Log corruption via unsanitized dynamic tags
Published Nov 24, 2025 · Updated Jan 7, 2026
Improper input validation in Fluent Bit before 4.0.12 allows remote attackers to corrupt logs or misroute records through crafted dynamic tags. The HTTP, Splunk, and Elasticsearch inputs accept tag_key record values without the sanitization applied to path-derived tags, preserving newlines, control characters, and traversal sequences. Reachability requires network access to an affected input or the ability to write source records, and the resulting effect depends on how downstream outputs use the tag.
Summary
What happened
Improper input validation in Fluent Bit before 4.0.12 allows remote attackers to corrupt logs or misroute records through crafted dynamic tags. The HTTP, Splunk, and Elasticsearch inputs accept tag_key record values without the sanitization applied to path-derived tags, preserving newlines, control characters, and traversal sequences. Reachability requires network access to an affected input or the ability to write source records, and the resulting effect depends on how downstream outputs use the tag.
The record
- CVE
- CVE-2025-12977
- Published
- Nov 24, 2025
- Updated
- Jan 7, 2026
- Vendor
- Fluent Bit
- Product
- Fluent Bit
- Classifications
- CWE-1287, T1565.002
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Nov 24, 2025CVE publishedPublication date reported by the CVE source.
- Jan 7, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <4.0.12
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo