{"id":"GHSA-q2j8-x8hf-63ch","summary":"Grav: Single invalid UTF-8 byte disables every rule in Security::detectXss(), bypassing the page-content XSS safety gate","details":"## Vulnerability Details\n\n**Component**: getgrav/grav core\n**File**: `system/src/Grav/Common/Security.php`\n**Function**: `detectXss()` (all six entries in the `$patterns` array use the PCRE `u` modifier), invoked from `Grav\\Common\\Data\\Validation::checkSafety()` (the save-time XSS gate for any non-`security.xss_whitelist` account's blueprint field, including the page `content` field) and `detectXssInEditorContent()` (the render-time backstop for GHSA-2c4f-86xc-cr74)\n**CWE**: CWE-79 (Stored XSS), root-caused by CWE-20 (Improper Input Validation — fails open on malformed input)\n**Severity**: High\n**CVSS**: 8.0 — CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N\n\n### Relationship to prior advisories\nThis project's `detectXss()`/`checkSafety()` stack has been patched at least three times for the \"page editor without super-admin rights stores an event handler that runs for site visitors\" bug class: GHSA-9695-8fr9-hw5q / GHSA-c2q3-p4jr-c55f / GHSA-w8cg-7jcj-4vv2 (unquoted-attribute bypasses), GHSA-269c-h76q-8cxw (quoted-attribute-boundary bypass), GHSA-2c4f-86xc-cr74 (render-time Twig-assembled bypass). All three patched the **regex logic**. This is a different, lower-level defect: the PHP regex *engine* silently refuses to evaluate the pattern at all once the input contains one invalid UTF-8 byte, independent of what the regex logic says — no amount of regex-logic hardening fixes this.\n\n### Root Cause\nEvery pattern in `$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\".\n\nMeanwhile, 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 `\u003cimg ... onerror=...\u003e` tag is untouched structurally; the payload still fires.\n\n### Vulnerable Code\n```php\n$patterns = [\n    'on_events' =\u003e '#\u003c(?:\"[^\"]*\"|\\'[^\\']*\\'|[^\u003e\"\\'])*?(?:[\\s\\x00-\\x20\\\"\\'\\/]|\"[^\"]*\"|\\'[^\\']*\\')on\\s*[a-z]+\\s*=#iu',\n    // ... five more, all with the /u modifier\n];\nforeach ($patterns as $name =\u003e $regex) {\n    if (!empty($enabled_rules[$name])) {\n        if (preg_match($regex, (string) $string) || preg_match($regex, $orig)) {\n            return $name;\n        }\n        // ...\n    }\n}\nreturn null; // reached even when the string contains \u003cimg onerror=...\u003e,\n             // as long as it also contains one invalid UTF-8 byte anywhere\n```\n\nDirectly reproducible against the exact regex:\n```php\n$regex = '#\u003c(?:\"[^\"]*\"|\\'[^\\']*\\'|[^\u003e\"\\'])*?(?:[\\s\\x00-\\x20\\\"\\'\\/]|\"[^\"]*\"|\\'[^\\']*\\')on\\s*[a-z]+\\s*=#iu';\nvar_dump(preg_match($regex, \"\u003cimg src=x onerror=alert(1)\u003e\"));          // int(1)  -- caught\nvar_dump(preg_match($regex, \"\u003cimg src=x \\x80onerror=alert(1)\u003e\"));      // bool(false), preg_last_error()==4\n```\n\n### Attack Scenario\n1. Attacker holds a page-edit (\"publisher\") account without super-admin rights.\n2. Sets page content to `Hello world \\x80\u003cimg src=x onerror=alert(document.cookie)\u003e` (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 — `$_POST` values are not UTF-8-validated by PHP).