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CVE-2026-27820

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

Published: Apr 16, 2026Updated: May 21, 2026 Sources: CVE List NVD BDUCWE-120
NVD MITRE
9.8CVSSCRITICAL
EPSS Score
0.6%
Top 57.9%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Patch Available

Description

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.

CVSS Vector Breakdown

AV:NAC:LPR:NUI:NS:UC:HI:HA:H
Exploitability
AV:NAttack Vector
Network
AC:LAttack Complexity
Low
PR:NPrivileges Required
None
UI:NUser Interaction
None
Scope
S:UScope
Unchanged
Impact
C:HConfidentiality
High
I:HIntegrity
High
A:HAvailability
High
Open in CVSS calculator →

Weaknesses

CWE-120CWE-131

Affected Products

Debian GNU/LinuxСообщество свободного программного обеспечения
On-premOS
Operating Systems / linux-distro
zlibЖан-Лу Гайи, Марк Адлер
Library
OSS Libraries / generic-library
zlibruby
Library
OSS Libraries / rubygems
zlibruby-lang
Library
OSS Libraries / generic-library
сообщество свободного программного обеспеченияoss-projectOperating Systemsaka сообщество свободного программного обеспечения, fsf
жан-лу гайи, марк адлерoss-projectOSS Librariesaka марка спенсер, шон иган и др.
rubyoss-projectJPOSS Libraries
ruby-langoss-projectOSS Librariesaka ruby, rexml

Attack Graph

Products CVE Techniques Tactics

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Exploitability

Official Patch Available

MITRE ATT&CK

2 techniques
Initial Access
Privilege Escalation
View detailed technique mapping

References

https://www.suse.com/security/cve/CVE-2026-27820.html
suse.com
https://github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w
github.com
https://www.ruby-lang.org/en/news/2026/03/05/buffer-overflow-zlib-cve-2026-27820/
ruby-lang.org
and 2 more references View all →

Timeline

Published
Apr 16, 2026
Last Updated
May 21, 2026

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