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ZlibApplication · Ruby Lang

CVE-2026-27820

CRITICAL · 9.8 CVSS v3.1 Published 2026-04-16
Fix available
A fix is available. Upgrade to 3.0.1 / 3.1.2 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A buffer overflow vulnerability exists in the zlib Ruby gem's Zlib::GzipReader component. The zstream_buffer_ungets function prepends caller-provided bytes to previously produced output but fails to verify the backing Ruby string has sufficient capacity before executing memmove to shift existing data, leading to memory corruption when buffer length exceeds capacity.

MitigationUpgrade the zlib gem to version 3.0.1, 3.1.2, or 3.2.3 (depending on your current major version) to address the missing capacity check in zstream_buffer_ungets.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
ZlibApplication
Affected:< 3.0.1>= 3.1.0, < 3.1.2>= 3.2.0, < 3.2.3

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check if zlib gem is installed
    Run `gem list zlib` or inspect your Gemfile/Gemfile.lock for the zlib gem entry
    Affected if The zlib gem is not present in your project dependencies
  2. Identify installed zlib gem version
    Run `gem list zlib` to see the installed version, or use Ruby: `ruby -e "require 'zlib'; puts Zlib::VERSION"`
    Affected if The displayed version falls within any of these ranges: < 3.0.1, >= 3.1.0 and < 3.1.2, >= 3.2.0 and < 3.2.3
  3. Detect Zlib::GzipReader usage in code
    Search your codebase for `Zlib::GzipReader` or `GzipReader.new` usage in your Ruby source files
    Affected if Your application code uses Zlib::GzipReader to process compressed input
  4. Check if processing untrusted compressed data
    Review where GzipReader is instantiated and trace whether the input source handles untrusted or external data
    Affected if Zlib::GzipReader processes compressed data from untrusted sources such as user uploads, external API responses, or network streams

You are affected if the zlib gem version is in a vulnerable range AND your code uses Zlib::GzipReader to process any compressed input, especially from untrusted sources.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 3.0.1 / 3.1.2 / 3.2.3 or later
Fixed in 3.0.13.1.23.2.3
Interim mitigation

Upgrade the zlib gem to version 3.0.1, 3.1.2, or 3.2.3 (depending on your current major version) to address the missing capacity check in zstream_buffer_ungets.

Recommended fix High confidence

zlib gem version 3.0.1, 3.1.2, or 3.2.3 (depending on your current major.minor branch)

  1. Identify the currently installed zlib gem version using `gem list zlib` or `bundle list`
  2. Determine which version branch you are on (3.0.x, 3.1.x, or 3.2.x) from the current version
  3. Run `gem install zlib -v <fixed_version>` where <fixed_version> is 3.0.1 (for 3.0.x), 3.1.2 (for 3.1.x), or 3.2.3 (for 3.2.x)
  4. If using Bundler, update the Gemfile: `gem 'zlib', '~> <major.minor>'` with the appropriate version constraint pointing to the fixed version, then run `bundle update zlib`
  5. Verify the upgrade was successful by running `gem list zlib` and confirming the version matches the expected fixed release
  6. Restart any Ruby applications or services using the gem to ensure they load the patched version
Caveat Patch releases typically contain only bug fixes and security patches with minimal risk of breaking changes; the upgrade is recommended due to the critical severity of this buffer overflow

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Zlib Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-27820 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data