Grav: Single invalid UTF-8 byte disables every rule in Security::detectXss(), bypassing the page-content XSS safety gate

## Vulnerability Details **Component**: getgrav/grav core **File**: `system/src/Grav/Common/Security.php` **Function**: `detectXss()` (all six entries in the `$patterns` array use the PCRE `u` modifier), invoked from `Grav\Common\Data\Validation::checkSafety()` (the save-time XSS gate for any non-`security.xss_whitelist` account's blueprint field, including the page `content` field) and `detectXssInEditorContent()` (the render-time backstop for GHSA-2c4f-86xc-cr74) **CWE**: CWE-79 (Stored XSS), root-caused by CWE-20 (Improper Input Validation — fails open on malformed input) **Severity**: High **CVSS**: 8.0 — CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N ### Relationship to prior advisories This project's `detectXss()`/`checkSafety()` stack has been patched at least three times for the "page editor without super-admin rights stores an event handler that runs for site visitors" bug class: GHSA-9695-8fr9-hw5q / GHSA-c2q3-p4jr-c55f / GHSA-w8cg-7jcj-4vv2 (unquoted-attribute bypasses), GHSA-269c-h76q-8cxw (quoted-attribute-boundary bypass), GHSA-2c4f-86xc-cr74 (render-time Twig-assembled bypass). All three patched the **regex logic**. This is a different, lower-level defect: the PHP regex *engine* silently refuses to evaluate the pattern at all once the input contains one invalid UTF-8 byte, independent of what the regex logic says — no amount of regex-logic hardening fixes this. ### Root Cause Every pattern in `$patterns` uses the PCRE `u` (UTF-8) modifier. PHP's documented behavior: if the subject string contains even one byte sequence that is not valid UTF-8, `preg_match()` does not "skip" that byte or report "no match" — it returns `false` for the **entire call**, with `preg_last_error() === PREG_BAD_UTF8_ERROR`. `detectXss()` only checks truthiness (`if (preg_match(...) || preg_match(...))`), so `false` and "0 matches" are indistinguishable to the calling code. A single stray byte anywhere in a field's value — not even near the actual payload — makes every one of the six checks silently report "no XSS found". Meanwhile, a real browser decoding the same bytes as UTF-8 (the encoding Grav serves pages as) does not fail open: it substitutes the invalid byte with one U+FFFD replacement character and renders the surrounding markup completely normally. The `<img ... onerror=...>` tag is untouched structurally; the payload still fires. ### Vulnerable Code ```php $patterns = [ 'on_events' => '#<(?:"[^"]*"|\'[^\']*\'|[^>"\'])*?(?:[\s\x00-\x20\"\'\/]|"[^"]*"|\'[^\']*\')on\s*[a-z]+\s*=#iu', // ... five more, all with the /u modifier ]; foreach ($patterns as $name => $regex) { if (!empty($enabled_rules[$name])) { if (preg_match($regex, (string) $string) || preg_match($regex, $orig)) { return $name; } // ... } } return null; // reached even when the string contains <img onerror=...>, // as long as it also contains one invalid UTF-8 byte anywhere ``` Directly reproducible against the exact regex: ```php $regex = '#<(?:"[^"]*"|\'[^\']*\'|[^>"\'])*?(?:[\s\x00-\x20\"\'\/]|"[^"]*"|\'[^\']*\')on\s*[a-z]+\s*=#iu'; var_dump(preg_match($regex, "<img src=x onerror=alert(1)>")); // int(1) -- caught var_dump(preg_match($regex, "<img src=x \x80onerror=alert(1)>")); // bool(false), preg_last_error()==4 ``` ### Attack Scenario 1. Attacker holds a page-edit ("publisher") account without super-admin rights. 2. Sets page content to `Hello world \x80<img src=x onerror=alert(document.cookie)>` (a raw invalid UTF-8 byte, deliverable via any non-JSON submission path — e.g. the bundled Form plugin's multipart/urlencoded field, or any blueprint-validated field populated from a raw POST body — `$_POST` values are not UTF-8-validated by PHP). 3. `Validation::checkSafety()` runs `detectXss()` on the value; every `preg_match()` call returns `false`, so `detectXss()` returns `null` ("no violation"). The payload saves unmodified. 4. Any visitor (including a super-admin browsing the public site) loads the page; the browser renders the intact `<img onerror=...>` element, executing the attacker's JavaScript in the visitor's session. ### Impact - **Type**: Stored XSS (CWE-79) - **Auth required**: Page-edit ("publisher") account, not super-admin - **Consequence**: Arbitrary JavaScript execution in any visitor's browser, including a super-admin who views the page — a cross-trust-boundary escalation from publisher to admin-equivalent action capability. ### Recommended Fix ```php public static function detectXss($string, ?array $options = null): ?string { if (null === $string || !is_string($string) || empty($string)) { return null; } // Fail closed: mb_check_encoding() validates the whole string up front // and returns a normal boolean — it never "fails open" the way a // /u-flagged preg_match() does on malformed input. if (!mb_check_encoding($string, 'UTF-8')) { return 'invalid_encoding'; } // ... rest unchanged } ``` `Validation::checkSafety()` only invokes `detectXss()` for accounts outside `security.xss_whitelist` (default `admin.super`), so this introduces no behavior change for whitelisted accounts. ### Verification Dynamically confirmed on grav 2.0.13: called the live `Security::detectXss()` directly (bootstrapped through Grav's own service container, not a standalone regex copy) — a clean payload was correctly flagged (`"on_events"`), the same payload plus one invalid UTF-8 byte returned `NULL` (bypass), and an ordinary safe string returned `NULL` as expected. Note: the JSON REST API (`api` plugin, the path Admin2's SPA uses to save pages) happens to reject raw invalid UTF-8 before it reaches `detectXss()`, because RFC 8259 requires JSON text to be valid UTF-8 and PHP's `json_decode()` enforces this — that's an incidental protection of the JSON layer, not a fix, and any non-JSON submission path (e.g. the bundled Form plugin's multipart/urlencoded fields) remains exposed. After applying the fix above, the same bypass payload correctly returns `"invalid_encoding"` (a violation), while an ordinary safe string still returns `NULL` (no regression). A ready-to-apply fix branch is prepared locally against this repo's `develop` branch (based on the `2.0.13` tag); happy to push it to a private fork once one is available for this advisory.