External risk intelligence

Fluent Bit input plugins allow tag manipulation for routing and filename control.

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2025-12977

Fluent Bit is commonly deployed as a log processor and forwarder in cloud-native environments, often acting as a gateway or edge service for collecting telemetry data. The in_http, in_splunk, and in_elasticsearch input plugins are designed to ingest data over the network, making these endpoints reachable in typical infrastructure-as-code and containerized deployment patterns.

Path Traversal

Treasuredata Fluent Bit

4.1.0

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

Fluent Bit input plugins, used for processing data from HTTP, Splunk, and Elasticsearch, have a vulnerability that could allow attackers to manipulate log routing or create malicious files. This occurs when special characters are injected into data keys, potentially impacting data integrity and how logs are managed. The primary concern is to confirm if your environment uses these specific plugins and is exposed to this type of input.

Special characters in data keys are not properly handled.
Could affect log integrity and routing logic.
Confirm relevance and exposure in your environment.

Attack Path

How an attacker could exploit the issue

Attackers can target Fluent Bit's input plugins if they have network access or can write records to Splunk or Elasticsearch. By manipulating tag keys with special characters, they can hijack data routing, inject forged records, or cause log misrouting, potentially leading to data corruption or unauthorized access.

Network access or record writing ability.
Injecting special characters into tag keys.
Data integrity and log routing compromise.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, an attacker with network access could inject special characters into tag keys, potentially leading to log misrouting, forged record injection, or impact on data integrity by affecting how logs are processed and stored.

Log integrity and routing.
Injection of special characters in tags.
Data misrouting or forged records.

Operational Fix

Recommended remediation, mitigation, and detection steps

The critical vulnerabilities in Fluent Bit's input plugins require immediate attention from teams managing data ingestion and logging pipelines. Identifying all deployments of affected Fluent Bit instances, confirming their network exposure, and assessing business criticality are the crucial first steps. Once identified, the accountable owner should be determined to plan and coordinate remediation, prioritizing efforts based on risk and potential impact to data integrity and routing.

Ownership: Platform and application teams.
Verify: Network reachability and business criticality.
Action: Plan coordinated remediation and data integrity checks.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Fluent Bit and where is it used?

Fluent Bit is an open-source log processor and forwarder frequently deployed in cloud-native environments. It acts as a central hub or edge service, collecting, parsing, and routing telemetry data from diverse sources to various storage destinations. Because it handles high volumes of log data, it is a critical component for maintaining observability pipelines in containerized infrastructure.

What does CVE-2025-12977 mean for log data?

This vulnerability is an Improper Validation of Specified Input (CWE-1287) affecting specific input plugins. It means Fluent Bit fails to sanitize tag_key inputs, allowing special characters like newlines or path traversal sequences. Attackers can exploit this to trick the system into misrouting logs, creating unauthorized files, or injecting forged records, which compromises the integrity and reliability of your log management system.

How do attackers trigger this vulnerability?

An attacker triggers the bug by sending specifically crafted input containing special characters through the in_http, in_splunk, or in_elasticsearch plugins. If the input source is trusted or restricted to internal logs, the risk is lower. The vulnerability is not triggered by standard, well-formed log messages that do not contain malicious control characters or directory navigation sequences in the tag_key fields.

Is my Fluent Bit instance at risk?

According to Halo Surface Signal, this vulnerability is likely relevant if your Fluent Bit instances act as gateways or edge services reachable over a network. Instances ingesting data from external sources or public-facing HTTP endpoints are at higher risk. Internal deployments might have reduced risk, but you should evaluate if your configuration allows data input from untrusted sources into the affected plugins.

What should I do first to address CVE-2025-12977?

Start by identifying all Fluent Bit instances in your environment and auditing their configurations to see if the in_http, in_splunk, or in_elasticsearch plugins are active. Once you have an inventory of these deployments, assess their network reachability and business criticality. Coordinate with your platform and application teams to prioritize these instances for updates and ensure your logging pipeline integrity is maintained.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.