Can evil websites get to your mqweb – understanding CORS

In the beginning was the html, and the html was good;  then we had html and scripts  which could only do things on the page, which was also good; then we had scripts which could reach out to other websites – and that’s where the problems began.  It was easy for evil developers to get you to click on an innocent looking page, which had a script which jumped into a different tab of your browser where you had your banking window open ,  or  to executed a script ; and steal all your money.

The browsers were improved to stop evil scripts from accessing a server, and then they were improved again so the server could say “stuff coming from this list of web sites is OK, I trust them.   One implementation is called CORS.  There is a good description here.   It says

Cross-Origin Resource Sharing (CORS) is a mechanism that uses additional HTTP headers to tell browsers to give a web application running at one origin, access to selected resources from a different origin. A web application executes a cross-origin HTTP request when it requests a resource that has a different origin (domain, protocol, or port) from its own.

I do not trust things that “just work”, I like to see evidence of it.   This lack of trust comes from working with a group of young testers who came to IBM Hursley to test their code.   All of their tests ran cleanly – and thought they had a few days spare to go to London sightseeing.  I stopped the server they were meant to be using (without telling them) and the tests carried on running successfully!   It turned out they had been testing their code with a dummy program acting as the server.    They removed this code, reran their tests  and most of them failed – and had to stay an extra week.

I also remember changing a config file, and being surprised when my changed worked first time.   After a cup of tea (an invaluable thinking aid) I put some spelling mistakes in the file ; and it carried on running.  Why? I was using the wrong config file.

I played with CORS and wanted to get things to fail, as well as to work.   This was a good choice, as I had many failures.

I’ll document how I got curl to work and demonstrate CORS , and document how I got a web browser to work – a real challenge

mqweb implements CORS, so you can configure mqweb to give a list of websites which may access your server.

The documentation  is not very clear.  It says

where allowedOrigins specifies the origin that you want to allow cross-origin requests from. You can use an asterisk surrounded by double quotation marks, “*”, to allow all cross-origin requests. You can enter more than one origin in a comma-separated list, surrounded by double quotation marks. To allow no cross-origin requests, enter empty quotation marks as the value for allowedOrigins.

My observations,

  • You cannot use generics, so* is the same as “*” – or allow all cross-origin requests
  • You must specify {scheme:address:port} so http or https,  the url with // at the front, and the specific port number
  • The match is an string equality test, so the case, spacing and values must be the same

How does an HTTP request work?

When you click on a web page, data is sent to the back end server.   The following data is exchanged

  • the request
  • request headers
  • your data going to the server
  • response headers
  • response data – such as the content of web page.

In more detail…


Request Headers

  • accept: for example  text/html, application/xml
  • accept-languages: en-GB
  • dnt:  1  this is “do not track me”
  • user-agent:  Chromium
  • cookie: bcookie=”….”

Response headers

  • status: 200
  • status: 200 OK
  • set-cookie ….
  • server: nginx
  • ….

Response data

This might be the web page to be displayed.  It can include script, images etc.

What is the origin of the page?

If your web page, invoked a script, for example from clicking a button, an “Origin” header is added, for example Origin: http://localhost:8884 which is the address of the web server hosting the page.   The backend server checks to see if this header is present, and looks up site in the authorised list.

If the Origin is acceptable, it sends down additional response headers (CORS Headers), so the browser (or your program) knows and can use the web site.

As part of the handshake, the browser can send up an “OPTIONS” request (instead of a get/delete etc), with a header saying the browser will be doing a GET/DELETE etc from this origin.    If there is a positive response, where the additional CORS headers are send back then the get/delete is allowed.   If the CORS headers are not present in the response, then the request will not be permitted.   This is called a preflight check – just like having your boarding pass checked at the gate before you get on the plane.

Does it work?

If there is no Origin header in the request, the backend server thinks it is all same domain, and does no CORS checks.

This CORS support is really aimed at web browsers, as the web browsers will automatically add headers for you.  If you are using curl or other tools to create your own request, you specify exactly the headers you want, so if you omit the Origins header, the server will not check.

My mqwebuser.xml file had <variable name=”mqRestCorsAllowedOrigins” value=”https://9999.0.0.1:19442,http://localhost:8884”/&gt;

So origins for https://9999.0.0.1:19442 and http://localhost:8884 are permitted.

