| Package | Affected versions | Patched versions |
|---|---|---|
| getgrav/grav | < 2.0.0-beta.2 | 2.0.0-beta.2 |
| grav | = 2.0.0≤ 1.8.0 |
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.
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.
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.
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Scan my site freeFixed 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.