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Fencer Vulns
critical 9.8

CVE-2026-27820 json

zlib: Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption

published 2026-04-16 6 mo ago modified 2026-10-26 13 d ago also GHSA-g857-hhfv-j68w , EUVD-2026-23278

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.6%
EPSS 0.6% chance of exploitation in 30 days, percentile 46%
public exploits
0
none known

Summary

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the Zlib::GzipReader. The zstream_buffer_ungets function prepends caller-provided bytes ahead of previously produced output but fails to guarantee the backing Ruby string has enough capacity before the memmove shifts the existing data. This can lead to memory corruption when the buffer length exceeds capacity. This issue has been fixed in versions 3.0.1, 3.1.2 and 3.2.3.

Patch / fix

  • rubygems zlib 3.0.1, 3.1.2, 3.2.3
  • ruby-lang zlib 3.0.1, 3.1.2, 3.2.3
  • ruby3.3 Debian 13 3.3.13-0+deb13u1

debian advisories DEBIAN-CVE-2026-27820

  • ruby2.3 Ubuntu Pro 16.04 LTS 2.3.1-2~ubuntu16.04.16+esm15

ubuntu advisories UBUNTU-CVE-2026-27820

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • ruby RHEL 9 3.3.12-8.module+el9.8.0+24568+2bbebe96
  • rubygems RHEL 9 3.5.22-8.module+el9.8.0+24568+2bbebe96
  • ruby-libs RHEL 9 3.3.12-8.module+el9.8.0+24568+2bbebe96
  • rubygem-pg RHEL 9 1.5.4-2.module+el9.8.0+24356+77305e91
  • rubygem-irb RHEL 9 1.13.1-8.module+el9.8.0+24568+2bbebe96
  • rubygem-rbs RHEL 9 3.4.0-8.module+el9.8.0+24568+2bbebe96
  • rubygem-rss RHEL 9 0.3.1-8.module+el9.8.0+24568+2bbebe96
  • rubygem-json RHEL 9 2.7.2-8.module+el9.8.0+24568+2bbebe96

red hat advisories RHSA-2026:50828RHSA-2026:50827RHSA-2026:50778RHSA-2026:50773RHSA-2026:50728RHSA-2026:8838

  • ruby2.5 SLE Module for Basesystem 15 SP7 2.5.9-150700.24.6.1

suse advisories SUSE-SU-2026:1066-1

  • ruby3.4 openSUSE Leap 16.0 3.4.10-160000.1.1

opensuse advisories openSUSE-SU-2026:21878-1

  • ruby AlmaLinux 8 3.3.12-8.module_el8.10.0+4241+4264e34b
  • rubygems AlmaLinux 8 3.5.22-8.module_el8.10.0+4241+4264e34b
  • ruby-libs AlmaLinux 8 3.3.12-8.module_el8.10.0+4241+4264e34b
  • rubygem-pg AlmaLinux 8 1.5.4-1.module_el8.10.0+3799+191214cc
  • rubygem-irb AlmaLinux 8 1.13.1-8.module_el8.10.0+4241+4264e34b
  • rubygem-rbs AlmaLinux 8 3.4.0-8.module_el8.10.0+4241+4264e34b
  • rubygem-rss AlmaLinux 8 0.3.1-8.module_el8.10.0+4241+4264e34b
  • rubygem-abrt AlmaLinux 8 0.4.0-1.module_el8.10.0+3799+191214cc

almalinux advisories ALSA-2026:50728ALSA-2026:50773ALSA-2026:50778ALSA-2026:50827ALSA-2026:50828

  • ruby4.0 Rocky Linux 10 4.0.6-36.el10_2
  • ruby Rocky Linux 10 3.3.12-14.el10_2
  • ruby Rocky Linux 9 4.0.6-34.module+el9.8.0+40271+b5850c84
  • rubygem-pg Rocky Linux 9 1.6.3-1.module+el9.8.0+40094+ce01217c
  • rubygem-mysql2 Rocky Linux 9 0.5.7-1.module+el9.8.0+40094+ce01217c
  • ruby Rocky Linux 9 3.3.12-8.module+el9.8.0+40270+7a73441e
  • rubygem-pg Rocky Linux 9 1.5.4-2.module+el9.7.0+40188+314ad260.0.1
  • rubygem-mysql2 Rocky Linux 9 0.5.5-3.module+el9.7.0+40032+36e56d0d

rocky linux advisories RLSA-2026:50773RLSA-2026:50778RLSA-2026:50827RLSA-2026:50828RLSA-2026:50728

