Vulnerable dependencies in Nokogiri

Summary

### Summary

Nokogiri [v1.13.2](https://github.com/sparklemotion/nokogiri/releases/tag/v1.13.2) upgrades two of its packaged dependencies:

- vendored libxml2 from v2.9.12 to [v2.9.13](https://download.gnome.org/sources/libxml2/2.9/libxml2-2.9.13.news) - vendored libxslt from v1.1.34 to [v1.1.35](https://download.gnome.org/sources/libxslt/1.1/libxslt-1.1.35.news)

Those library versions address the following upstream CVEs:

- libxslt: [CVE-2021-30560](https://nvd.nist.gov/vuln/detail/CVE-2021-30560) (CVSS 8.8, High severity) - libxml2: [CVE-2022-23308](https://nvd.nist.gov/vuln/detail/CVE-2022-23308) (Unspecified severity, see more information below)

Those library versions also address numerous other issues including performance improvements, regression fixes, and bug fixes, as well as memory leaks and other use-after-free issues that were not assigned CVEs.

Please note that this advisory only applies to the CRuby implementation of Nokogiri `< 1.13.2`, and only if the _packaged_ libraries are being used. If you've overridden defaults at installation time to use _system_ libraries instead of packaged libraries, you should instead pay attention to your distro's `libxml2` and `libxslt` release announcements.

### Mitigation

Upgrade to Nokogiri `>= 1.13.2`.

Users who are unable to upgrade Nokogiri may also choose a more complicated mitigation: compile and link an older version Nokogiri against external libraries libxml2 `>= 2.9.13` and libxslt `>= 1.1.35`, which will also address these same CVEs.

### Impact

#### libxslt [CVE-2021-30560](https://nvd.nist.gov/vuln/detail/CVE-2021-30560)

- CVSS3 score: 8.8 (High) - Fixed by https://gitlab.gnome.org/GNOME/libxslt/-/commit/50f9c9c

All versions of libxslt prior to v1.1.35 are affected.

Applications using **untrusted** XSL stylesheets to transform XML are vulnerable to a denial-of-service attack and should be upgraded immediately.

#### libxml2 [CVE-2022-23308](https://nvd.nist.gov/vuln/detail/CVE-2022-23308)

- As of the time this security advisory was published, there is no officially published information available about this CVE's severity. The above NIST link does not yet have a published record, and the libxml2 maintainer has declined to provide a severity score. - Fixed by https://gitlab.gnome.org/GNOME/libxml2/-/commit/652dd12 - Further explanation is at https://mail.gnome.org/archives/xml/2022-February/msg00015.html

The upstream commit and the explanation linked above indicate that an application may be vulnerable to a denial of service, memory disclosure, or code execution if it parses an **untrusted** document with parse options `DTDVALID` set to true, and `NOENT` set to false.

An analysis of these parse options:

- While `NOENT` is off by default for Document, DocumentFragment, Reader, and Schema parsing, it is on by default for XSLT (stylesheet) parsing in Nokogiri v1.12.0 and later. - `DTDVALID` is an option that Nokogiri does not set for any operations, and so this CVE applies only to applications setting this option explicitly.

It seems reasonable to assume that any application explicitly setting the parse option `DTDVALID` when parsing **untrusted** documents is vulnerable and should be upgraded immediately.