CVE-2026-42612

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.

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.

• 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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-42612 has a CVSS score of 8.5 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.0-beta.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42612? CVE-2026-42612 is a high-severity cross-site scripting (XSS) vulnerability in getgrav/grav (composer), affecting versions < 2.0.0-beta.2. It is fixed in 2.0.0-beta.2. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-42612? CVE-2026-42612 has a CVSS score of 8.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of getgrav/grav are affected by CVE-2026-42612? getgrav/grav (composer) versions < 2.0.0-beta.2 is affected.
4. Is there a fix for CVE-2026-42612? Yes. CVE-2026-42612 is fixed in 2.0.0-beta.2. Upgrade to this version or later.
5. Is CVE-2026-42612 exploitable, and should I be worried? Whether CVE-2026-42612 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42612 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42612? Upgrade getgrav/grav to 2.0.0-beta.2 or later.