There are many security vulnerabilities that have permitted full machine takeovers in an automated fashion for a long time now. Generally speaking though such attacks only work against very small fractions of the Internet, and, no matter how big Drupal may be in absolute terms, in relative terms, it is not that large. Heartbleed was so bad because it was so widespread. So many things use OpenSSL. That you could lose private keys was just icing on the cake; the arbitrary memory read was bad enough on its own, and the difficulty of detecting it also factored into its high rating. So, yes, I'd still rate Heartbleed far, far above this. Or the Ruby YAML attack.
(POODLE was by contrast well-named; annoying, yappy, ultimately not significant enough to warrant its own entry in the Great Security Vulnerability list. Enough to worry about and mitigate and it if helps bury SSLv3, hey, great, but not a thing for universal panic the way Heartbleed or Shellshock were.)