Comprehensive Technical Analysis of CVE-2026-25150 (Qwik Prototype Pollution Vulnerability)

1. Vulnerability Assessment and Severity Evaluation

Vulnerability Overview

CVE-2026-25150 is a prototype pollution vulnerability in the @builder.io/qwik-city middleware of the Qwik JavaScript framework, specifically within the formToObj() function. The flaw arises from insufficient sanitization of form field names containing dot notation (e.g., user.name), allowing attackers to inject malicious properties such as __proto__, constructor, or prototype into Object.prototype.

Severity Evaluation (CVSS 9.3 - Critical)

CVSS Metric	Score	Justification
Attack Vector (AV)	Network (N)	Exploitable remotely via HTTP requests.
Attack Complexity (AC)	Low (L)	No authentication required; straightforward exploitation.
Privileges Required (PR)	None (N)	Unauthenticated attackers can exploit.
User Interaction (UI)	None (N)	No user interaction needed.
Scope (S)	Unchanged (U)	Affects the vulnerable component but may impact other parts of the application.
Confidentiality (C)	High (H)	Potential for sensitive data exposure via polluted properties.
Integrity (I)	High (H)	Arbitrary property injection can lead to authentication bypass or privilege escalation.
Availability (A)	High (H)	Denial of Service (DoS) possible via polluted properties.

Overall CVSS Score: 9.3 (Critical) The high severity stems from:

• Unauthenticated remote exploitation (no credentials required).
• Potential for privilege escalation, authentication bypass, or DoS.
• Widespread impact on applications using affected Qwik versions.

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2. Potential Attack Vectors and Exploitation Methods

Exploitation Mechanism

The vulnerability is triggered when an attacker submits a crafted HTTP POST request with malicious form field names that include prototype pollution gadgets. The formToObj() function processes these fields to construct nested objects but fails to sanitize dangerous property names.

Example Exploit Payload

POST /api/form-handler HTTP/1.1
Host: vulnerable-app.com
Content-Type: application/x-www-form-urlencoded

__proto__.isAdmin=true&__proto__.authToken=attacker-controlled-value

Impact:

• Pollutes Object.prototype with attacker-controlled properties.
• Any object inheriting from Object.prototype (i.e., nearly all JavaScript objects) will now have these properties.
• Can lead to:
  • Authentication bypass (e.g., isAdmin=true).
  • Privilege escalation (e.g., modifying security-related properties).
  • Denial of Service (DoS) (e.g., polluting toString or valueOf to crash the application).

Advanced Exploitation Scenarios

1. Authentication Bypass

   • If an application checks user.isAdmin, an attacker can pollute Object.prototype.isAdmin to true, causing all objects to inherit this property.
   • Example:

     if (user.isAdmin) { /* Grant admin access */ }
     

     After pollution, user.isAdmin resolves to true even if the original object did not have this property.

2. Remote Code Execution (RCE) via Gadget Chains

   • If the application uses a vulnerable library (e.g., lodash, jquery) that relies on Object.prototype properties, an attacker may chain prototype pollution with other vulnerabilities to achieve RCE.
   • Example:

     __proto__.toString = () => { require('child_process').exec('malicious-command'); }
     

3. Denial of Service (DoS)

   • Polluting critical methods (e.g., toString, valueOf) can crash the application.
   • Example:

     __proto__.toString = () => { while(true); }
     

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3. Affected Systems and Software Versions

Vulnerable Software

• Qwik Framework (versions prior to 1.19.0).
• @builder.io/qwik-city middleware (specifically the formToObj() function).

Affected Environments

• Web applications using Qwik for server-side rendering (SSR) or client-side form handling.
• Node.js backend services processing form submissions via @builder.io/qwik-city.
• Serverless functions (e.g., AWS Lambda, Vercel, Netlify) running Qwik middleware.

Unaffected Versions

• Qwik v1.19.0 and later (patched).
• Applications not using @builder.io/qwik-city middleware.

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4. Recommended Mitigation Strategies

Immediate Actions

1. Upgrade to Qwik v1.19.0 or Later

   • The patch (commit 5f65bae2bc33e6ca0c21e4cfcf9eae05077716f7) introduces proper sanitization of form field names.
   • Upgrade command:

     npm install @builder.io/qwik@latest
     # or
     yarn upgrade @builder.io/qwik@latest
     

2. Apply Input Sanitization (Temporary Workaround)

   • If upgrading is not immediately possible, implement a server-side filter to block dangerous property names:

     function sanitizeFormData(formData) {
       const dangerousProps = ['__proto__', 'constructor', 'prototype'];
       for (const key in formData) {
         if (dangerousProps.includes(key)) {
           throw new Error("Malicious input detected");
         }
       }
       return formData;
     }
     

3. Use Object.create(null) for Safe Object Creation

   • Replace direct object creation with Object.create(null) to avoid prototype inheritance:

     const safeObj = Object.create(null);
     

4. Implement Content Security Policy (CSP)

   • Restrict inline scripts and external sources to mitigate potential XSS vectors that could chain with prototype pollution.

