CVE-2026-27820: zlib is a Ruby interface for the zlib compression/decompression library

This month

CRITICAL 9.8REMOTEUPGRADE

CVE-2026-27820 is a critical-severity flaw (CVSS 9.8) affecting zlib. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 th

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 54%)
CISA exploited list	No
Weakness	CWE-120, CWE-131
Published	2026-04-16
Last modified	2026-10-26

Publisher (CNA): GitHub, Inc. · Vendor: ruby-lang

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-16 18:16 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

ruby-lang zlib UPGRADE

Affected: < 3.0.1 · >= 3.1.0 and < 3.1.2 · >= 3.2.0 and < 3.2.3

Fixed in: 3.0.1, 3.1.2, 3.2.3

Safe to downgrade to: a release before 3.1.0

1. Check installed version

apt-cache policy zlib || rpm -q zlib || zlib --version

2. UPGRADE to 3.0.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade zlib

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade zlib

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.1.0 (Debian / Ubuntu)

apt-cache madison zlib sudo apt install zlib=<VERSION_BEFORE_3.1.0> sudo apt-mark hold zlib

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list zlib sudo dnf downgrade zlib-<VERSION_BEFORE_3.1.0> sudo dnf versionlock add zlib

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-2026-27820?

zlib: upgrade to 3.0.1, 3.1.2, 3.2.3. Exact commands for each OS are on this page.

Is CVE-2026-27820 being exploited?

CVE-2026-27820 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-2026-27820 affect?

CVE-2026-27820 affects zlib. Severity is critical (CVSS 9.8). 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 ca

How urgent is CVE-2026-27820?

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

Software pages

Ruby vulnerabilities · zlib vulnerabilities

References

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