The woes of sanitizing SVGs

A detailed history of SVG sanitization vulnerabilities in Scratch (the coding platform) from 2019 to 2026, covering XSS attacks, HTTP leaks via CSS @import, url(), image-set(), and CSS nesting bypass bugs. Each fix added more complexity to a custom sanitization pipeline, yet new bypasses kept emerging. The author argues this approach is fundamentally unsustainable and presents an alternative used in TurboWarp (a Scratch fork): sandboxing SVGs inside an iframe with a strict Content-Security-Policy, letting the browser enforce security rather than trying to out-parse every possible CSS trick. Two currently unfixed vulnerabilities are also disclosed, including one found by Claude Opus after publication.