\n3. `Validation::checkSafety()` runs `detectXss()` on the value; every `preg_match()` call returns `false`, so `detectXss()` returns `null` (\"no violation\"). The payload saves unmodified.\n4. Any visitor (including a super-admin browsing the public site) loads the page; the browser renders the intact `\u003cimg onerror=...\u003e` element, executing the attacker's JavaScript in the visitor's session.\n\n### Impact\n- **Type**: Stored XSS (CWE-79)\n- **Auth required**: Page-edit (\"publisher\") account, not super-admin\n- **Consequence**: Arbitrary JavaScript execution in any visitor's browser, including a super-admin who views the page — a cross-trust-boundary escalation from publisher to admin-equivalent action capability.\n\n### Recommended Fix\n```php\npublic static function detectXss($string, ?array $options = null): ?string\n{\n    if (null === $string || !is_string($string) || empty($string)) {\n        return null;\n    }\n\n    // Fail closed: mb_check_encoding() validates the whole string up front\n    // and returns a normal boolean — it never \"fails open\" the way a\n    // /u-flagged preg_match() does on malformed input.\n    if (!mb_check_encoding($string, 'UTF-8')) {\n        return 'invalid_encoding';\n    }\n\n    // ... rest unchanged\n}\n```\n`Validation::checkSafety()` only invokes `detectXss()` for accounts outside `security.xss_whitelist` (default `admin.super`), so this introduces no behavior change for whitelisted accounts.\n\n### Verification\nDynamically 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).\n\nA 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.","aliases":["CVE-2026-75834"],"modified":"2026-09-17T21:00:04.950465351Z","published":"2026-09-17T20:44:39Z","database_specific":{"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-09-17T20:44:39Z","nvd_published_at":null,"cwe_ids":["CWE-79"]},"references":[{"type":"WEB","url":"https://github.com/getgrav/grav/security/advisories/GHSA-q2j8-x8hf-63ch"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-75834"},{"type":"PACKAGE","url":"https://github.com/getgrav/grav"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/grav-before-stored-xss-via-invalid-utf-8-byte"}],"affected":[{"package":{"name":"getgrav/grav","ecosystem":"Packagist","purl":"pkg:composer/getgrav/grav"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.0.14"}]}],"versions":["0.8.0","0.9.0","0.9.1","0.9.10","0.9.11","0.9.12","0.9.13","0.9.14","0.9.15","0.9.16","0.9.17","0.9.18","0.9.19","0.9.2","0.9.20","0.9.21","0.9.22","0.9.23","0.9.24","0.9.25","0.9.26","0.9.27","0.9.28","0.9.29","0.9.3","0.9.30","0.9.31","0.9.32","0.9.33","0.9.34","0.9.35","0.9.36","0.9.37","0.9.38","0.9.39","0.9.4","0.9.40","0.9.41","0.9.42","0.9.43","0.9.44","0.9.45","0.9.5","0.9.6","0.9.7","0.9.8","0.9.9","1.0.0","1.0.0-rc.1","1.0.0-rc.2","1.0.0-rc.3","1.0.0-rc.4","1.0.0-rc.5","1.0.0-rc.6","1.0.1","1.0.10","1.0.2","1.0.3","1.0.4","1.0.5","1.0.6","1.0.7","1.0.8","1.0.9","1.1.0","1.1.0-beta.1","1.1.0-beta.2","1.1.0-beta.3","1.1.0-beta.4","1.1.0-beta.5","1.1.0-rc.1","1.1.0-rc.2","1.1.0-rc.3","1.1.1","1.1.10","1.1.11","1.1.