HTTP Desync Attack (Request Smuggling)

NOTE to respect the nondisclosure policy of the program, the actual vulnerable asset is not disclosed and the same has been referenced as my.vulnerable.com wherever necessary.

I had found an HTTP Desync (Request Smuggling) vulnerability affecting one of the cryptocurrency payment system (https://my.vulnerable.com) along with 121 other hosts utilizing the Distil Bot protection.

It was observed that the front end server is using the Content-Length while the back end server is making use of Transfer-Encoding header to determine the length of an HTTP request. This desynchronization in determining the length of request between the servers could be abused into escalating a Mass Account Takeover scenario by utilizing the POST parameters of urlencoded form to log requests of legit users.

Please NOTE that in an effort to provide the key proof of concept in the most basic and realistic form, the entire demonstration was carried out by utilizing two different machines operating at two different public IP addresses (one to depict victim and other for attacker) by making the use of Burp's native intruder instead of Turbo intruder. It should also be noted that the following form of testing would cause realtime disruption against the genuine users and so during the submission of this report, the entire assessment was carried out within the staging environment instead of production systems. Inbuilt Burp's intruder could be utilized in such cases since the staging environment would suffer relatively low traffic.

Now one of the consequences of the above vulnerability is that an attacker can leverage it to log mass user requests. Before proceeding, let's understand one of the basic functionality of https://my.vulnerable.com, which is if I make a POST request as -:

Then if I navigate at https://my.vulnerable.com/user/vendor/legal I can see the value of Business Address field being saved there (in this case it's blahblah).

This simply means, if I put the user_vendor_legal%5Baddress%5D param at the end of the POST body and make a request like this -:

Please NOTE that above is a single request (not multiple) that attacker would initiate. Observe the last param user_vendor_legal%5Baddress%5D at the end of the POST body. Means, after making this above ambiguous request, whosoever be the next legit user, making any GET or POST request, then their GET/POST request would get appended as the value of last user_vendor_legal%5Baddress%5D param of smuggled request. Afterwards, when the attacker will navigate to https://my.vulnerable.com/user/vendor/legal then he/she will see victim's HTTP request being logged as Business Address within his/her Legal Entity section -:

Above screenshots shows one of the logged HTTP POST request of any random user captured during the assessment. Observe the presence of token value of the user within their X-D-Token request header.

While testing the logging of requests, by making them saved at my Legal Entity section of https://my.vulnerable.com/user/vendor/legal, I observed that I was getting saved with user's request meant for many other hosts as well such as www.******.com, www.*****.be, api.******.com etc. All these belonged to other organizations different from the target program.

Then I observed that all above hosts including my.vulnerable.com are pointing to the same IP of 192.225.xxx.x which belongs to Distil Networks, a leading Bot manager (now acquired by Imperva). This simply meant that not only for my.vulnerable.com but we can steal/log user requests of all the applications whose back end web server lies behind the 192.225.xxx.x. Whois reverse IP lookup said that there are 121 other websites using 192.225.xxx.x.

Above screenshots shows one of the logged HTTP GET request of any random user captured during the assessment. Observe the Host of captured request is www.****.com which is different from my.vulnerable.com.

Similarly, I was able to log following requests in the form of POST data. Observe the presence of cookies in some these requests -:

Upon this I stumbled on a thought that if by making use of my.vulnerable.com, an attacker is capable of logging requests of other 121 websites behind the Distil, then whether the vice versa would also be possible ? Whether an attacker can make use of their input fields to entirely log the requests of my.vulnerable.com users.

So I randomly went to one of the www.******.com and found that it is also vulnerable to the exact same CL.TE based vulnerability by which the my.vulnerable.com is, which is a CL.TE dualchunk left alive HTTP Request Smuggling vulnerability, where two Transfer Encoding headers are specified in the attack request like this -:

Transfer-Encoding: chunked

Transfer-encoding: identity

See the following Turbo Intruder PoC screenshot for www.******.com -:

I Emailed the www.******.com support team regarding the vulnerability and informed them to fix it up before a bad guy could exploit it.

Now one of the challenges faced during the assessment was that I'm only able to log the first 255 characters of a user's HTTP request. This is because there is a server side input length validation for the user_vendor_legal%5Baddress%5D parameter which does NOT allow saving a string of more than 255 characters. To workaround this, there were two solutions -:

Solution #1

During my assessment, I didn't look around the Send, Exchanges etc functionalities. There may exist any POST field which would allow saving of more than 255 characters. Secondly, I registered a free account. There maybe other account types which may have more functionalities and so there might be any input field which would allow saving of more than 255 characters.

But instead of looking around the account type alternatives or utilizing more time in exploring the Send, Exchanges etc functionalities, I thought to submit this report anyways. Because even with the current scenario along with this restriction, exploitability of the attack is not much affected, and following damage can still be done which I think is enough to assess the severity of the reported issue -:

i) Logging users Password reset requests because it's a GET request of the form -:

And the relevant data (reset token) comes within the first 255 characters.

ii) Logging users PIN reset requests because it's a GET request of the form -:

And the relevant data (reset token) comes within the first 255 characters.

iii) Logging other GET requests containing sensitive Token values. For example, earlier while browsing at my.vulnerable.com, I observed this request -:

This token can also be stolen, as it comes within the first 255 characters.

iv) Similarly, while connecting or signing in with my.vulnerable.com from task.vulnerable.com, following request is generated -:

So these values of sso & sig parameters can also be stolen (where task.vulnerable.com is an another asset of the same organization).

v) And obviously as demonstrated earlier, sensitive header or cookie value from within the request can also be accessed, if it falls within the first 255 characters.

Solution #2

Let us assume that there is no input field in the my.vulnerable.com which allows more than 255 characters. But interestingly, as demonstrated in the previous section that if by making the use of my.vulnerable.com an attacker is capable of logging requests of other 121 websites, and on the other hand vice versa is also possible. Means, if there is any input field which allows saving of more than 255 characters in any of those other 121 hosts, then an attacker can make use of them to entirely log the requests of my.vulnerable.com users along with the full cookie header.

The interesting part of this vulnerability is, no user (victim) interaction is required. Any victim from anywhere will simply request https://my.vulnerable.com or any of those 121 websites but still their session would be hijacked. Another thing is that in this manner an attacker can log an enormous number of users HTTP requests on the web. All that an attacker needs to do is just make the above request exhibiting ambiguity, wait for a second or two, check your Legal Entity section at https://my.vulnerable.com/user/vendor/legal and every time you will see someone's cookies and tokens logged in there.