CVE-2025-12978
📋 TL;DR
Fluent Bit's in_http, in_splunk, and in_elasticsearch input plugins have a tag validation flaw where tag prefixes can be incorrectly matched as full tags. This allows authenticated attackers to manipulate log routing, potentially redirecting records to unintended destinations. Organizations using Fluent Bit with these plugins exposed to untrusted networks are affected.
💻 Affected Systems
- Fluent Bit
📦 What is this software?
Fluent Bit by Treasuredata
⚠️ Risk & Real-World Impact
Worst Case
Attackers could redirect all logs to malicious endpoints, completely compromising log integrity, injecting forged data, and causing alert flooding that disrupts security monitoring.
Likely Case
Selective log manipulation where attackers redirect specific log types to bypass monitoring or inject misleading data into security systems.
If Mitigated
With proper network segmentation and authentication, impact is limited to authorized users who could still manipulate logs they have access to.
🎯 Exploit Status
Exploitation requires authenticated access to the input endpoints or exposure to untrusted networks.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.1.0
Vendor Advisory: https://fluentbit.io/announcements/v4.1.0/
Restart Required: Yes
Instructions:
1. Download Fluent Bit 4.1.0 or later from official sources. 2. Stop Fluent Bit service. 3. Replace binary with patched version. 4. Restart Fluent Bit service. 5. Verify version with 'fluent-bit --version'.
🔧 Temporary Workarounds
Network Access Restriction
linuxRestrict network access to Fluent Bit input endpoints using firewall rules
iptables -A INPUT -p tcp --dport [FLUENTBIT_PORT] -s [TRUSTED_NETWORK] -j ACCEPT
iptables -A INPUT -p tcp --dport [FLUENTBIT_PORT] -j DROP
Authentication Enforcement
allEnsure strong authentication is enabled on all input plugins
# Configure authentication in fluent-bit.conf
[INPUT]
Name http
Port 8888
HTTP_User user
HTTP_Passwd secure_password
🧯 If You Can't Patch
- Implement strict network segmentation to isolate Fluent Bit endpoints from untrusted networks
- Enable and audit authentication on all input plugins, using strong credentials and regular rotation
🔍 How to Verify
Check if Vulnerable:
Check Fluent Bit version with 'fluent-bit --version' and verify if below 4.1.0 with vulnerable plugins enabledCheck Version:
fluent-bit --versionVerify Fix Applied:
Confirm version is 4.1.0 or higher with 'fluent-bit --version' and test tag validation with crafted inputs📡 Detection & Monitoring
Log Indicators:
- Unexpected tag patterns in Fluent Bit logs
- Logs appearing in unexpected output destinations
- Authentication failures on input endpoints
Network Indicators:
- Unusual traffic patterns to Fluent Bit input ports
- Requests with crafted tag prefixes
SIEM Query:
source="fluent-bit" AND (tag="*prefix*" OR dest="unexpected_endpoint")