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GHSA-9695-8fr9-hw5q
No affected components available
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.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker needs basic access or low-level privileges. No user interaction is needed for the attacker to exploit this vulnerability. The vulnerability can affect other systems as well, not just the initial system. There is a high impact on the confidentiality of the information. There is a low impact on the integrity of the data.
Exploitation activity has been observed. Apply available patches or mitigations urgently.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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