Nokogiri is an open source XML and HTML library for the Ruby programming language. Nokogiri `1.13.8` and `1.13.9` fail to check the return value from `xmlTextReaderExpand` in the method `Nokogiri::XML::Reader#attribute_hash`. This can lead to a null pointer exception when invalid markup is being parsed. For applications using `XML::Reader` to parse untrusted inputs, this may potentially be a vector for a denial of service attack. Users are advised to upgrade to Nokogiri `>= 1.13.10`. Users may be able to search their code for calls to either `XML::Reader#attributes` or `XML::Reader#attribute_hash` to determine if they are affected.