Vulnerability Instance

Buffer overflow vulnerability in Zlib::GzipReader
A buffer overflow vulnerability exists in Zlib::GzipReader.
This vulnerability has been assigned the CVE identifier
CVE-2026-27820. We recommend upgrading the zlib gem.

## Details

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

Thanks to calysteon for reporting this issue. Also thanks to
nobu for creating the patch.