A critical vulnerability was found in the Ruby on Rails framework. Because of this the most Ruby on Rails sites can be hacked relatively easily. This afternoon the online system for Dutch government services digid.nl was brought down for a period of time by its administrators because of this vulnerability.
Every Ruby on Rails environment with XML not deactivated is vulnerable, even if yaml-support is switched off. In the standard setup XLM is activated on these environments. The fix is upgrading the Ruby on Rails framework to one of the following versions: 3.2.11, 3.1.10, 3.0.19, or 2.3.15.
Technically the vulnerability works as follows: By inserting an XML-document in the HTTP postbody, and by including a yaml value in forementioned XML-document, Ruby objects can be created. This can be used in practically all circumstanced to launch an SQL injection. In some circumstances it can be used to execute system commands. More details here, here en here.