flb_gzip_compress in flb_gzip.c in Fluent Bit before 1.6.4 has an out-of-bounds write because it does not use the correct calculation of th…

published 2021-01-03 5 y ago modified 2026-09-08 2 mo ago also BIT-fluent-bit-2020-35963 , EUVD-2020-23546

cvss: 7.8; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.3%; EPSS 1.3% chance of exploitation in 30 days, percentile 71%
public exploits: 0; none known

flb_gzip_compress in flb_gzip.c in Fluent Bit before 1.6.4 has an out-of-bounds write because it does not use the correct calculation of the maximum gzip data-size expansion.

Patch / fix

fixed in 2 links 1

bitnami fluent-bit 1.6.4
treasuredata fluent_bit 1.6.4

github.com/fluent/fluent-bit@cadff53 commit

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2020-35963 updated 2025-11-21 cvss 7.8

fluent-bit: Out of bounds write in flb_gzip_compress in flb_gzip.c

Red Hat Advanced Cluster Management for Kubernetes 2 rhacm2/acm-grafana-rhel8
not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.33% chance of exploitation in the next 30 days, higher than 71% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

bugs.chromium.org https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=27261 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
NVD	3.1	7.8	CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
NVD	2.0	6.8	AV:N/AC:M/Au:N/C:P/I:P/A:P
OSV	3.1	7.8	CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Affected packages

ecosystem	package	affected	fixed in
bitnami	fluent-bit	<1.6.4	1.6.4

Classification

Weakness

CWE-787 Out-of-bounds Write

References

github.com /fluent/fluent-bit/commit/cadff53c093210404aed01c4cf586adb8caa07af
patch, third-party-advisory, advisory
bugs.chromium.org /p/oss-fuzz/issues/detail?id=27261
exploit, issue-tracking, third-party-advisory, advisory
fluentbit.io /announcements/v1.6.4/
release-notes, vendor-advisory, advisory
nvd.nist.gov /vuln/detail/CVE-2020-35963
web

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary