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

$patterns = [
    'on_events' => '#<(?:"[^"]*"|\'[^\']*\'|[^>"\'])*?(?:[\s\x00-\x20\"\'\/]|"[^"]*"|\'[^\']*\')on\s*[a-z]+\s*=#iu',
    // ... five more, all with the /u modifier
];
foreach ($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:

$regex = '#<(?:"[^"]*"|\'[^\']*\'|[^>"\'])*?(?:[\s\x00-\x20\"\'\/]|"[^"]*"|\'[^\']*\')on\s*[a-z]+\s*=#iu';
var_dump(preg_match($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

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.