  • ruby Alpine 3.21 3.3.13-r0
  • ruby Alpine 3.24, Alpine 3.23, Alpine 3.22 3.4.9-r0

alpine advisories ALPINE-CVE-2026-27820

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • ruby Azure Linux 3.0 3.3.5-8

azure linux advisories AZL-83450

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

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-2026-27820 updated cvss 5.6

zlib: zlib: Memory corruption via buffer overflow in Zlib::GzipReader

  • Red Hat Enterprise Linux 6 ruby, ruby-ri, ruby-irb, ruby-libs, ruby-rdoc, ruby-tcltk Red Hat Enterprise Linux 7 ruby, ruby-irb, rubygems, ruby-libs, ruby-tcltk, rubygem-json, +6 more Red Hat Enterprise Linux 8 ruby, ruby-irb, rubygems, ruby-libs, rubygem-pg, rubygem-irb, +25 more Red Hat Enterprise Linux 9 ruby, rubygems, ruby-libs, rubygem-irb, rubygem-rbs, rubygem-rss, +13 more
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Enterprise Linux AppStream (v. 8) ruby 3.3.12-8.module+el8.10.0+24567+c4e5368e, rubygems 3.5.22-8.module+el8.10.0+24567+c4e5368e, ruby-libs 3.3.12-8.module+el8.10.0+24567+c4e5368e, rubygem-pg 1.5.4-1.module+el8.10.0+21226+b78a28c4, rubygem-rbs 3.4.0-8.module+el8.10.0+24567+c4e5368e, rubygem-rss 0.3.1-8.module+el8.10.0+24567+c4e5368e, +18 more
    fixed: vendor fix RHSA-2026:50728
  • Red Hat Enterprise Linux AppStream (v. 9) ruby 3.3.12-8.module+el9.8.0+24568+2bbebe96, ruby 4.0.6-34.module+el9.8.0+24564+e55418dc, rubygems 3.5.22-8.module+el9.8.0+24568+2bbebe96, ruby-libs 3.3.12-8.module+el9.8.0+24568+2bbebe96, ruby-libs 4.0.6-34.module+el9.8.0+24564+e55418dc, rubygem-pg 1.5.4-2.module+el9.8.0+24356+77305e91, +40 more
    fixed: vendor fix RHSA-2026:50828
  • Red Hat Enterprise Linux AppStream (v. 10) ruby 3.3.12-14.el10_2, ruby4.0 4.0.6-36.el10_2, rubygems 3.5.22-14.el10_2, ruby-libs 3.3.12-14.el10_2, rubygem-rbs 3.4.0-14.el10_2, rubygem-rss 0.3.1-14.el10_2, +18 more Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) ruby-doc 3.3.12-14.el10_2, ruby4.0-doc 4.0.6-36.el10_2
    fixed: vendor fix RHSA-2026:50778
  • Red Hat Hardened Images ruby3.4 3.4.8-31.1.hum1, ruby4.0 4.0.0-33.3.hum1, ruby3.3 3.3.10-23.1.hum1, ruby3.4-libs 3.4.8-31.1.hum1, ruby4.0-libs 4.0.0-33.3.hum1, ruby3.3-libs 3.3.10-23.1.hum1, +6 more
    fixed: vendor fix RHSA-2026:7305
  • Red Hat Hardened Images rubygem3.3-rbs 3.4.0-23.1.hum1, rubygem3.3-rss 0.3.1-23.1.hum1, rubygem3.4-rbs 3.8.0-31.1.hum1, rubygem3.4-rss 0.3.1-31.1.hum1, rubygem4.0-rss 0.3.2-33.3.hum1, rubygem3.3-irb 1.13.1-23.1.hum1, +45 more
    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

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
6 mo ago

Exploit prediction

0.56% chance of exploitation in the next 30 days, higher than 46% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 1.7 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U
CNA 4.0 1.7 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
GitHub 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected packages

ecosystem package affected fixed in
rubygems zlib
>=3.2.0, <3.2.3
>=3.1.0, <3.1.2
<3.0.1
3.2.3
3.1.2
3.0.1

Affected products

  • ruby zlib

Classification

Weakness
  • CWE-120 Buffer Copy without Checking Size of Input
  • CWE-131 Incorrect Calculation of Buffer Size

References

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.