CVE-2026-27820: Buffer overflow vulnerability in Zlib::GzipReader | Ruby

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.