Open Redirect in tjenkinson/url-toolkit

Reported on Jul 8th 2021

✍️ Description

url-toolkit mishandles certain uses of backslash such as https:/\ and interprets the URI as a relative path. Browsers accept backslashes after the protocol, and treat it as a normal slash, while url-toolkit sees it as a relative path. Which will lead to SSRF attacks, open redirects, or other undesired behavior.

Similar to CVE-2021-27515, CVE-2021-27516

🕵️‍♂️ Proof of Concept

  1. Create the following PoC file:
// poc.js
var urlToolkit = require("url-toolkit")
var u= urlToolkit.parseURL("https:/\/\/\")
  1. Execute the following commands in another terminal:
npm i url-toolkit # Install affected module
node poc.js #  Run the PoC
  1. Check the Output:
  scheme: 'https:',
  netLoc: '//',
  path: '/',
  params: '',
  query: '',
  fragment: ''

💥 Impact

Depending on library usage and attacker intent, impacts may include allow/block list bypasses, SSRF attacks, open redirects, or other undesired behavior.

We have contacted a member of the tjenkinson/url-toolkit team and are waiting to hear back 18 days ago
tjenkinson/url-toolkit maintainer
18 days ago


I will try and take a look at this later. If not hopefully this weekend.


tjenkinson/url-toolkit maintainer
18 days ago

There is definitely an issue here but it's not related to \ I think. The problem is just having a url like https:///, which results in netLoc being //.

Looks like Chrome will convert an http://///// to

tjenkinson/url-toolkit maintainer
18 days ago

I'm wondering if we should throw an error if netLoc is empty or if netLoc should be http:////// in the above case

tjenkinson/url-toolkit maintainer
18 days ago

*if netLoc is just //

tjenkinson/url-toolkit maintainer
16 days ago

I just released which documents the difference between "URL Living Standard" and "RFC 1808". I don't think it's a bug given it's the correct result according to that RFC (if I've understood correctly).

Also the description here is not quite right. It's not an issue related to backslash, but too many forward slashes.

Thanks for reporting. If there's more of an issue here and I misunderstood please let me know.


tjenkinson/url-toolkit maintainer has invalidated this vulnerability 16 days ago

The library implements "RFC 1808", and not "URL Living Standard", and I think the output is valid with respect to "RFC 1808".

I've done a new release to document this difference in the readme.

The disclosure bounty has been dropped
The fix bounty has been dropped
Tom Jenkinson confirmed that a fix has been merged on 2c8cff 16 days ago
Tom Jenkinson has been awarded the fix bounty
Tom Jenkinson
16 days ago


Well it was a doc update as mentioned above and this was a reference to that...