GHSA-9695-8FR9-HW5Q
Vulnerability from github – Published: 2026-05-05 21:27 – Updated: 2026-05-13 13:52Summary
A 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.
Details
The detectXss() function relies on a blacklist pattern to filter malicious attributes. The specific regex pattern used to match on* events is flawed:
'on_events' => '#(<[^>]+[a-z\x00-\x20\"\'\/])(on[a-z]+|xmlns)\s*=[\s|\'\"].*[\s|\'\"]>#iUu'
This 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.
PoC
An attacker with publisher-level access can reproduce this by injecting the following payload into any vulnerable content field:
<img src=x onerror=eval(atob(/YWxlcnQoZG9jdW1lbnQuY29va2llKQ/.source))>
Execution Details:
The 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.
Impact
- Vulnerability Type: Stored Cross-Site Scripting (XSS)
- Impacted Parties: Any user (including administrators) who views the compromised content published by the attacker.
- Consequences: Attackers can execute malicious scripts in a victim's browser, leading to session hijacking (cookie theft), unauthorized actions.
Maintainer note — fix applied (2026-04-24)
Fixed in Grav core on the 2.0 branch: commit 5a12f9be8 — will ship in 2.0.0-beta.2.
What changed: the on_events regex in Security::detectXss() no longer requires quotes or whitespace around =. The previous form:
'on_events' => '#(<[^>]+[\s\x00-\x20\"\'\/])(on\s*[a-z]+|xmlns)\s*=[\s|\'\"].*[\s|\'\"]>#iUu'
required [\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:
'on_events' => '#<[^>]*?[\s\x00-\x20\"\'\/](on\s*[a-z]+|xmlns)\s*=#iu'
Detecting 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.
This same regex bypass was the detection-layer half of GHSA-c2q3-p4jr-c55f and GHSA-w8cg-7jcj-4vv2; the fix here knocks both down.
Files:
- system/src/Grav/Common/Security.php.
- tests/unit/Grav/Common/Security/DetectXssTest.php — 18 cases: unquoted PoCs, quoted-form regression, safe-content negatives.
{
"affected": [
{
"package": {
"ecosystem": "Packagist",
"name": "getgrav/grav"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "2.0.0-beta.2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-42612"
],
"database_specific": {
"cwe_ids": [
"CWE-79"
],
"github_reviewed": true,
"github_reviewed_at": "2026-05-05T21:27:15Z",
"nvd_published_at": "2026-05-11T16:17:34Z",
"severity": "HIGH"
},
"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\u0027on_events\u0027 =\u003e \u0027#(\u003c[^\u003e]+[a-z\\x00-\\x20\\\"\\\u0027\\/])(on[a-z]+|xmlns)\\s*=[\\s|\\\u0027\\\"].*[\\s|\\\u0027\\\"]\u003e#iUu\u0027\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\u003cimg src=x onerror=eval(atob(/YWxlcnQoZG9jdW1lbnQuY29va2llKQ/.source))\u003e\n```\n\u003cimg width=\"1889\" height=\"482\" alt=\"image1\" src=\"https://github.com/user-attachments/assets/0f1a339b-25a8-4b6e-91af-8c59e6a39297\" /\u003e\n\u003cimg width=\"3055\" height=\"920\" alt=\"image2\" src=\"https://github.com/user-attachments/assets/12680058-bbb3-4446-b58e-515533bb4e90\" /\u003e\n\u003cimg width=\"2909\" height=\"1339\" alt=\"image3\" src=\"https://github.com/user-attachments/assets/c7ed7e61-8dcf-402d-8589-98d18978c71a\" /\u003e\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\u0027s browser, leading to session hijacking (cookie theft), unauthorized actions.\n\n\n---\n\n## Maintainer note \u2014 fix applied (2026-04-24)\n\nFixed in Grav core on the `2.0` branch: commit [`5a12f9be8`](https://github.com/getgrav/grav/commit/5a12f9be8) \u2014 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\u0027on_events\u0027 =\u003e \u0027#(\u003c[^\u003e]+[\\s\\x00-\\x20\\\"\\\u0027\\/])(on\\s*[a-z]+|xmlns)\\s*=[\\s|\\\u0027\\\"].*[\\s|\\\u0027\\\"]\u003e#iUu\u0027\n```\n\nrequired `[\\s|\u0027\"]` immediately after the `=`, so `\u003cimg src=x onerror=alert(1)\u003e` 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\u0027on_events\u0027 =\u003e \u0027#\u003c[^\u003e]*?[\\s\\x00-\\x20\\\"\\\u0027\\/](on\\s*[a-z]+|xmlns)\\s*=#iu\u0027\n```\n\nDetecting the attribute name + `=` is enough for a tripwire \u2014 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) \u2014 18 cases: unquoted PoCs, quoted-form regression, safe-content negatives.",
"id": "GHSA-9695-8fr9-hw5q",
"modified": "2026-05-13T13:52:24Z",
"published": "2026-05-05T21:27:15Z",
"references": [
{
"type": "WEB",
"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": "WEB",
"url": "https://github.com/getgrav/grav/commit/5a12f9be8314682c8713e569e330f11805d0a663"
},
{
"type": "PACKAGE",
"url": "https://github.com/getgrav/grav"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "Grav Vulnerable to Publisher-Level Stored XSS via Unquoted Event Attributes"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Published Proof of Concept: A public proof of concept is available for this vulnerability.
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- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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