CVE-2025-12977 affects Fluent Bit, an open-source log processor and forwarder widely used for collecting and routing logs in cloud-native and hybrid environments. The vulnerability resides in the input plugins in_http, in_splunk, and in_elasticsearch, which fail to properly sanitize the tag_key input parameter. Tags in Fluent Bit influence how logs are routed and processed, and some output plugins derive filenames or content based on these tags. An attacker with network access or the ability to inject records into Splunk or Elasticsearch can supply malicious tag_key values containing special characters such as newline characters or directory traversal sequences (e.g., ../). These unsanitized inputs can result in newline injection, allowing attackers to manipulate log formats; path traversal, enabling unauthorized file access or overwriting; forged record injection, compromising log integrity; and log misrouting, disrupting monitoring and alerting systems. The vulnerability does not require authentication or user interaction, increasing its risk. The CVSS 3.1 score of 9.1 reflects the high impact on confidentiality and integrity, with network vector and low attack complexity. No patches or known exploits are currently available, but the flaw's nature makes it a significant threat to environments relying on Fluent Bit for log management.

For European organizations, the impact of CVE-2025-12977 can be severe, especially those relying on Fluent Bit for centralized logging, security monitoring, and compliance reporting. Exploitation could lead to unauthorized modification or injection of log data, undermining the integrity of security logs and audit trails critical for incident detection and forensic analysis. Path traversal could allow attackers to overwrite or access sensitive files on logging servers, potentially leading to data breaches or disruption of logging infrastructure. Misrouting of logs can cause loss of visibility into security events, delaying detection of other attacks. Industries with strict regulatory requirements such as finance, healthcare, and critical infrastructure in Europe could face compliance violations and reputational damage. The network-based attack vector and lack of required authentication make it easier for attackers to exploit this vulnerability remotely, increasing the risk to cloud environments and hybrid deployments common in European enterprises.

European organizations should immediately audit their Fluent Bit deployments to identify usage of the in_http, in_splunk, and in_elasticsearch input plugins. Until official patches are released, organizations should implement strict input validation and sanitization at the network perimeter or proxy level to block tag_key values containing newline characters, path traversal sequences, or other special characters. Restrict network access to Fluent Bit input endpoints to trusted sources only, employing network segmentation and firewall rules. Monitor logs for unusual tag_key values or routing anomalies that could indicate exploitation attempts. Consider temporarily disabling vulnerable input plugins if feasible or isolating Fluent Bit instances handling untrusted data. Engage with Fluent Bit maintainers and subscribe to security advisories for timely patch releases. Additionally, enhance monitoring and alerting on log integrity and routing failures to detect exploitation early.