Impact

Versions 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 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().

Patches

0867302
0b74695

An 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.

Workarounds

The 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.

References

#5539
koajs/koa#1800
https://expressjs.com/en/4x/api.html#res.location