Improper Handling of `callbackUrl` parameter in next-auth

3 years ago

### Impact

An attacker can send a request to an app using NextAuth.js with an invalid `callbackUrl` query parameter, which internally we convert to a `URL` object. The URL instantiation would fail due to a malformed URL being passed into the constructor, causing it to throw an unhandled error which led to our **API route handler timing out and logging in to fail**. This has been remedied in the following releases:

next-auth v3 users before version 3.29.5 are impacted. (We recommend upgrading to v4, as v3 is considered unmaintained. See our [migration guide](https://next-auth.js.org/getting-started/upgrade-v4))

next-auth v4 users before version 4.5.0 are impacted.

### Patches

We’ve released patches for this vulnerability in:

– v3 – `3.29.5`
– v4 – `4.5.0`

You can do:

“`sh
npm i next-auth@latest
“`

“`sh
yarn add next-auth@latest
“`

“`sh
pnpm add next-auth@latest
“`

(This will update to the latest v4 version, but you can change `latest` to `3` if you want to stay on v3. This is not recommended.)

### Workarounds

If for some reason you cannot upgrade, the workaround requires you to rely on [Advanced Initialization](https://next-auth.js.org/configuration/initialization#advanced-initialization). Here is an example:

**Before:**

“`js
// pages/api/auth/[…nextauth].js
import NextAuth from “next-auth”

export default NextAuth(/* your config */)
“`

**After:**

“`js
// pages/api/auth/[…nextauth].js
import NextAuth from “next-auth”

function isValidHttpUrl(url) {
try {
return /^https?:/.test(url).protocol);
} catch {
return false;
}
}

export default async function handler(req, res) {
if (
req.query.callbackUrl &&
!isValidHttpUrl(req.query.callbackUrl)
) {
return res.status(500).send(”);
}

return await NextAuth(req, res, /* your config */)
}
“`

### References

This vulnerability was discovered not long after https://github.com/nextauthjs/next-auth/security/advisories/GHSA-q2mx-j4x2-2h74 was published and is very similar in nature.