### ImpactVersions of Express.js prior to 4.19.2 and pre-release alpha and beta versions before 5.0.0-beta.3 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()`.### Patcheshttps://github.com/expressjs/express/commit/0867302ddbde0e9463d0564fea5861feb708c2ddhttps://github.com/expressjs/express/commit/0b746953c4bd8e377123527db11f9cd866e39f94An initial fix went out with `express@4.19.0`, we then patched a feature regression in `4.19.1` and added improved handling for the bypass in `4.19.2`.### WorkaroundsThe fix for this involves pre-parsing the url string with either `require('node:url').parse` or `new URL`. These are steps you can take on your own before passing the user input string to `res.location` or `res.redirect`.### Referenceshttps://github.com/expressjs/express/pull/5539https://github.com/koajs/koa/issues/1800https://expressjs.com/en/4x/api.html#res.location