GHSA-4mf4-73j6-mvrw

### Impact Many djust built-in component template tags (`djust.components.templatetags.*`) render a developer/user-supplied URL into an `href` / `action` attribute, HTML-escaping it with `conditional_escape` but **never validating the URL scheme**. HTML escaping prevents attribute breakout but does not neutralize a `javascript:` URI (which needs no escapable characters), so a URL value of `javascript:alert(document.cookie)` lands verbatim in `<a href="javascript:alert(document.cookie)">` and executes in the victim's session on click.

Affected sinks (executing href/action contexts; not exhaustive): breadcrumb (`djust_components.py:1535`; `_advanced.py:1999,2022`), `dj_nav` link/dropdown/brand (`djust_components.py:5632,5779,5807,5830`), citation URL (`:6936`), cookie-consent privacy link (`:8240`), error-page action (`_advanced.py:1728`), form `action` (`_forms.py:1239`). `<img src>` sinks are lower risk (javascript: does not execute via img src). No scheme validation exists anywhere in the component tags; a docstring at `djust_components.py:781` even acknowledges the `javascript:` risk without mitigating it.

**Severity Medium:** XSS in the victim's authenticated session whenever an app renders one of these components with a URL sourced from user-controllable data; for `href` requires a click, worst case is the stored variant (URL persisted from one user, rendered to another). Conditional on developer usage, hence Medium rather than High.

Reproduced: `breadcrumb(items=[{"label":"Home","url":"javascript:alert(document.cookie)"}])` emits `<a ... href="javascript:alert(document.cookie)">Home</a>`; `JaVaScRiPt:` case variant also passes; `https://` URLs are preserved.

### Patches Add a `safe_url()` helper that neutralizes non-allowlisted schemes (`javascript:`, `vbscript:`, `data:`) to `#` while preserving http/https/mailto/tel/relative/anchor URLs, and route every href/action/formaction/xlink:href sink through it (then HTML-escape). Add a regression + an audit_ast/system-check rule flagging un-routed URL-attribute interpolation.

### Workarounds Do not pass user-controllable URLs to the affected built-in component tags; pre-validate URL schemes in application code before binding them to component arguments.

### References Reproducer + finding writeup retained privately by the maintainer.