Denial of service or RCE from libxml2 and libxslt

Nokogiri is affected by series of vulnerabilities in libxml2 and libxslt,
which are libraries Nokogiri depends on. It was discovered that libxml2 and
libxslt incorrectly handled certain malformed documents, which can allow
malicious users to cause issues ranging from denial of service to remote code
execution attacks.

For more information, the Ubuntu Security Notice is a good start:
http://www.ubuntu.com/usn/usn-2994-1/