Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption (CVE-2026-27820)

Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption (CVE-2026-27820) | HOL Guard CVE

Why this deserves its current priority

CVSS is 9.8. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps zlib (rubygems), zlib (rubygems), zlib (rubygems), zlib (rubygems) and additional mapped packages. Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
zlibrubygems	>=3.2.0,<3.2.3	3.2.3
zlibrubygems	>=3.1.0,<3.1.2	3.1.2
zlibrubygems	<3.0.1	3.0.1
zlibrubygems	>=3.2.0 <3.2.3	3.2.3
zlibrubygems	>=3.1.0 <3.1.2	3.1.2
zlibrubygems	>=0 <3.0.1	3.0.1

Recommended response

Check inventory. Check lockfiles and deployed manifests for zlib, zlib, zlib.
Review the reported fix. Update zlib to 3.2.3; zlib to 3.1.2; zlib to 3.0.1; zlib to 3.2.3; zlib to 3.1.2; zlib to 3.0.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Apr 16, 2026
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Jun 11, 2026

Sources and claim methodology

Upstream source 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](https://hackerone.com/calysteon) ### References * https://hackerone.com/reports/3467067

Quoted source text, attributed separately from HOL analysis.