugin’s security.users setting, under a certain configuration pattern the authentication check is not enforced correctly; specifically, if security.users is supplied without a required shared key parameter, the listener may accept unauthenticated connections. In practice the guarded authentication gate can be bypassed and the plugin will accept and process records from unauthenticated senders.
What an attacker can do
By exploiting the bypass an attacker who can reach the forward listener port can:- Inject forged log records that pollute audit trails or conceal malicious behavior.
- Flood logging backends and alerting systems with noise or bogus events (alert fatigue, missed true positives).
- Manipulate routing decisions by supplying crafted tags/keys so that records are routed to unintended sinks or file paths.
- In chained attacks or misconfigurations, enable path traversal, file overwrite, or amplify opportunities for further exploitation where other Fluent Bit flaws exist.
Timeline and publish details (verified)
- Vulnerability disclosed publicly and CVE assigned: November 24, 2025.
- Vendor advisory: Fluent Bit project posted a security advisory and released fixed builds in the 4.1.x and backported 4.0.x lines. The vendor advisory and release notes list the missing-authentication issue as remediated.
- Independent trackers and coordination: CERT/CC and other vulnerability databases indexed the CVE; several publications and security vendors published write-ups and risk assessments shortly after the disclosure.
Severity, scoring and exploitability — what the data says
- The NVD description confirms the bypass of the security.users authentication mechanism, but at the time of publication NVD did not supply a complete CVSS v3.x vector string on its own page. The record does appear in public CVE feeds.
- Several aggregators and vulnerability trackers list a moderate-to-high practical severity driven more by impact on integrity and reliability of logs than by direct code execution. One widely cited composite shows a CVSS v3 rating in the mid-range (around 6.5/10) reflecting network vector and no required privileges or user interaction for the bypass condition.
- Exploitability is straightforward in the right environment: if the forward port is reachable and the misconfiguration exists, an attacker can send traffic without needing credentials. However, the CVE is not a classical remote code execution (RCE) by itself; its damage model is centered on data integrity, detection evasion, and telemetry poisoning—effects that are strategic and operationally severe in monitored environments.
Who and what is affected
- Affected component: Fluent Bit — specifically the in_forward input plugin when configured with security.users under the vulnerable pattern.
- Confirmed affected versions: disclosures point at the 4.1.0 line and earlier 4.0.x commits that were backported; vendor releases patched the issues in 4.1.1 and backported fixes to 4.0.12 (or specific patch-level updates listed in the vendor advisory). Operators should verify exact build numbers in their environment.
- Exposure profile: any system running Fluent Bit with the forward input listening on a network-accessible interface (including container host networking, NodePort services, or unprotected cloud ports) is at risk if misconfigured. Default container deployments that expose the forward port publicly or over untrusted networks are particularly vulnerable.
Why this matters to WindowsForum readers and enterprise admins
- Fluent Bit is widely deployed in mixed Windows/Linux hybrid cloud environments as a telemetry collector for both platform and application logs. Tampering with log integrity defeats one of the fundamental pillars of security operations: reliable telemetry for detection and incident response.
- An attacker who can inject or suppress logs can mask lateral movement, persistence, and exfiltration, making detection significantly harder even if other defenses are in place.
- Because Fluent Bit is frequently run in privileged container contexts or as a system-level agent, a compromised telemetry pipeline can be abused to reroute sensitive data or manipulate operational processes that depend on logs (for example, automated responses or alert-driven orchestration).
Practical detection and hunting guidance
Short, high-signal detection steps to run now:- Inspect Fluent Bit configurations where in_forward is enabled. Confirm whether security.users is set and, crucially, whether the configuration also uses a Shared_Key (or equivalent shared secret) as required by your policy. If security.users is configured without the shared key, assume the listener may accept unauthenticated traffic.
- Hunt for anomalous sources connected to forward ports: unexpected IP addresses connecting to forward listener ports, sudden bursts of write volume to logging outputs, or unusual tag patterns that route to unexpected sinks.
- Examine logs for implausible records—e.g., service logs that appear out of order, repeated identical entries intended to drown true alerts, or records with tags/fields that map to sensitive sinks. Flag records originating from unknown container IDs or node names.
- Monitor for suspicious filesystem activity if you use outputs like Out_File or file-based outputs—combined issues disclosed with Fluent Bit include path traversal and out_file path risks. If you see file writes to unexpected paths driven by incoming tags, investigate immediately.
Remediation and mitigation — immediate steps
- Patch first: upgrade Fluent Bit to the vendor-released patched builds (the advisory lists fixes in the 4.1.x and backported 4.0.x lines). Confirm the exact patched version for your build and apply it promptly. Re-deploy agent containers or packages as part of an immutable update process.
- Harden network exposure:
- Block access to the in_forward listener from untrusted networks.
- Ensure the port is not exposed via NodePort, public LoadBalancer, or host networking without proper firewall controls.
- Use network policies (Kubernetes NetworkPolicy, host firewall rules) or cloud security groups to restrict inbound sources to known collectors.
- Enforce authentication and secrets:
- Where possible, deploy mutual TLS (mTLS) or a transport-layer security mechanism in front of Fluent Bit or use a secure, authenticated transport.
- Confirm security.users is configured correctly and that shared keys or equivalent credentials are present and rotated regularly. Do not rely on configuration that appears to enable auth but omits required keys.
