Grav before 2.0.15 contains a stored cross-site scripting vulnerability in the detectXss() function where unpaired quotes in unquoted attribute values bypass event-handler detection. Authenticated editors can inject event handlers like onerror= that pass validation and execute in visitor browsers when page content is rendered.

Affected: Packagist: getgrav/grav

## Affected versions and vulnerable location

- Confirmed on grav core at `78ebfc1` (tag 2.0.13). - Detector: `system/src/Grav/Common/Security.php:290`, the `on_events` regex, run via `patternMatches()` (`:315-330`). - The `on_events` pattern at HEAD: `#<(?:"[^"]*"|'[^']*'|[^>"'])*?(?:[\s\x00-\x20"'/]|"[^"]*"|'[^']*')on\s*[a-z]+\s*=#iu` - Sole save-time guard for non-super content: `Validation::checkSafety()` (`system/src/Grav/Common/Data/Validation.php:160` scalars, `:165` arrays), invoked per field from `BlueprintSchema::validate` -> `Validation::checkSafety` (`system/src/Grav/Common/Data/BlueprintSchema.php:248`). `security.xss_whitelist: [admin.super]` exempts only super-admins (`Validation.php:148`).

## Root cause (distinct from GHSA-269c)

GHSA-269c hardened the tag-body scan to be quote-aware so a `>` inside a paired quoted attribute value is treated as data, not a tag close. That same quote-awareness opened a new gap: the regex treats ANY `"` or `'` as a string delimiter, but HTML only enters a quoted-value state when a quote appears immediately after `=`. A single unpaired quote sitting inside an unquoted attribute value is, to the browser, just a value character; to the regex it is an unterminated string that neither `[^>"']` nor `"[^"]*"` can consume, so the lazy tag-body scan cannot advance past it to reach the following ` on...=` handler. No alignment matches and `detectXss()` returns null.

## Proof (executed)

The detector was replicated verbatim (the `on_e…