KatexRenderer rendered the raw math source through {@html} whenever renderToString threw. throwOnError only suppresses KaTeX ParseError, so a RangeError (maximum call stack size exceeded, reachable with deeply-nested brace input) escaped into the catch and re-exposed the unescaped source. Because the math tokenizer captures everything between the delimiters verbatim, that source can carry an HTML/JS payload which then executed in the viewer's browser on the application origin, a stored, cross-user XSS reachable through normal chat/channel/shared-chat rendering. Escape the fallback so the source is shown as text and is never injected as HTML. Valid math is unaffected, it still renders through the success path.
Co-authored-by: maxntv <maxntv@users.noreply.github.com>