I used a configuration file for curl (because command parameters did not work passing the headers) and had in curl.parms

cacert ./cacert.pem
cert ./colinpaice.pem:password
key ./colinpaice.key.pem
cookie cookie.jar.txt
cookie-jar cookie.jar.txt
request OPTIONS
header “Access-Control-Request-Method: DELETE”
header “Access-Control-Request-Headers: Content-Type”
header “Origin: https://9999.0.0.1:19442
header “ibm-mq-rest-csrf-token : COLINCSRF”

I used the command

curl –verbose –config curl.parms –url https://localhost:9445/ibmmq/rest/v1/messaging/qmgr/QMA/queue/DEEPQ/message

The headers were displayed, and I had

OPTIONS /ibmmq/rest/v1/messaging/qmgr/QMA/queue/DEEPQ/message HTTP/1.1
> Host: localhost:9445
> User-Agent: curl/7.58.0
> Accept: */*
> Cookie: LtpaToken2_….
> Access-Control-Request-Method: DELETE
> Access-Control-Request-Headers: Content-Type
> Origin: https://9999.0.0.1:19442
> ibm-mq-rest-csrf-token : COLINCSRF

The response headers were

< HTTP/1.1 200 OK
< X-Powered-By: Servlet/3.1
< X-XSS-Protection: 1;mode=block
< X-Content-Type-Options: nosniff
< Content-Security-Policy: default-src ‘none’; script-src ‘self’ ‘unsafe-inline’ ‘unsafe-eval’; connect-src ‘self’; img-src ‘self’; style-src ‘self’ ‘unsafe-inline’; font-src ‘self’
< Cache-Control: no-cache, no-store, must-revalidate
< Access-Control-Allow-Credentials: true
< Access-Control-Allow-Origin: https://9999.0.0.1:19442
< Access-Control-Allow-Max-Age: 90
< Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE
< Access-Control-Allow-Headers: Accept-Charset, Accept-Language, Authorization, Content-Type, ibm-mq-rest-csrf-token, ibm-mq-md-correlationId, ibm-mq-md-expiry, ibm-mq-md-persistence, ibm-mq-md-replyTo, ibm-mq-rest-mft-total-transfers

The response header Access-Control-Allow-Origin: https://9999.0.0.1:19442 shows that requests with origin https://9999.0.0.1:19442 is acceptable.

When I used a different “Origin”,   I did not get any Access-Control-Allow-* headers.  So from the absence,  I could tell the request was not support from the different origin.

The Access-Control-Allow-Headers is a list of header names which can be sent, so the header ibm-mq-rest-csrf-token : COLINCSRF is valid, but “COLIN:value”  is not valid, because ibm-mq-rest-csrf-token  is in the Access-Control-Allow-Headers list, and COLIN is not in the list.

The Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE says my application can use any of the methods in the list.

Using a web browser.

If you use HTML in a local file, then the “Origin” is null, and so does not match any elements in the authorised list.  I had to set up my own web server – which was easy to do using Python.

Using the web page at the bottom of this posting, I pointed my web browser at the web server.   It displayed a button.  I pressed it, and it invoked a script. Using the developer mode ( Al+Ctrl +i) in Chrome could see network flows etc.

The request headers had

  • Host: localhost:9445 This is where my webserver is hosted
  • ibm-mq-rest-csrf-token : 99  I specified this header value
  • Origin: http://localhost:8884  this overrode the value I had specified in the headers.

The response headers included

  • Access-Control-Allow-Credentials: true
  • Access-Control-Allow-Origin: http://localhost:8884
  • Access-Control-Expose-Headers: Content-Language, Content-Length, Content-Type, Location, ibm-mq-qmgrs, ibm-mq-md-messageId, ibm-mq-md-correlationId, ibm-mq-md-expiry, ibm-mq-md-persistence, ibm-mq-md-replyTo, ibm-mq-rest-mft-total-transfers
  • ibm-mq-md-expiry: unlimited
  • ibm-mq-md-messageId: 414d5120514d412020202020202020204c27165d04a98a25
  • ibm-mq-md-persistence: persistent

We can see from

  • the Access-Control-* headers we know this has validated for http://localhost:8884
  • the Access-Control-Expose-Headers we can see what headers will be accepted
  • ibm-mq-md-persistence:  persistence. For the returned messages, it was a persistent message

The web page used

<TITLE>Call a  mqweb rest API </TITLE>
  function local()
         method :'DELETE',
         headers: {
                    'Origin': 'https://localhost:888499'
                    , 'Access-Control-Request-Headers' : 'Content-Type'
                    ,'ibm-mq-rest-csrf-token' : '99'
    .then((response) => response.text() )
    .then(x => {  document.write("OUTPUT:"+x);    } )
    .catch(error => { console.log("Booo:" + error);});
</script> /head> <body> <button onclick="local()"">press me</button> </body> </html>


For information about “fetch(…,…)” see here.

For information about the “.then(…) ” see here.

3 thoughts on “Can evil websites get to your mqweb – understanding CORS

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s