Updated advisory posts against rubysec/ruby-advisory-db@5a41723

Author: jasnow

advisories/_posts/2026-03-05-CVE-2026-27820.md

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+---
+layout: advisory
+title: 'CVE-2026-27820 (zlib): Buffer overflow vulnerability in Zlib::GzipReader'
+comments: false
+categories:
+- zlib
+advisory:
+  gem: zlib
+  cve: 2026-27820
+  url: https://www.ruby-lang.org/en/news/2026/03/05/buffer-overflow-zlib-cve-2026-27820
+  title: Buffer overflow vulnerability in Zlib::GzipReader
+  date: 2026-03-05
+  description: |
+    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.
+  patched_versions:
+  - "~> 3.0.1"
+  - "~> 3.1.2"
+  - ">= 3.2.3"
+  related:
+    url:
+    - https://www.ruby-lang.org/en/news/2026/03/05/buffer-overflow-zlib-cve-2026-27820
+    - https://rubygems.org/gems/zlib/versions/3.2.3
+    - https://rubygems.org/gems/zlib/versions/3.1.2
+    - https://rubygems.org/gems/zlib/versions/3.0.1
+---