Add security advisory for CVE-2026-27820

Author: hsbt

en/news/_posts/2026-03-04-buffer-overflow-zlib-cve-2026-27820.md

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+---
+layout: news_post
+title: "CVE-2026-27820: DoS vulnerability in REXML"
+author: "hsbt"
+translator:
+date: 2026-03-04 09:00:00 +0000
+tags: security
+lang: en
+---
+
+A buffer overflow vulnerability exists in `Zlib::GzipReader`. This vulnerability has been assigned the CVE identifier [CVE-2026-27820](https://www.cve.org/CVERecord?id=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
+
+[calysteon](https://hackerone.com/calysteon)
+
+## History
+
+* Originally published at 2026-03-04 09:00:00 (UTC)