GHSA-wjv4-x9w8-wm3h

### Summary

`Nokogiri::XML::Document#root=` validated only that the new root was a `Nokogiri::XML::Node`, allowing a DTD node to be set as the document root. The result is a heap use-after-free during garbage collection or finalization, leading to an invalid memory read or potentially a segfault.

Nokogiri 1.19.4 restricts `Document#root=` to element nodes, raising `TypeError` for any other node type.

This memory-safety issue affects only the CRuby implementation (libxml2). The JRuby implementation was not affected; the same input validation was added there for behavioral parity.

### Severity

The Nokogiri maintainers have evaluated this as low severity. This is only triggered by a programming error. It requires application code to assign a non-element node such as a DTD as the document root via `Document#root=`. Nokogiri 1.19.4 now raises `TypeError` instead of allowing a use-after-free. It cannot be triggered by untrusted input or through normal use of the public API.

### Mitigation

Upgrade to Nokogiri 1.19.4 or later.

As a workaround, applications that cannot upgrade should avoid assigning a DTD (or any non-element node) via `Document#root=`.

### Credit

This issue was responsibly reported by Zheng Yu from depthfirst.com.