CVE-2025-12977: Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins fail to sanitize tag_key inputs

This month

CRITICAL 9.1REMOTEDOWNGRADE

CVE-2025-12977 is a critical-severity flaw (CVSS 9.1) 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 fail to sanitize tag_key inputs. An attacker with network access or the ability to write records into Splunk or Elasticsearch can supply tag_key values containing special characters such as newlines or ../ that are treated as valid tags. Because tags influence routing and some outputs derive filenames or contents from tags, this can allow newline injection, path traversal, forged r

Why this priority

Critical severity
Remotely exploitable, no login, no user action

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

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.8% (top 45%)
CISA exploited list	No
Weakness	CWE-1287
Published	2025-11-24
Last modified	2026-10-18

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

Last updated 2026-10-18 · 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
2026-10-18 20:10 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-2025-12977?

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

Is CVE-2025-12977 being exploited?

CVE-2025-12977 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-2025-12977 affect?

CVE-2025-12977 affects fluent bit. Severity is critical (CVSS 9.1). Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins fail to sanitize tag_key inputs. An attacker with network access or the ability to write records into Splunk or Elasticsearch can supply tag_key values containing special cha

How urgent is CVE-2025-12977?

Priority 51/100. Critical severity; Remotely exploitable, no login, no user action

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-2025-12969 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-2025-12978 MEDIUM 5.4 DOWNGRADE Fluent Bit in_http, in_splunk, and in_elasticsearch input plugins contain a flaw in the tag_key validation log
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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