Nokogiri: Possible Use-After-Free when directly using `NokogirI::XML::XPathContext` beyond document lifetime

Summary

Nokogiri::XML::XPathContext did not keep its source document alive for garbage collection. If an XPathContext outlived its document and the document was collected, evaluating an XPath expression could read invalid memory and potentially segfault.

This is only reachable when application code constructs an XPathContext directly and lets the document become unreachable while continuing to use the context. The normal Document#xpath, #css, and related search methods are not affected, and it is not triggerable by malicious document input.

Nokogiri 1.19.4 makes XPathContext keep its source document alive for as long as the context exists.

Only the CRuby implementation is affected. JRuby is not affected.

Severity

The Nokogiri maintainers have evaluated this as low severity. Reaching it requires an unusual API-usage pattern that does not arise during normal use. The application must construct an XML::XPathContext directly and continue using it after allowing its source document to be garbage-collected. Nokogiri 1.19.4 makes this pattern safe with no change to the public API. The context now keeps its source document alive for as long as it exists.

Mitigation

Upgrade to Nokogiri 1.19.4 or later.

As a workaround, ensure the source document remains referenced for as long as any XPathContext created from it is in use. The standard Document#xpath, #css, and related search methods already do this and are unaffected.

Credit

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