Grav vulnerable to Cross-Site Scripting (XSS) Stored endpoint `/admin/pages/[page]` parameter `data[header][template]` in Advanced Tab

Summary

A Stored Cross-Site Scripting (XSS) vulnerability was identified in the /admin/pages/[page] endpoint of the Grav application. This vulnerability allows attackers to inject malicious scripts into the data[header][template] parameter. The script is saved within the page's frontmatter and executed automatically whenever the affected content is rendered in the administrative interface or frontend view.

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Details

Vulnerable Endpoint: POST /admin/pages/[page]
Parameter: data[header][template]

The application fails to properly sanitize user input in the data[header][template] field, which is stored in the YAML frontmatter of the page. An attacker can inject JavaScript code using this field, and the payload is rendered and executed when the page is accessed, especially within the Admin Panel interface.

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PoC

Payload:

<script>alert('PoC-XXS73')</script>

Steps to Reproduce:

1. Log in to the Grav Admin Panel and navigate to Pages.

2. Create a new page or edit an existing one.

3. In the Advanced > Template field (which maps to data[header][template]), insert the payload:

4. Save the page.

5. Return to the Pages section and click on the three-dot menu of the affected page:

6. The stored XSS payload is triggered, and the script is executed in the browser:

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Impact

Stored XSS vulnerabilities can have serious consequences, including:

• Session hijacking: Capturing admin session cookies or tokens

• Malware delivery: Executing scripts that load malicious resources

• Credential theft: Creating fake login prompts to steal usernames/passwords

• Data exposure: Reading sensitive metadata or page contents

• Privilege escalation: Performing actions as an authenticated user

• Website defacement: Altering visual or functional elements of the site

• Reputation damage: Undermining user trust in the application

by CVE-Hunters