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CRITICAL severity

Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruptionGHSA-g857-hhfv-j68w

CRITICAL

GHSA-g857-hhfv-j68w is a critical-severity (CVSS 9.8) CWE-120 vulnerability in zlib. A fix is available for zlib — see the affected versions and patch details below.

Also known asCVE-2026-27820
Published
Updated
Affected
3 pkgs
Patched
3 / 3
Exploits
None indexed
Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-g857-hhfv-j68w.

EPSS Exploitation Probability

via FIRST.org ↗
0.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs47th percentile — riskier than 47% of all scored CVEsHighest risk
0.00%0.42%0.83%1.25%0.0%0.6%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected
💎zlib💎zlib💎zlib

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

Details

A buffer overflow vulnerability exists in 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.

Recommended action

We recommend to update the zlib gem to version 3.2.3 or later. In order to ensure compatibility with bundled version in older Ruby series, you may update as follows instead:

  • For Ruby 3.2 users: Update to zlib 3.0.1
  • For Ruby 3.3 users: Update to zlib 3.1.2

You can use gem update zlib to update it. If you are using bundler, please add gem "zlib", ">= 3.2.3" to your Gemfile.

Affected versions

zlib gem 3.2.2 or lower

Credits

calysteon

References

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
💎RubyGemszlib≥ 3.2.0&&< 3.2.33.2.3bundle update zlib --conservative
💎RubyGemszlib≥ 3.1.0&&< 3.1.23.1.2bundle update zlib --conservative
💎RubyGemszliball versions3.0.1bundle update zlib --conservative

Affected Products

1 product · 3 configurations
Application
zlibruby-lang
≥ 3.2.0 && < 3.2.3
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for zlib, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update zlib to 3.2.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g857-hhfv-j68w is resolved across your whole dependency graph.

  3. Workarounds

    Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

A buffer overflow vulnerability exists in the Zlib::GzipReader component of the Ruby zlib interface. This flaw, caused by insufficient memory capacity during data manipulation, could lead to memory corruption and system instability. This vulnerability is considered of a Moderate severity this happens because the high…

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Source: Red Hat security advisory for GHSA-g857-hhfv-j68w (CC BY 4.0)
ProductFixed inAdvisory
Red Hat Enterprise Linux 10ruby4.0-0:4.0.6-36.el10_2RHSA-2026:50773
Red Hat Enterprise Linux 10ruby-0:3.3.12-14.el10_2RHSA-2026:50778
Red Hat Enterprise Linux 8ruby:3.3-8100020260727154509.489197e6RHSA-2026:50728
Red Hat Enterprise Linux 9ruby:4.0-9080020260727092509.9RHSA-2026:50827
Red Hat Enterprise Linux 9ruby:3.3-9080020260728061751.9RHSA-2026:50828
Red Hat Hardened Imagesruby3-3-main-3.3.10-23.1.hum1RHSA-2026:7305
Red Hat Hardened Imagesruby3-4-main-3.4.8-31.1.hum1RHSA-2026:7307
Red Hat Hardened Imagesruby4-0-main-4.0.0-33.3.hum1RHSA-2026:8838

Frequently Asked Questions

### Details A buffer overflow vulnerability exists in `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. ### Recommended action We recommend to update the `zlib` gem to version 3.2.3 or later. In order to ensure compatibility with bundled version in older Ruby series, you may update as follows instead: * For Ruby 3.2 users: Update t
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