CVE-2025-4776

Executive Summary

CVE-2025-4776 is a Stored Cross-Site Scripting (XSS) vulnerability discovered in the Phlox WordPress theme, affecting all versions up to and including 2.17.7. The flaw resides in the insufficient sanitization of the data-caption HTML attribute, allowing authenticated attackers with at least Contributor-level access to inject arbitrary web scripts. When a user visits a page containing the malicious payload, the injected script executes in the context of the victim's browser, potentially leading to session hijacking, data theft, or defacement. The vulnerability has a CVSS score of 6.4 (Medium), and as of this report, no official patch has been released. Given the widespread use of the Phlox theme, this poses a significant risk to WordPress sites.

Supply Chain & Package Security

This vulnerability is specific to the WordPress ecosystem and the Phlox theme. It is not related to any NPM, PyPI, Go, or Ruby packages. The risk is introduced through the theme's codebase, which is distributed via the WordPress theme repository and third-party marketplaces. Administrators should verify the integrity of their theme files and monitor for updates from the vendor.

CVSS Breakdown & Threat Level

• CVSS Score: 6.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
• Attack Vector (AV): Network – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): Low – An attacker needs Contributor-level access or higher.
• User Interaction (UI): Required – A victim must visit the compromised page.
• Scope (S): Changed – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low – Limited data exposure possible.
• Integrity (I): Low – Limited ability to modify content.
• Availability (A): None – No impact on availability.

The threat level is elevated due to the authenticated nature of the attack, which bypasses many perimeter defenses, and the potential for widespread exploitation across sites using the Phlox theme.

Deep Dive & Mechanics

The vulnerability is a classic Stored XSS, where user-supplied input is stored on the server and later rendered in a web page without proper escaping. In the Phlox theme, the data-caption attribute is used to store caption text for images or other media elements. The theme fails to sanitize this attribute, allowing an attacker to inject malicious JavaScript.

Attack Flow:

1. An authenticated user with Contributor-level access creates or edits a post/page.
2. The attacker inserts a media element (e.g., an image) and sets the data-caption attribute to a malicious payload, such as "><script>alert('XSS')</script>.
3. The theme stores this payload in the database without sanitization.
4. When any user (including administrators) views the page, the browser renders the data-caption attribute, executing the injected script.

The root cause is the lack of output escaping for the data-caption attribute. WordPress provides functions like esc_attr() for safe attribute output, but the Phlox theme does not use them in this context.

Vulnerable vs. Patched Code

Vulnerable Code (Phlox <= 2.17.7):

// In a template file, e.g., single.php or a shortcode handler
echo '<div data-caption="' . $caption . '">...</div>';

Here, $caption is directly concatenated into the HTML attribute without escaping.

Patched Code (Hypothetical):

echo '<div data-caption="' . esc_attr($caption) . '">...</div>';

The esc_attr() function encodes special characters like ", ', <, >, and &, preventing script injection.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress site running Phlox theme <= 2.17.7.
• An account with Contributor-level privileges or higher.

Exploitation Steps:

1. Log in to the WordPress admin panel.
2. Navigate to Posts > Add New.
3. Insert an image or media element.
4. In the caption field (or directly in the HTML view), set the data-caption attribute to:

   "><script>alert('XSS')</script>
   

5. Publish or save the post.
6. Wait for a victim (e.g., an admin) to view the post. The script executes in their browser.

Advanced Payload:

"><img src=x onerror=alert(document.cookie)>

This payload steals cookies when the image fails to load.

Blue Team

Detection:

• Monitor for posts/pages containing suspicious data-caption attributes.
• Look for unusual JavaScript in post content or media captions.
• Use web application firewalls (WAF) to block common XSS patterns.

Prevention:

• Apply input validation and output escaping to all user-supplied data.
• Restrict Contributor-level permissions to trusted users only.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.

Lab Setup:

1. Install a local WordPress instance (e.g., using XAMPP or Docker).
2. Install Phlox theme version 2.17.7.
3. Create a Contributor user and log in.
4. Follow the Red Team steps to reproduce the vulnerability.
5. Use browser developer tools to inspect the rendered HTML and confirm the XSS.

Indicators of Compromise (IOCs)

• File-based IOCs:

  • Modified or new posts/pages containing data-caption with script tags or event handlers.
  • Suspicious entries in the wp_posts table with post_content containing data-caption and JavaScript.

• Network-based IOCs:

  • HTTP requests to the WordPress admin with unusual data-caption parameters.
  • Outbound connections from the server to attacker-controlled domains (if payload exfiltrates data).

• Log-based IOCs:

  • WordPress audit logs showing creation/editing of posts by Contributor users with unusual content.
  • Web server logs with requests containing XSS patterns in query strings or POST data.

Detection Engineering

YARA Rule

rule CVE_2025_4776_Phlox_XSS {
    meta:
        description = "Detects stored XSS payloads in Phlox theme data-caption attribute"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-4776"
    strings:
        $xss1 = /data-caption\s*=\s*"[^"]*<script[^>]*>/i
        $xss2 = /data-caption\s*=\s*"[^"]*on\w+\s*=\s*[^"]*>/i
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-4776 Phlox Theme Stored XSS Detection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit stored XSS in Phlox theme via data-caption attribute
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/wp-admin/post.php'
        cs-uri-query: 'action=editpost'
        sc-status: 200
        cs-request: '*data-caption*'
    keywords:
        - '<script'
        - 'onerror'
        - 'onload'
        - 'onclick'
    condition: selection and keywords
falsepositives:
    - Legitimate use of JavaScript in captions (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-4776 Phlox Theme Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; content:"data-caption"; http_client_body; pcre:"/(<script|on\w+\s*=)/i"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Phlox theme until a patched version is released. Switch to a different, secure theme.
2. Restrict Contributor-level accounts to only trusted users. Consider disabling registration or requiring admin approval.
3. Manually patch the theme by adding esc_attr() to the data-caption output in all template files. Locate the vulnerable code (likely in single.php, page.php, or shortcode handlers) and apply the fix.

Long-term Measures:

• Monitor the Phlox vendor for an official patch and apply it immediately upon release.
• Implement a Web Application Firewall (WAF) with rules to block XSS payloads.
• Enforce Content Security Policy (CSP) headers to limit script execution.
• Conduct regular security audits of all WordPress themes and plugins.
• Educate content creators about the risks of XSS and safe coding practices.

Workaround: If the theme cannot be disabled, use a WordPress security plugin (e.g., Wordfence) to add virtual patching for this vulnerability.

Valters IT