Would it be possible for messenger apps to simply ignore <script> tags (and accept that this will break a small fraction of SVGs)? Or is that not a sufficient defense?
The only reliable solution would be an allowlist of safe elements and attributes, but it would quickly cause compat issues unless you spend time curating the rules. I did not find an existing lib doing it at the time, and it was too much effort to maintain it ourselves.
The solution I ended up implementing was having a sandboxed Chromium instance and communicating with it through the dev tools to load the SVG and rasterize it. This allowed uploading SVG files, but it was then served as rasterized PNGs to other users.
But you can use an `img` tag (`<img src="evil.svg">`) and that'll basically Just Work, or use a CSP. I wouldn't rely on sanitizing, but I'd still sanitize.
That doesn't help too much if evil.svg is hosted on the same domain (with default "Content-Type: image/svg+xml" header), because attacker can send a direct link to the file.