How I could Steal Your Google Bug Hunter Account with Two Clicks in IE

This post is another evidence to show how difficult to parse a URL correctly. IE has URL parsing problem, this idea is originated from Sergey Bobrov. And then successfully exploited by filedescriptor in github. Credits to them. Now, this post show how we can defeat the login mechanism that Google Bug Hunter Dashboard site uses,  and steal your access token by two clicks in Internet Explorer.

Google has a vulnerability reward program, that is hosted with domain name to serve the dashboard for bug hunters. The site URL is So for any bug hunters/pentester/hacker who see this, this would usually mean potential weak spots, because session/cookies cannot be shared cross domain, i.e. sessions in cannot share with sessions in . There must be some login mechanism implemented to log user in from to If we can find a weak spot in the process, we can own it.

Login Mechanism

The dashboard for bughunter look like this.


Once we click Sign In, the traffic happens under the hood look like this.


  2. Which will then redirect to[token]
  3. Which will then redirect to[state token]
  4. Final destination is, and in step 4, the cookies/session of user will be set finally.

In this flow, the most important part of auth token here is state parameter. If we are able to steal the state parameter, then we are able to takeover the researcher’s account with the token. You can see in this flow, there is so many redirect is going on under the hood, what if we can make final destination in step 4 to be our owned server, this would essentially mean leaking researcher’s token in the referrer header. So, by using this method, I have tried to change


It didn’t work, the state token returned in step 3 won’t be valid if I entered any other domain other than, this frustrated me of cause. And this is the moment where I choose to submit the report to Google Security Team and later found out my report is invalid because the state token returned in step 3 is invalid to login bug hunter dashboard, if i entered different domain name. It doesn’t sounds good because I made a mistake to submit the report without actually confirming the bug, so I decided to fix my mistake, by keep playing with this mechanism.

URL Encoding

Upon further investigation, I found out that the url decoding is inconsistent in these three domain.

  1. In Step 1, won’t perform any url decode in the redirect
  2. In Step 2, would perform at most 2 times in url decode, which mean, it will turn %25%36%32 to b, as well as %62 to b. Two times at most.
  3. In Step 3, would perform at most 1 time in url decode, which mean, it will turn %25%36%32 to %62 only. Nothing more, and this is the main subject of this post. Have a look on the traffic of step 3


Can you see the weird behaviour? become in Step 3. And this would work perfectly in Firefox and Chrome, as they will decode the Location Header correctly, and redirect user to, but not the same case for IE. IE in Windows 7,8.1 will redirect user to If you want to don’t know why, you must have missed the link I provided in the beginning of the article. Since no one claimed, attacker could have claimed it and at least launch Open Redirect attack. But Google Security doesn’t like Open Redirect, no reward/fix will be implemented, if it is just Open Redirect.

How can we exploit this behaviour? First thing come to my mind is Referer Leak, at first, I thought by using the payload

Attack Payload:

would be enough for Referer Leakage, as I thought 302 redirect in Location Header would atomically include the originated Link in the Referer Header, but I was wrong, 302 redirect won’t include its original site in Referer Header and this is the time I chose to update my submitted report to Google. Again, this is bad, because at that moment I do not know 302 redirect would not include the referer header if its just 302 redirect. I am telling everyone about this as a reminder to all of us, especially myself, confirm the vulnerability before submitting.

I got this report wrong twice, I have to be right in one last time. So I have to keep digging about this flaw, it has to be exploitable. Finally, this page was presented to me when use my attack payload .


Wait, why would this page appear? It turns out whenever I have more than 1 logged in Google Account, this page would prompt me to choose one of them before logging in . How could this help us in this case, it turns out it helps a lot! Like I said, 302 won’t include referer header of originated site. But this is not the same case if any user action is involved, like clicking a link/button/image. If user clicked on something, and got redirected to other site, the referer remains.

If user visit Site A and Click on Site B which then Redirect to Site C which then Redirect to Site D. All of the redirect request to B,C,D would include site A URL in Referer Header.

