CVE-2025-12977: Fluent Bit Path Traversal Vulnerability

CVE-2025-12977 Overview

CVE-2025-12977 is a critical input validation vulnerability in Fluent Bit affecting the in_http, in_splunk, and in_elasticsearch input plugins. These plugins fail to properly sanitize tag_key inputs, allowing attackers with network access or the ability to write records into Splunk or Elasticsearch to supply malicious tag values containing special characters such as newlines or path traversal sequences (../). Since tags influence routing logic and some output plugins derive filenames or content from these tags, successful exploitation can lead to newline injection, path traversal, forged record injection, or log misrouting.

Critical Impact

Attackers can manipulate log routing, inject forged records, or perform path traversal attacks by exploiting unsanitized tag_key values in Fluent Bit input plugins, compromising data integrity across cloud logging infrastructure.

Affected Products

• Treasuredata Fluent Bit version 4.1.0
• Fluent Bit in_http input plugin
• Fluent Bit in_splunk input plugin
• Fluent Bit in_elasticsearch input plugin

Discovery Timeline

• 2025-11-24 - CVE-2025-12977 published to NVD
• 2025-11-28 - Last updated in NVD database

Technical Details for CVE-2025-12977

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-1287) in Fluent Bit's input plugin architecture. The in_http, in_splunk, and in_elasticsearch plugins accept a tag_key configuration parameter that determines which field from incoming records should be used as the routing tag. However, the plugins fail to validate or sanitize the extracted tag values before using them in downstream processing.

When an attacker supplies records containing specially crafted values in the field designated by tag_key, those values are accepted verbatim as valid tags. Since Fluent Bit's internal routing mechanism trusts these tags implicitly, malicious payloads propagate through the logging pipeline. Output plugins that use tags to construct file paths, determine destinations, or format output records become vulnerable to manipulation.

The attack is particularly dangerous in cloud environments where Fluent Bit aggregates logs from multiple sources. An attacker who can inject records into monitored data sources (such as Splunk or Elasticsearch indices) or send HTTP requests to the in_http endpoint gains the ability to influence how all subsequent log processing occurs.

Root Cause

The root cause is insufficient input validation in the tag extraction logic within the affected input plugins. When processing incoming records, the plugins extract the value from the configured tag_key field and use it directly as a Fluent Bit tag without validating that the value conforms to expected tag format constraints. The code fails to reject or sanitize special characters including newlines (\n), path traversal sequences (../), and other potentially dangerous character combinations that have semantic meaning in downstream contexts.

Attack Vector

The attack requires network access to send malicious records to one of the vulnerable input plugins. For the in_http plugin, an attacker can craft HTTP POST requests containing records with malicious tag_key values. For in_splunk and in_elasticsearch plugins, an attacker who can write records into the monitored Splunk or Elasticsearch instances can inject poisoned records that will be processed by Fluent Bit.

Once a malicious tag value enters the system, the impact depends on the output configuration. If file-based outputs derive filenames from tags, path traversal sequences can write logs to arbitrary filesystem locations. Newline injection can corrupt log formats or inject forged entries. Tag manipulation can cause logs to be routed to unintended destinations, potentially bypassing security monitoring or causing data loss.

For technical details on exploitation scenarios, refer to the Oligo Security Vulnerability Analysis.

Detection Methods for CVE-2025-12977

Indicators of Compromise

• Fluent Bit log entries containing tags with unexpected special characters such as ../, ..\\, or newline sequences
• Unexpected files appearing in directories outside the configured log output paths
• Log files with corrupted formatting or records that appear injected between legitimate entries
• Routing anomalies where logs are delivered to incorrect destinations

Detection Strategies

• Monitor Fluent Bit input plugins for incoming records containing suspicious characters in fields that map to tag_key configuration
• Implement file integrity monitoring on directories near log output paths to detect path traversal attempts
• Audit Fluent Bit configurations to identify deployments using tag_key with untrusted input sources
• Enable verbose logging to capture the full content of processed tags for forensic analysis

Monitoring Recommendations

• Deploy network monitoring to inspect HTTP POST requests to Fluent Bit in_http endpoints for malicious payloads
• Configure alerts for any tag values containing path separators or control characters
• Implement SentinelOne Singularity to detect anomalous file system activity indicative of path traversal exploitation
• Review Splunk and Elasticsearch write permissions to limit which principals can inject records that Fluent Bit will process

How to Mitigate CVE-2025-12977

Immediate Actions Required

• Upgrade Fluent Bit to version 4.1 or later, which includes fixes for this vulnerability
• Review all Fluent Bit configurations using in_http, in_splunk, or in_elasticsearch plugins with tag_key settings
• Restrict network access to Fluent Bit input endpoints to trusted sources only
• Audit Splunk and Elasticsearch index permissions to limit write access to trusted principals

Patch Information

Security patches addressing this vulnerability have been released in Fluent Bit version 4.1 and backported to version 4.0. Organizations should upgrade to the patched versions immediately. For detailed patch information and upgrade instructions, see the Fluent Bit Security Blog Post.

Workarounds

• Disable the tag_key configuration option on affected input plugins if dynamic tagging from record content is not required
• Implement a Lua filter plugin to sanitize tag values before they reach output plugins
• Deploy Fluent Bit behind a reverse proxy that validates and sanitizes incoming requests
• Use static tag configurations instead of dynamic tag_key extraction from untrusted sources

# Example: Use static tags instead of dynamic tag_key extraction
# Before (vulnerable configuration):
# [INPUT]
#     Name http
#     Listen 0.0.0.0
#     Port 9880
#     Tag_Key source

# After (hardened configuration with static tag):
[INPUT]
    Name http
    Listen 0.0.0.0
    Port 9880
    Tag static.http.input