KaTeX is a fast, easy-to-use JavaScript library for TeX math rendering on the web. KaTeX users who render untrusted mathematical expressions with renderToString could encounter malicious input using \htmlData that runs arbitrary JavaScript, or generate invalid HTML. Users are advised to upgrade to KaTeX v0.16.21 to remove this vulnerability. Users unable to upgrade should avoid use of or turn off the trust option, or set it to forbid \htmlData commands, forbid inputs containing the substring "\\htmlData" and sanitize HTML output from KaTeX.