CVE-2025-12978: Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins contain a flaw in the tag_key validation log

Routine

MEDIUM 5.4DOWNGRADE

CVE-2025-12978 is a medium-severity flaw (CVSS 5.4) affecting fluent bit. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins contain a flaw in the tag_key validation logic that fails to enforce exact key-length matching. This allows crafted inputs where a tag prefix is incorrectly treated as a full match. A remote attacker with authenticated or exposed access to these input endpoints can exploit this behavior to manipulate tags and redirect records to unintended destinations. This compromises the authent

Why this priority

No elevated risk signals yet

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.4% (top 65%)
CISA exploited list	No
Published	2025-11-24
Last modified	2026-10-08

Publisher (CNA): CERT/CC · Vendor: treasuredata

Last updated 2026-10-08 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-11-24 15:15 UTCPublished CVE record published
2026-10-08 10:10 UTCUpdated Record last modified

treasuredata fluent bit DOWNGRADE

Affected: 4.1.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.1.0

1. Check installed version

apt-cache policy fluent-bit || rpm -q fluent-bit || fluent-bit --version

2. No fixed release: DOWNGRADE to a release before 4.1.0 (Debian / Ubuntu)

apt-cache madison fluent-bit sudo apt install fluent-bit=<VERSION_BEFORE_4.1.0> sudo apt-mark hold fluent-bit

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list fluent-bit sudo dnf downgrade fluent-bit-<VERSION_BEFORE_4.1.0> sudo dnf versionlock add fluent-bit

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-12978?

fluent bit: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2025-12978 being exploited?

CVE-2025-12978 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-12978 affect?

CVE-2025-12978 affects fluent bit. Severity is medium (CVSS 5.4). Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins contain a flaw in the tag_key validation logic that fails to enforce exact key-length matching. This allows crafted inputs where a tag prefix is incorrectly treated as a full

How urgent is CVE-2025-12978?

Priority 28/100. No elevated risk signals yet.

Software pages

Elasticsearch vulnerabilities · Splunk vulnerabilities

Related CVEs

CVE-2025-12970 HIGH 8.8 DOWNGRADE The extract_name function in Fluent Bit in_docker input plugin copies container names into a fixed size stack
CVE-2025-12977 CRITICAL 9.1 DOWNGRADE Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins fail to sanitize tag_key inputs
CVE-2025-12969 MEDIUM 6.5 DOWNGRADE Fluent Bit in_forward input plugin does not properly enforce the security.users authentication mechanism under
CVE-2025-12972 MEDIUM 5.3 DOWNGRADE Fluent Bit out_file plugin does not properly sanitize tag values when deriving output file names
CVE-2025-29478 MEDIUM 5.5 MITIGATE An issue in fluent-bit v.3.7.2 allows a local attacker to cause a denial of service via the cfl_list_size in c

References

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