{"id":"CVE-2026-42612","aliases":["GHSA-9695-8fr9-hw5q"],"url":"https://o3.security/vulnerability/CVE-2026-42612","summary":"Grav: Publisher-Level Stored XSS via Unquoted Event Attributes","details":"### Summary\nA stored Cross-Site Scripting (XSS) vulnerability in `getgrav/grav` allows publisher-level accounts to execute arbitrary JavaScript. The issue arises from a blacklist bypass in the `detectXss()` function when handling unquoted HTML event attributes.\n\n### Details\nThe `detectXss()` function relies on a blacklist pattern to filter malicious attributes. The specific regex pattern used to match `on*` events is flawed:\n```php\n'on_events' => '#(<[^>]+[a-z\\x00-\\x20\\\"\\'\\/])(on[a-z]+|xmlns)\\s*=[\\s|\\'\\\"].*[\\s|\\'\\\"]>#iUu'\n```\nThis pattern fails to properly identify `on*` event handlers that are constructed without quotation marks. This allows an attacker to completely bypass the filter. *Note: It is highly recommended to replace this blacklist approach with a robust, established HTML sanitization library.*\n\n### PoC\nAn attacker with publisher-level access can reproduce this by injecting the following payload into any vulnerable content field:\n```html\n<img src=x onerror=eval(atob(/YWxlcnQoZG9jdW1lbnQuY29va2llKQ/.source))>\n```\n<img width=\"1889\" height=\"482\" alt=\"image1\" src=\"https://github.com/user-attachments/assets/0f1a339b-25a8-4b6e-91af-8c59e6a39297\" />\n<img width=\"3055\" height=\"920\" alt=\"image2\" src=\"https://github.com/user-attachments/assets/12680058-bbb3-4446-b58e-515533bb4e90\" />\n<img width=\"2909\" height=\"1339\" alt=\"image3\" src=\"https://github.com/user-attachments/assets/c7ed7e61-8dcf-402d-8589-98d18978c71a\" />\n\n\n**Execution Details:**\nThe `onerror` event is written without quotes to bypass the regex. Because unquoted attributes are restricted in their character usage (e.g., the `=` symbol cannot be used easily), the payload leverages `atob()` and regex `.source` to decode the base64 string `YWxlcnQoZG9jdW1lbnQuY29va2llKQ` (which translates to `alert(document.cookie)`). The `atob()` function conveniently auto-completes the necessary `=` padding for the base64 string.\n\n### Impact\n- **Vulnerability Type:** Stored Cross-Site Scripting (XSS)\n- **Impacted Parties:** Any user (including administrators) who views the compromised content published by the attacker.\n- **Consequences:** Attackers can execute malicious scripts in a victim's browser, leading to session hijacking (cookie theft), unauthorized actions.\n\n\n---\n\n## Maintainer note — fix applied (2026-04-24)\n\nFixed in Grav core on the `2.0` branch: commit [`5a12f9be8`](https://github.com/getgrav/grav/commit/5a12f9be8) — will ship in **2.0.0-beta.2**.\n\n**What changed:** the `on_events` regex in `Security::detectXss()` no longer requires quotes or whitespace around `=`. The previous form:\n\n```\n'on_events' => '#(<[^>]+[\\s\\x00-\\x20\\\"\\'\\/])(on\\s*[a-z]+|xmlns)\\s*=[\\s|\\'\\\"].*[\\s|\\'\\\"]>#iUu'\n```\n\nrequired `[\\s|'\"]` immediately after the `=`, so `<img src=x onerror=alert(1)>` slid past. The new regex drops the value-matching tail entirely and just flags the presence of an `on*=` attribute anywhere inside a tag:\n\n```\n'on_events' => '#<[^>]*?[\\s\\x00-\\x20\\\"\\'\\/](on\\s*[a-z]+|xmlns)\\s*=#iu'\n```\n\nDetecting the attribute name + `=` is enough for a tripwire — the trade-off is occasional false positives on legitimate attribute *values* containing `on*=` substrings, which the maintainer can hand-approve.\n\nThis same regex bypass was the detection-layer half of GHSA-c2q3-p4jr-c55f and GHSA-w8cg-7jcj-4vv2; the fix here knocks both down.\n\n**Files:**\n- [`system/src/Grav/Common/Security.php`](https://github.com/getgrav/grav/blob/2.0/system/src/Grav/Common/Security.php).\n- [`tests/unit/Grav/Common/Security/DetectXssTest.php`](https://github.com/getgrav/grav/blob/2.0/tests/unit/Grav/Common/Security/DetectXssTest.php) — 18 cases: unquoted PoCs, quoted-form regression, safe-content negatives.","published":"2026-05-11T15:22:00.325Z","modified":"2026-08-12T03:51:19.830997373Z","cvss":{"score":8.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:L/A:N"},"epss":{"score":0.00238,"percentile":0.14533,"asOf":"2026-08-24"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Packagist","name":"getgrav/grav","fixedVersion":"2.0.0-beta.2"}],"fix":{"url":"https://github.com/getgrav/grav/commit/5a12f9be8314682c8713e569e330f11805d0a663","label":"getgrav/grav@5a12f9b"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42612.json"},{"type":"ADVISORY","url":"https://github.com/getgrav/grav/security/advisories/GHSA-9695-8fr9-hw5q"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-42612"},{"type":"FIX","url":"https://github.com/getgrav/grav/commit/5a12f9be8314682c8713e569e330f11805d0a663"},{"type":"PACKAGE","url":"https://github.com/getgrav/grav"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:19.830997373Z"}}