Grav Vulnerable to Publisher-Level Stored XSS via Unquoted Event Attributes

Summary

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.