CVE-2024-39926

Executive Summary

CVE-2024-39926 is a stored cross-site scripting (XSS) vulnerability discovered in Vaultwarden (formerly Bitwarden_RS) version 1.30.3. The flaw resides in the admin dashboard, allowing an authenticated attacker to inject malicious HTML or JavaScript. However, the application's default Content Security Policy (CSP) significantly mitigates exploitation, restricting script execution. The vulnerability has a CVSS score of 0.0 (pending official assessment) but is estimated to be low severity due to the CSP restrictions and required authentication. No public proof-of-concept (PoC) or exploit code is available at this time.

Supply Chain & Package Security

Vaultwarden is an open-source Rust-based implementation of the Bitwarden server API. The vulnerability affects the admin dashboard component, which is part of the core application. No supply chain compromise has been identified; the issue is a code-level flaw in input sanitization.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): High (H) – Attacker must be authenticated as an admin.
• User Interaction (UI): Required (R) – An admin must view the injected content.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): Low (L) – Limited data exposure possible.
• Integrity (I): Low (L) – Limited content manipulation possible.
• Availability (A): None (N) – No impact on availability.

Threat Level: Low (due to CSP mitigation and authentication requirement).

Deep Dive & Mechanics

The vulnerability is a stored XSS in the admin dashboard of Vaultwarden. The admin dashboard displays user-submitted data (e.g., organization names, user details) without proper sanitization. An authenticated attacker with admin privileges can inject malicious HTML or JavaScript into fields that are later rendered in the dashboard.

The default Content Security Policy (CSP) of Vaultwarden restricts script execution to same-origin sources only, blocking inline scripts and external script loads. This means that while HTML injection is possible (e.g., <img src=x onerror=alert(1)>), JavaScript execution is blocked by the CSP. However, HTML injection alone can still be used for phishing attacks (e.g., injecting fake login forms) or defacement.

The root cause is insufficient output encoding in the admin dashboard templates. The application fails to escape user-controlled data before rendering it in HTML context.

Vulnerable vs. Patched Code

Vulnerable Code (1.30.3):

// In admin dashboard template rendering
let user_input = get_user_input(); // e.g., organization name
let html = format!("<div>{}</div>", user_input); // No escaping

Patched Code (Hypothetical):

// Properly escape HTML entities
let user_input = get_user_input();
let escaped = html_escape::encode_text(&user_input);
let html = format!("<div>{}</div>", escaped);

The patch would involve using a proper HTML escaping function (e.g., html_escape::encode_text in Rust) for all user-controlled data displayed in the admin dashboard.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to a Vaultwarden instance (version 1.30.3)
• Valid admin credentials

Steps:

1. Log in to the Vaultwarden admin dashboard.
2. Navigate to a field that accepts user input (e.g., organization name, user display name).
3. Inject the following payload:

   <img src=x onerror="alert('XSS')">
   

   Note: Due to CSP, JavaScript may not execute. For HTML injection, use:

   <h1>Injected Content</h1>
   

4. Save the changes.
5. When another admin views the dashboard, the injected content is rendered.

Expected Outcome: The injected HTML is displayed in the admin dashboard. JavaScript execution is blocked by CSP.

Blue Team

Detection:

• Monitor admin dashboard logs for unusual input patterns (e.g., HTML tags in user fields).
• Use web application firewall (WAF) rules to block common XSS payloads.

Mitigation:

• Apply input validation and output encoding.
• Strengthen CSP to block all inline content (e.g., script-src 'none').

Indicators of Compromise (IOCs)

• Network: HTTP requests to /admin endpoint with HTML tags in POST data.
• Logs: Admin dashboard logs showing unusual characters (e.g., <, >, ").
• File: Modified database entries containing HTML/JavaScript in user fields.

Detection Engineering

YARA Rule

rule CVE_2024_39926_Vaultwarden_XSS {
    meta:
        description = "Detects stored XSS payloads in Vaultwarden admin dashboard"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-39926"
    strings:
        $html_tag = /<[a-zA-Z\/][^>]*>/
        $xss_payload = /<script[^>]*>.*<\/script>/i
        $event_handler = /on\w+\s*=\s*['\"][^'\"]*['\"]/i
    condition:
        any of them
}

Sigma Rule

title: Vaultwarden Admin Dashboard XSS Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential XSS payloads in Vaultwarden admin dashboard requests
author: Valters IT
date: 2024-01-01
logsource:
    category: web
    product: vaultwarden
detection:
    selection:
        http.method: POST
        http.url: /admin*
        http.body|contains: 
            - '<script'
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate HTML in user fields (rare)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-39926 Vaultwarden Admin XSS Attempt"; flow:to_server; content:"POST"; http_method; content:"/admin"; http_uri; pcre:"/<[a-zA-Z\/][^>]*>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Restrict admin dashboard access to trusted IPs only.
2. Short-term: Apply input validation and output encoding for all user-controlled data in admin templates.
3. Long-term: Upgrade to a patched version once available. Monitor Vaultwarden GitHub for updates.
4. Compensating Controls: Strengthen CSP to script-src 'none'; and use Content Security Policy reporting to detect violations.

Valters IT
DevSecOps & Cyber Threat Intelligence