Grav v2.0.0 contains a cross-site scripting vulnerability (fixed in 2.0.1). The XSS blueprint validator (Security::detectXss()) runs on raw page content before Twig processing. When Twig content processing is enabled (twig_content.process_enabled: true), an attacker with page-write API permission can use Twig's string concatenation operator (~) to dynamically construct event handler names, dangerous tag names, or dangerous protocols at render time (e.g. {% set x = "on" ~ "error" %}). The validator sees only the harmless Twig expression and allows the content, but after Twig rendering the output (rendered via {{ page.content|raw }}) contains an active payload such as <img src=1 onerror=alert(1)>, executing arbitrary JavaScript in visitors' browsers.

Affected: Packagist: getgrav/grav

## Summary

The XSS blueprint validator (`Security::detectXss()`) runs on the **raw page content before Twig processing**. An attacker can use Twig's string concatenation operator (`~`) to dynamically construct an event handler name at render time. The validator sees `{{ "on" ~ "error" }}` - a harmless Twig expression - and allows the content. After Twig processes the template, the output contains `<img src=1 onerror=alert(1)>` which is rendered via `{{ content|raw }}` and executes in the victim's browser.

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## Details

**The two-stage attack** exploits the separation between validation and rendering:

**Stage 1 - what the XSS validator sees** (raw page content):

```twig {% set x = "on" ~ "error" %} <img src=1 {{ x }}=alert(document.domain)> ```

The `detectXss()` function scans this string. The `on_events` regex looks for `<[^>]*?[\s\x00-\x20\"\'\/](on\s*[a-z]+|xmlns)\s*=` inside HTML tags. In `{{ x }}`, the `{` character is not in the boundary set `[\s\x00-\x20\"\'\/]`, and `x` is not `on`. **No match - passes validation.**

**Stage 2 - what Twig produces** (after rendering):

```html <img src=1 onerror=alert(document.domain)> ```

The validator never re-inspects Twig output. The theme template renders this via `{{ page.content|raw }}` (confirmed in `quark2/templates/default.html.twig:5`), so no auto-escaping occurs.

**Why `{% set %}` and `~` are allowed** - `system/config/security.yaml:125-145`:

```yaml allowed_tags: - set # ← allows variable assignment…