CVE-2025-12969
📋 TL;DR
This vulnerability allows remote attackers to bypass authentication in Fluent Bit's in_forward input plugin under certain configurations, enabling them to send unauthenticated log data. Attackers can inject forged logs, flood systems, or manipulate routing, compromising log integrity. Organizations using Fluent Bit with forward input exposed are affected.
💻 Affected Systems
- Fluent Bit
📦 What is this software?
Fluent Bit by Treasuredata
⚠️ Risk & Real-World Impact
Worst Case
Attackers inject malicious log data that triggers false alerts, overwhelms monitoring systems, or manipulates downstream processing decisions, potentially causing service disruption or misleading security teams.
Likely Case
Unauthenticated log injection leading to data integrity issues, alert fatigue from log flooding, and potential manipulation of log-based routing decisions.
If Mitigated
Limited impact if proper network segmentation and authentication controls are already in place, though authentication bypass remains possible.
🎯 Exploit Status
Exploitation requires network access to the forward input port and specific misconfigurations.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.1.0 or 4.0.12
Vendor Advisory: https://fluentbit.io/blog/2025/10/28/security-vulnerabilities-addressed-in-fluent-bit-v4.1-and-backported-to-v4.0/
Restart Required: Yes
Instructions:
1. Download Fluent Bit 4.1.0 or 4.0.12 from official sources. 2. Stop Fluent Bit service. 3. Replace binary or update package. 4. Restart Fluent Bit service.
🔧 Temporary Workarounds
Network Access Restriction
linuxRestrict network access to Fluent Bit forward input port using firewall rules.
sudo iptables -A INPUT -p tcp --dport 24224 -s trusted_ip_range -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 24224 -j DROP
Disable Forward Input
allTemporarily disable in_forward input plugin if not required.
Comment out or remove [INPUT] section with Name forward in Fluent Bit configuration
🧯 If You Can't Patch
- Implement strict network segmentation to limit access to Fluent Bit forward port to trusted sources only.
- Use additional authentication layers like TLS client certificates or IP whitelisting.
🔍 How to Verify
Check if Vulnerable:
Check Fluent Bit version and configuration for in_forward plugin with security.users enabled.Check Version:
fluent-bit --versionVerify Fix Applied:
Verify Fluent Bit version is 4.1.0 or 4.0.12 or higher, and test authentication enforcement.📡 Detection & Monitoring
Log Indicators:
- Unexpected log sources or formats in Fluent Bit logs
- Authentication failure messages followed by successful data ingestion
Network Indicators:
- Unusual traffic to Fluent Bit forward port from unauthorized IPs
- High volume of log data from unexpected sources
SIEM Query:
source="fluent-bit" AND (event="authentication_failure" OR src_ip NOT IN allowed_ips) AND dest_port=24224