To be fair it's a flash drop in replacement. It isn't supposed to be secure by design, just like flash.

I expect it to fair much better than Flash. 808/1020 (79%) of the CVEs reported against flash were for memory errors (buffer overruns and things) that allowed remote code execution. So, assuming the Ruffle developers haven't been using "unsafe", just writing it in Rust immediately removes 80% of the security problems that were with Flash.

Also, many of the security problems with Flash were deliberate (by design). For example, Flash was designed to send your browser fingerprint to advertising sites. Ruffle obviously doesn't do that.

It's running in a sandbox afaik, and the goal is to replace Flash but stay secure. Since it uses Web Assembly, you can't use Ruffle in any way to create any security threat, you couldn't create without using Ruffle. (Different to Flash, which created tons of new security threats, even leading to the plugin being disable).