This is very important here, because this mean the state token of will be leaked to for IE user in Windows 7,8.1. And how useful is the state token? It turns out it is everything you need in order to have access to BugHunter Dashboard.

I made a few tests, and turns out anyone with the state token in like the image I provided above, could takeover the user’s bughunter account. I checked the source code of the “Account Choosing” page.


The important state token is loaded in the page of “Account Choosing”, this mean, if I could know the URL of Account Choosing Page, then I could takeover the victim’s bughunter account.

A quick recap of the vulnerability here.

  1. Visit
  2. If user has more than one logged in google account, it will present a page look[state token]
  3. User click on the Continue button, for fire fox and chrome user, they will be redirected to the correct site But for IE user, they will be redirected to, which is unclaimed. The referer header would be include the url in previous point.
  4. Owner of will now be presented with the url of[state token] in referer header, and he can now replay the visit, by using this link, and log in as the victim.

This is it, a small url decoding problem could cause you an account compromise. Upon further investigation, any combo + * is vulnerable to this attack as well.

When Server Side Request Forgery combine with Cross Site Scripting


Hi, this is my third post about my Bug Hunting Journey. This finding is an interesting one, as it demonstrates how to chain different type of bugs in order to completely take over victim’s account. This finding allows me to take over your Yahoo account with just an click of a link. Sounds scary? Read on.

GET /d/xxxxxxxx/xxxxxxx?appid=YMail&crumb=kPun5CrTk1R&wssid=&clientId=mailsearch&timezone=Asia%2FHong_Kong&allowGoogleDocs=1&ymreqid=a5551380-75ce-13d6-1c16-c00001016500&

Take some time to read the request, and try to find something that is unusual in the request.

Not sure what you think it is unusual, but the parameter url look quite fun to me. Because it seems I can control the destination of the url, by changing to, then I could make a Server Side Request Forgery.

So I did the test, make it points to my own server, I set up nc -lnvp 80 to listen for the traffic. The traffic went through and my Yahoo Cookies Header is included in the GET request from the Yahoo Server.

Let’s do a quick recap, first, set up a nc -lnvp 80 in my server, lets say my server ip address is

Then, make a get request to my server, by changing to http%3A123.123.123.123

Finally, a request from Yahoo Owned server has made a request to my, and included all of my yahoo header in the request.

So I can further confirm it is vulnerable to internal scanning as well, by changing the url to, it failed, so i used another trick, http://::80, this is ipv6, and bypassed the check, and then I was able to perform internal port scanning and other behavior as well.

Remember I said in the title that I could own your yahoo account by only one click? It is not a click bait, it is true, because when this attack combines with XSS, that recently discovered by klikki, then he can do more than just viewing the email of victim, he can actually takeover their account by forcing them to make the SSRF request to attacker owned server and get a copy of their cookies.

Bypassing Google Email Domain Check to Deliver Spam Email on Google’s Behalf

Welcome to this blog again, this is my second write up about my Bug Hunting Journey, if you missed the first write up about how I could buy anything for free in Yahoo, please go here to have a look.

Now, this write up is about how I use Google’s service to send email with any domain names, and with arbitrary title/content. You can safely say that I can hijack Google’s email service to send any email to anyone I want. Sounds severe right? I agree with you, but Google Security Team treat that as a low severity bug as they believe anyone could have send spam email, so this finding does not qualify for reward. Ok, they got a point, the process to find it is fun anyway. Let’s see how I manage to hijack their email service.

Google FireBase

Google’s FIrebase is a super useful service that could save developers tons of effort. It provides authentication service that could integrate with any platform you want, like web, iOS, android. So developers do not need to build an authentication mechanism from scratch.

There is some functions that firebase must provide in order to have a competent service, sending password reset email is one of those must have functions. From the screenshot below, we can see it allows us to user our own domain name and own template to deliver the password reset email. Firebase is nice, nice enough to allow developers to specify their own domain name to deliver the email. Before using custom domain name, we have to prove to Google that we actually own the domain.

Email Template
Email Template


But here is the problem, I can specify any domain name without proving I am the owner. Actually this is a classic example of how developers would make careless mistake, when we look at the front end, we cannot tamper with the domain name. However if we capture the traffic and look at the request body, and carefully change the domain name, we can actually bypass the check and use any domain name instantly.

