CVE-2026-75834: Grav before 2.0.14 Stored XSS via Invalid UTF-8 Byte

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.xsswhitelist 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, pregmatch() does not "skip" that byte or report "no match" — it returns false for the entire call, with preglasterror() === PREGBADUTF8ERROR. detectXss() only checks truthiness (if (pregmatch(...) || pregmatch(...))), 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 $patterns = [ 'onevents' => '#<(?:"[^"]"|\'[^\']\'|[^>"\'])?(?:[\s\x00-\x20\"\'\/]|"[^"]"|\'[^\']\')on\s[a-z]+\s=#iu', // ... five more, all with the /u modifier ]; foreach ($patterns as $name => $regex) { if (!empty($enabledrules[$name])) { if (pregmatch($regex, (string) $string) || pregmatch($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 $regex = '#<(?:"[^"]"|\'[^\']\'|[^>"\'])?(?:[\s\x00-\x20\"\'\/]|"[^"]"|\'[^\']\')on\s[a-z]+\s=#iu'; vardump(pregmatch($regex, "<img src=x onerror=alert(1)>")); // int(1) -- caught vardump(pregmatch($regex, "<img src=x \x80onerror=alert(1)>")); // bool(false), preglasterror()==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 pregmatch() 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 === $string || !isstring($string) || empty($string)) { return null; }

// Fail closed: mbcheckencoding() validates the whole string up front // and returns a normal boolean — it never "fails open" the way a // /u-flagged pregmatch() does on malformed input. if (!mbcheckencoding($string, 'UTF-8')) { return 'invalidencoding'; }

// ... rest unchanged } Validation::checkSafety() only invokes detectXss() for accounts outside security.xsswhitelist (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 ("onevents"), 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 jsondecode() 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 "invalidencoding" (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.