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The source view builds its own anchors for Markdown links and, unlike rendered Markdown, its output never passes through ammonia — so a paste containing `[click](javascript:alert(1))` shipped a working `javascript:` URL. Escaping does not help here: it keeps the attribute intact, which is exactly what makes the URL work, and only the CSP stood between that and script execution. The target is the link scope's own text, so it is not known when the scope opens. Hold the anchor until the text arrives and emit it only for a scheme that goes somewhere (http, https, mailto, or a relative target); anything else is shown as the text it already reads as. Browsers drop whitespace and control characters before resolving a URL, so the scheme is compared with them taken out. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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The source view builds its own anchors for Markdown links and, unlike rendered Markdown, its output never passes through ammonia — so a paste containing
[click](javascript:alert(1))shipped a workingjavascript:URL. Escaping does not help here: it keeps the attribute intact, which is exactly what makes the URL work, and only the CSP stood between that and script execution.The target is the link scope's own text, so it is not known when the scope opens. Hold the anchor until the text arrives and emit it only for a scheme that goes somewhere (http, https, mailto, or a relative target); anything else is shown as the text it already reads as. Browsers drop whitespace and control characters before resolving a URL, so the scheme is compared with them taken out.