In the request body, I notice there is a {“email”:””}. So I change that to {“email”:””}, and trigger the password reset. The final result is this.

Hijacked Domain Name
Hijacked Domain Name

So I documented all these finding and send it to Google Security Team and have it fixed within a week.

Of cause is very obviously a scam domain name, but imagine I used, with nicely crafted message, it is not difficult to deliver spam email effectively by abusing this bug.

Abusing Multistage Logic Flaw to Buy Anything for Free at

Welcome, this is my first write-up of my Bug Bounty Journey that explain how I earn $3000 reward from Yahoo, hope you enjoy.


I have started my Bug Bounty Journey after finishing my OSCP lab session in June, I started to check on first, it didn’t went well for a month, but after I read the book “The Web Application Hackers Handbook”, I suddenly realize there might be a multistage logic flaw during Yahoo Deals checkout process, so I decided to take a look and fuzz around and see what things could go wrong in the process. And I finally come up with a bug that allows me to buy arbitrary items with arbitrary price on and

Let start digging

Yahoo! Deals, Shop, Auctions, are quite prmoinent service in Hong Kong, they provide a platform that allows user to trade items with Yahoo Payment system. I started to buy an item at, I followed along the process and monitor the traffic by Burp Suite.

Buying Page in Yahoo
Buying Page in Yahoo
Entering Delivery Content in Yahoo
Entering Delivery Content in Yahoo

When I checked on the checkout traffic, here is things start interesting. Here is the request made when I pressed “Check Out Now”

Checkout in Cart Burp Interception
Checkout in Cart Burp Interception

If we look closely, you can see the orderInfo has something important, paymentAmount is sent in plain text, in this case, it is 450 HKD. I found this interesting and decided to manipulate this amount through out the process. Fortunately, Yahoo system keeps validating the paymentAmount, when I change the paymentAmount to other values, it would return this page.

Checkout Error when trying to manipulate the paymentAmount
Checkout Error when trying to manipulate the paymentAmount

I keep experimenting and could not find a way to bypass its validation on price, until I finally reached the part where I would redirect to a third party payment system to finish my credit card payment. Due to its sensitive nature, I am not going to include the name of third party payment system here.

Here is the Burp Interception of the redirection

Details of Redirection of Third Party Payment Checkout Process
Details of Redirection of Third Party Payment Checkout Process

As we look closely to the Referrer and  POST parameter “amount”, we can see the amount is once again transmitted in plain text, so I intuitively manipulate the amount to “5” in both places, and the result is a surprise.

A Payment Page that is stating to accept HKD 5
A Payment Page that is stating to accept HKD 5

From this point onward, everything went smooth, I entered my credit card info, pass the mobile phone SMS authentication, and after finishing, I was presented with this page.

Successful Checkout Page in Yahoo
Successful Checkout Page in Yahoo

I have a final check on my credit card statement, the payment amount was indeed 5 HKD.

The End

I believe the consequences of this bug is self-explanatory, I am going to pass on explaining how would this affect Yahoo. This kind of vulnerability is a classic case of lacking validation on client supplied data. And this could lead to disastrous result like bypassing login mechanism, transferring money to an unauthenticated account, and in this case, purchasing items with client supplied price.

Thank you very much for reading my first bug bounty write-up, I hope you enjoyed it.

If you are one of the bug bounty hunter and live in Hong Kong, please let me know and contact me asap, cause I am desparately to find a local friend that could share ideas in Hunting Bugs.

Feel free to leave your thoughts below, and there will be a few more write-ups coming in next few weeks.


Jul 15th 2016: Reported to Yahoo

Jul 15th 2016: Receive Responsefrom junot that asked to confirm credit card was indeed charged for HKD 5

Jul 16th 2016: Provide credit card statement to Yahoo

Jul 20th 2016: Receive Response from junot that they have fixed the bug and ask to confirm it has been fixed

Jul 27th 2016: Receive response from junot about receiving Bug Bounty award $3000USD