Express.js Open Redirect in malformed URLs

Vulnerability Description

Express.js minimalist web framework for node. Versions of Express.js prior to 4.19.0 and all pre-release alpha and beta versions of 5.0 are affected by an open redirect vulnerability using malformed URLs. When a user of Express performs a redirect using a user-provided URL Express performs an encode [using `encodeurl`](https://github.com/pillarjs/encodeurl) on the contents before passing it to the `location` header. This can cause malformed URLs to be evaluated in unexpected ways by common redirect allow list implementations in Express applications, leading to an Open Redirect via bypass of a properly implemented allow list. The main method impacted is `res.location()` but this is also called from within `res.redirect()`. The vulnerability is fixed in 4.19.2 and 5.0.0-beta.3.

References

• https://github.com/expressjs/express/security/advisories/GHSA-rv95-896h-c2vc
• https://github.com/koajs/koa/issues/1800
• https://github.com/expressjs/express/pull/5539
• https://github.com/expressjs/express/commit/0867302ddbde0e9463d0564fea5861feb708c2dd
• https://github.com/expressjs/express/commit/0b746953c4bd8e377123527db11f9cd866e39f94
• https://expressjs.com/en/4x/api.html#res.location