- Apply compensating controls:
- Rate-limit logging endpoints and create alerting for abnormal ingestion volumes.
- Add input-side validation where feasible (drop records with unexpected tags/fields).
- Use trusted message signing or integrity checks in downstream pipelines where your environment supports them.
- Validate after patching:
- After upgrades, validate behavior under test with known-good clients and attempt to reproduce the bypass in a lab (do not run tests on production logging endpoints).
- Confirm downstream logs and metadata are free of forged records and confirm the expected routing rules apply.
Longer-term remediation and hardening recommendations
- Replace unauthenticated transports: prefer TLS with client certificates or established secure broker patterns (e.g., push over an authenticated message broker) rather than leaving forward listeners unprotected.
- Implement defense in depth for telemetry: treat logs as a separate trust boundary—apply access controls, immutability where appropriate (append-only logging), and independent retention to prevent single-point manipulation.
- Centralize and harden logging agent configuration via an immutable configuration pipeline (GitOps) and reduce runtime config drift that can lead to insecure patterns.
- Consider adding server-side validation rules in collectors to discard inputs with malformed tags or disallowed fields; this raises the effort for blind injection attacks.
Detection signatures and SIEM rules (examples)
- SIEM Rule: Alert when a node that does not normally generate specific event types (for example, an infrastructure-only node) suddenly emits application audit logs.
- IDS Rule: Flag connections to forward listener ports from IPs not whitelisted in your network policy.
- EDR/Host Rule: Flag spawn events or writes to logging output files initiated by fluent-bit process that appear at unusual times or in unexpected directories (for out_file outputs).
- Integrity Rule: Alert if tag-derived file paths are created outside configured directories (protects against path-traversal payloads combined with tag injection).
Risk assessment: what can go wrong if you don’t patch?
- Silent deception: Attackers can insert false positives or false negatives into your telemetry so that detection engines generate noise while real malicious activity is concealed.
- Post-compromise stealth: If the attacker can alter logs during or after an intrusion, forensic trails are unreliable and incident response is severely hamstrung.
- Audit and compliance failures: Systems that depend on tamper-proof logging for compliance can fail regulatory obligations if records are corrupted or forged.
- Amplification with other bugs: CVE-2025-12969 sits alongside several other Fluent Bit issues (path traversal, buffer overflows, tag-handling bugs) disclosed in the same advisory window; combined these can materially raise the attack surface and enable higher-impact outcomes.
How to validate your environment (step-by-step)
- Inventory: list Fluent Bit instances, their versions, and which inputs are enabled (especially in_forward).
- Configuration review: for every instance with in_forward enabled, check the security.users stanza and confirm that a Shared_Key or equivalent secret is present. If not, treat the instance as vulnerable until patched and reconfigured.
- Network scan: identify which instances have the forward listener reachable from untrusted networks. Block or restrict access immediately.
- Deploy patch: upgrade to the officially released patched Fluent Bit build in a maintenance window, then restart agents. Confirm the new version in running container images or package manifests.
- Post-patch validation: test with an authorized client and attempt an unauthenticated send from a controlled test host to confirm that authentication is now enforced. Do not attempt to test with production data.
- Forensics check: search historical logs for evidence of anomalous injection, spike patterns, or routing anomalies coinciding with periods where the misconfiguration may have existed.
Communications and governance points
- Notify stakeholders: Operations, Security, and Cloud teams must be informed and involved. Logging integrity is a cross-functional concern that touches compliance, security monitoring, and application reliability.
- Incident response readiness: If you detect signs of prior injection, escalate to incident response—tampered logs require a different preservation and validation approach.
- Patch policy: Update deployment playbooks to include Fluent Bit as a monitored dependency for future security and quality rollouts. Automate version checks where possible.
Final assessment and practical takeaway
CVE-2025-12969 is not a hyper-technical remote-code-execution worm on its own, but it is a materially dangerous vulnerability because it subverts the trust on which security monitoring depends. The practical consequences—alert fatigue, forged audit trails, and loss of forensic integrity—are exactly the kind of silent failures that let real attackers persist and operate undetected.- Immediate action: patch Fluent Bit, restrict forward listener exposure, and verify configurations for security.users and shared keys.
- Medium-term action: treat telemetry as a hardened, independent trust boundary: add authenticity, segmentation, and independent retention to your logging pipeline.
Appendix — Key references and notes for verification
- The NVD entry for CVE-2025-12969 documents the in_forward authentication bypass and lists references used by national vulnerability databases. Confirm the NVD entry for authoritative tracking information.
- Fluent Bit’s official release notes and security advisory describe the fixes shipped in the 4.1.x and backported 4.0.x lines; consult those release notes for exact fixed-version numbers and upgrade instructions.
- CERT/CC and other VU databases provide coordinated vulnerability advisories and additional contextual risk notes; cross-check those entries to understand potential attack surface implications.
- Independent reporting and analysis from technical outlets and security vendors provide operational context and recommended mitigations—use these to prioritize patching and detection work.
The core security imperative is clear: follow the vendor patches now, lock down network exposure, and treat logging pipelines as a hardened service rather than a convenience. Injected or lost logs are not mere noise— they are the blind spots attackers rely on. Patch, verify, and monitor.
Source: MSRC Security Update Guide - Microsoft Security Response Center