Long-Term Recommendations

• Code Audits for Prototype Pollution

  • Review all form-handling logic for similar vulnerabilities.
  • Use static analysis tools (e.g., Semgrep, ESLint) to detect unsafe property assignments.

• Adopt Secure Coding Practices

  • Avoid using obj[key] for dynamic property assignment; use Object.defineProperty() with writable: false.
  • Use libraries like lodash.merge with caution (known to be vulnerable to prototype pollution).

• Monitor for Exploitation Attempts

  • Log and alert on suspicious form submissions containing __proto__, constructor, or prototype.

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5. Impact on the Cybersecurity Landscape

Broader Implications

1. Increased Risk of Supply Chain Attacks

   • Qwik is a popular framework for high-performance web applications. A critical vulnerability in such a widely used library increases the attack surface for supply chain compromises.
   • Attackers may target CI/CD pipelines to inject malicious dependencies.

2. Prototype Pollution as a Persistent Threat

   • This vulnerability highlights the growing prevalence of prototype pollution in JavaScript ecosystems.
   • Similar flaws have been found in Express.js, Lodash, and jQuery, indicating a systemic issue in JavaScript’s object model.

3. Exploitation in Serverless and Edge Computing

   • Qwik is often used in serverless architectures (e.g., Vercel, Netlify). A successful exploit could lead to lateral movement in cloud environments.

4. Regulatory and Compliance Risks

   • Organizations using affected versions may violate data protection regulations (e.g., GDPR, CCPA) if exploitation leads to unauthorized data access.

Historical Context

• CVE-2019-10744 (Lodash Prototype Pollution) – Similar vulnerability in Lodash’s merge function.
• CVE-2020-7703 (jQuery Prototype Pollution) – Affected jQuery’s extend method.
• CVE-2021-23337 (Express.js Prototype Pollution) – Exploitable via query parameters.

This trend underscores the need for better static analysis tools and secure-by-default frameworks.

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6. Technical Details for Security Professionals

Root Cause Analysis

The vulnerability exists in the formToObj() function within @builder.io/qwik-city, which processes form data into nested objects. The function fails to sanitize property names when splitting dot-notation keys (e.g., user.name → { user: { name: value } }).

Vulnerable Code (Simplified)

function formToObj(formData) {
  const result = {};
  for (const [key, value] of Object.entries(formData)) {
    const keys = key.split('.');
    let current = result;
    for (let i = 0; i < keys.length - 1; i++) {
      const k = keys[i];
      if (!current[k]) current[k] = {}; // ❌ No sanitization
      current = current[k];
    }
    current[keys[keys.length - 1]] = value;
  }
  return result;
}

Problem:

• If key = "__proto__.isAdmin", the code assigns result.__proto__.isAdmin = true, polluting Object.prototype.

Patched Code (Qwik v1.19.0)

function formToObj(formData) {
  const result = Object.create(null); // ❌ Avoids prototype inheritance
  const dangerousProps = new Set(['__proto__', 'constructor', 'prototype']);

  for (const [key, value] of Object.entries(formData)) {
    if (dangerousProps.has(key)) continue; // ✅ Sanitization
    const keys = key.split('.');
    let current = result;
    for (let i = 0; i < keys.length - 1; i++) {
      const k = keys[i];
      if (dangerousProps.has(k)) continue; // ✅ Additional check
      if (!current[k]) current[k] = Object.create(null);
      current = current[k];
    }
    current[keys[keys.length - 1]] = value;
  }
  return result;
}

Fixes:

1. Uses Object.create(null) to avoid prototype inheritance.
2. Explicitly blocks dangerous property names (__proto__, constructor, prototype).

Exploitation Detection

• Log Analysis:

  • Look for HTTP requests containing __proto__, constructor, or prototype in form data.
  • Example SIEM query (Splunk):

    index=web_logs method=POST form_data="*__proto__*" OR form_data="*constructor*" OR form_data="*prototype*"
    

• Runtime Protection:

  • Use Node.js runtime protection tools (e.g., Snyk, Sqreen) to detect prototype pollution attempts.
  • Deploy Web Application Firewalls (WAFs) with rules to block malicious form submissions.

Proof of Concept (PoC)

// Simulate a vulnerable form handler
const formData = {
  "__proto__.isAdmin": "true",
  "user.name": "attacker"
};

const result = formToObj(formData);
console.log({}.isAdmin); // Output: "true" (Object.prototype polluted)

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Conclusion

CVE-2026-25150 is a critical prototype pollution vulnerability in the Qwik framework that enables unauthenticated remote exploitation with severe consequences, including privilege escalation, authentication bypass, and DoS. The flaw stems from insufficient input sanitization in the formToObj() function and has been patched in Qwik v1.19.0.

Key Takeaways for Security Professionals:

1. Immediately upgrade to Qwik v1.19.0 or later.
2. Audit all form-handling logic for similar vulnerabilities.
3. Monitor for exploitation attempts via WAFs and SIEM rules.
4. Adopt secure coding practices to prevent prototype pollution.
5. Stay vigilant for emerging threats in JavaScript frameworks.

This vulnerability underscores the critical need for secure-by-default frameworks and proactive security testing in modern web development.