It is the same vulnerability at a fundamental level (it's virtually the same code), but it isn't exploitable out of the box in the same way Rails was, at least not on its own. However, there is a web framework, Grape, that was exploitable in exactly the same way that Rails was due to MultiXml's vulnerability.
And, really, technically, it was ActiveSupport that had this vulnerability. Even outside of Rails, had you used Hash.from_xml on untrusted user input you would have run into exactly the same issues.
That's correct. But I'm planning to remove this functionality anyway, just to be safe. It's too easy to accidentally open this vulnerability and it's unclear what the valid use-case is for parsing YAML or symbols from XML.
When I implemented this functionality, it was only to be compatible with the Rails parser.
If you're consuming even one third-party XML API using multi_xml, that means you're open to RCE if that API provider is malicious or itself compromised, as well as man in the middle attacks if you're not consuming the API via SSL.
Harder to exploit, perhaps, but given the large number of Rails apps that themselves are likely to be unpatched right now, pivoting to RCE on every customer of a SaaS provider seems like a very viable attack vector. Strongly recommend that everybody look at this seriously.