Fluent Bit Flaws Open the Door to Log Hijacking and Cloud Takeover

A new chain of five critical Fluent Bit vulnerabilities could let attackers tamper with logs, crash agents, or even execute code across cloud and Kubernetes environments.

Fluent Bit is embedded in billions of containers and powers observability pipelines for banks, AI platforms, manufacturers, and major cloud providers. 

“The vulnerabilities create pathways for attackers to disrupt cloud services, tamper with data, and gain deeper access to the same Cloud and Kubernetes infrastructure,” said Oligo Security researchers.

Breaking Down the Fluent Bit CVEs

The newly disclosed Fluent Bit vulnerabilities expose critical weaknesses across its ingestion and processing pipeline, giving attackers multiple pathways to manipulate logs or execute code.

CVE-2025-12972 

CVE-2025-12972 is a path-traversal flaw in Fluent Bit caused by unsanitized tag values being used to generate output filenames. 

When the File parameter is not set, Fluent Bit derives filenames from incoming tags, allowing attackers to embed ../ sequences and write logs to arbitrary filesystem locations. 

With partial control over the log content, attackers can overwrite system files, plant malicious code, tamper with logs, or even achieve remote code execution in environments where Fluent Bit runs with elevated permissions.

CVE-2025-12970  

CVE-2025-12970 is a stack buffer overflow in Fluent Bit’s Docker Metrics input plugin, caused by copying container names into a fixed 256-byte buffer without checking their length. 

An attacker who can create or influence container names — through Docker APIs or tampered configuration files — can supply a name longer than 256 characters and trigger the overflow. 

This results in memory corruption that can crash the Fluent Bit agent or enable remote code execution. 

Because the plugin often runs with access to the Docker socket, successful exploitation could give attackers control of the logging agent and access to sensitive logs or node-level privileges.

CVE-2025-12978 

CVE-2025-12978 is caused by Fluent Bit comparing only the length — not the full name — of a tag_key field, allowing a payload with just the first character of the expected key to be treated as a match. 

This lets attackers spoof trusted tags without knowing the actual tag_key and route their logs through filters and outputs meant for other services or tenants. 

By controlling the tag value, attackers can bypass filters, inject misleading data, and disrupt downstream monitoring. Any deployment using tag_key with HTTP, Elasticsearch, or Splunk inputs is affected.

CVE-2025-12977 

CVE-2025-12977 occurs because Fluent Bit fails to sanitize tags derived from user-controlled fields when using the tag_key option. 

These dynamic tags can include characters like newlines, control sequences, or ../, which many output plugins embed directly into filenames, log entries, or routing logic. 

As a result, attackers can corrupt logs, inject forged entries, break parsing, or trigger path traversal depending on configuration. 

This flaw expands the attack surface by turning ordinary log fields into vectors for injection and downstream manipulation.

CVE-2025-12969  

CVE-2025-12969 allows silent authentication bypass in Fluent Bit’s in_forward input when security.users is configured without a shared_key. 

In this setup, Fluent Bit fails to enforce authentication, letting attackers send logs without credentials even though operators believe user-based authentication is enabled. 

This enables false telemetry injection, log tampering, alert flooding, and concealment of malicious activity. The flaw is risky in multi-tenant or cloud environments where forwarders are exposed to network access.

This vulnerability chain underscores the importance of securing the tools responsible for collecting and routing operational data. 

Mitigation Strategies for the Fluent Bit CVEs

Addressing the Fluent Bit vulnerabilities requires a layered and proactive approach to hardening log pipelines. 

Because these flaws affect routing, authentication, and file handling, organizations must combine patching with stronger environmental controls to reduce exploitation risk.

• Upgrade Fluent Bit to versions 4.1.1 or 4.0.12 to apply fixes for tag sanitization, authentication enforcement, and safer input handling.
• Restrict and segment network access to Fluent Bit inputs by limiting which services can reach in_forward, HTTP, or other exposed plugins.
• Use static tags and validate all incoming log data to prevent untrusted inputs from influencing routing, filenames, or downstream processing.
• Lock down output paths and filesystems by setting fixed file names, mounting configuration directories as read-only, and enforcing SELinux/AppArmor policies.
• Run Fluent Bit in a hardened environment using least-privilege permissions, container isolation, mTLS for log forwarding, and secure credential management.
• Rate-limit and monitor logging traffic to detect anomalous tag patterns, unexpected file writes, agent crashes, or surges in inbound logs.
• Apply downstream safeguards in SIEM and storage systems with schema validation, integrity checks, and redundant logging paths to prevent or detect log tampering.

Securing Fluent Bit is essential, because compromised telemetry undermines every downstream detection and response process.

The Rising Threat to Logging and Telemetry Systems

These vulnerabilities underscore a broader shift in the threat landscape: the infrastructure that powers observability — long treated as low-risk, behind-the-scenes plumbing — has become a high-value target for attackers. 

As organizations lean more heavily on logs, metrics, and traces for detection and response, compromising the telemetry pipeline can be just as damaging as compromising an application or database. 

Fluent Bit’s placement at the intersection of untrusted input and sensitive operational data makes weaknesses in its routing or file-handling logic especially dangerous. 

A single flaw in this layer can distort visibility, erode forensic integrity, and blind security teams at the moment they rely on telemetry the most.

This erosion of visibility highlights why modern defenses must assume compromise by default — a core principle at the heart of zero-trust security.