12","1.1.13","1.1.14","1.1.15","1.1.16","1.1.17","1.1.2","1.1.3","1.1.4","1.1.5","1.1.6","1.1.7","1.1.8","1.1.9","1.1.9-rc.1","1.1.9-rc.2","1.1.9-rc.3","1.2.0","1.2.0-rc.1","1.2.0-rc.2","1.2.0-rc.3","1.2.1","1.2.2","1.2.3","1.2.4","1.3.0","1.3.0-rc.1","1.3.0-rc.2","1.3.0-rc.3","1.3.0-rc.4","1.3.0-rc.5","1.3.1","1.3.10","1.3.2","1.3.3","1.3.4","1.3.5","1.3.6","1.3.7","1.3.8","1.3.9","1.4.0","1.4.0-beta.1","1.4.0-beta.2","1.4.0-beta.3","1.4.0-rc.1","1.4.0-rc.2","1.4.1","1.4.2","1.4.3","1.4.4","1.4.5","1.4.6","1.4.7","1.4.8","1.5.0","1.5.0-beta.1","1.5.0-beta.2","1.5.0-rc.1","1.5.1","1.5.10","1.5.2","1.5.3","1.5.4","1.5.5","1.5.6","1.5.7","1.5.8","1.5.9","1.6.0","1.6.0-beta.1","1.6.0-beta.2","1.6.0-beta.3","1.6.0-beta.4","1.6.0-beta.5","1.6.0-beta.6","1.6.0-beta.7","1.6.0-beta.8","1.6.0-rc.1","1.6.0-rc.2","1.6.0-rc.3","1.6.0-rc.4","1.6.1","1.6.10","1.6.11","1.6.12","1.6.13","1.6.14","1.6.15","1.6.16","1.6.17","1.6.18","1.6.19","1.6.2","1.6.20","1.6.21","1.6.22","1.6.23","1.6.24","1.6.25","1.6.26","1.6.27","1.6.28","1.6.29","1.6.3","1.6.30","1.6.31","1.6.4","1.6.5","1.6.6","1.6.7","1.6.8","1.6.9","1.7.0","1.7.0-beta.1","1.7.0-beta.10","1.7.0-beta.2","1.7.0-beta.3","1.7.0-beta.4","1.7.0-beta.5","1.7.0-beta.6","1.7.0-beta.7","1.7.0-beta.8","1.7.0-beta.9","1.7.0-rc.1","1.7.0-rc.10","1.7.0-rc.11","1.7.0-rc.12","1.7.0-rc.13","1.7.0-rc.14","1.7.0-rc.15","1.7.0-rc.16","1.7.0-rc.17","1.7.0-rc.18","1.7.0-rc.19","1.7.0-rc.2","1.7.0-rc.20","1.7.0-rc.3","1.7.0-rc.4","1.7.0-rc.5","1.7.0-rc.6","1.7.0-rc.7","1.7.0-rc.8","1.7.0-rc.9","1.7.1","1.7.10","1.7.12","1.7.13","1.7.14","1.7.15","1.7.16","1.7.17","1.7.18","1.7.19","1.7.20","1.7.21","1.7.22","1.7.23","1.7.24","1.7.25","1.7.26","1.7.26.1","1.7.27","1.7.27.1","1.7.28","1.7.29","1.7.29.1","1.7.3","1.7.30","1.7.31","1.7.32","1.7.33","1.7.34","1.7.35","1.7.36","1.7.37","1.7.37.1","1.7.38","1.7.39","1.7.39.1","1.7.39.2","1.7.39.3","1.7.39.4","1.7.4","1.7.40","1.7.41","1.7.41.1","1.7.41.2","1.7.42","1.7.42.1","1.7.42.2","1.7.42.3","1.7.43","1.7.44","1.7.45","1.7.46","1.7.47","1.7.48","1.7.49","1.7.49.1","1.7.49.2","1.7.49.3","1.7.49.4","1.7.49.5","1.7.5","1.7.51","1.7.52","1.7.53","1.7.53.1","1.7.53.2","1.7.53.3","1.7.6","1.7.7","1.7.8","1.7.9","1.8.0-beta.1","1.8.0-beta.10","1.8.0-beta.11","1.8.0-beta.12","1.8.0-beta.13","1.8.0-beta.14","1.8.0-beta.15","1.8.0-beta.16","1.8.0-beta.17","1.8.0-beta.18","1.8.0-beta.19","1.8.0-beta.2","1.8.0-beta.20","1.8.0-beta.21","1.8.0-beta.22","1.8.0-beta.23","1.8.0-beta.24","1.8.0-beta.25","1.8.0-beta.26","1.8.0-beta.27","1.8.0-beta.28","1.8.0-beta.29","1.8.0-beta.3","1.8.0-beta.4","1.8.0-beta.5","1.8.0-beta.6","1.8.0-beta.7","1.8.0-beta.8","1.8.0-beta.9","2.0.0","2.0.0-beta.1","2.0.0-beta.2","2.0.0-beta.3","2.0.0-beta.4","2.0.0-rc.1","2.0.0-rc.10","2.0.0-rc.2","2.0.0-rc.3","2.0.0-rc.4","2.0.0-rc.5","2.0.0-rc.6","2.0.0-rc.7","2.0.0-rc.8","2.0.0-rc.9","2.0.1","2.0.10","2.0.11","2.0.12","2.0.13","2.0.2","2.0.3","2.0.4","2.0.5","2.0.6","2.0.7","2.0.8","2.0.9"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-q2j8-x8hf-63ch/GHSA-q2j8-x8hf-63ch.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N"},{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"}]}