In the previous section, we explored different techniques for detecting Server-Side Request Forgery (SSRF) based on the application's scenarios. Now, let's delve into the exploitation techniques associated with SSRF, which come into play once SSRF has been confirmed within the application. These techniques aim to assess the vulnerability's risk or impact. The SSRF exploitation process can be divided into two main parts.
Exploiting Application Internal infrastructure:
- Companies utilize various architectural patterns for running applications, including reverse proxies, load balancers, cache servers, and different routing methods. It is crucial to determine if an application is running on the same host. URL bypass techniques can be employed to invoke well-known URLs and Protocols like localhost (127.0.0.1) and observe the resulting responses. Malicious payloads can sometimes trigger error messages or responses that inadvertently expose internal IP addresses, providing valuable insights into the internal network.
- Another approach involves attempting connections to well-known ports on localhost or leaked IP addresses and analyzing the responses received on different ports.
- Application-specific information, such as the operating system, application server version, load balancer or reverse proxy software/platform, and vulnerable server-side library versions, can aid in targeting specific payloads for exploitation. It is also worthwhile to check if the application permits access to default sensitive files located in predefined locations. For example, on Windows systems, accessing critical files like win.ini, sysprep.inf, sysprep.xml, and NTLM hashes can be highly valuable. A comprehensive list of Windows files is available at https://github.com/soffensive/windowsblindread/blob/master/windows-files.txt. On Linux, an attacker may exfiltrate file:////etc/passwd hashes through SSRF.
- If the application server runs on Node.js, a protocol redirection attack can be attempted by redirecting from an attacker's HTTPS server endpoint to HTTP. For instance, using a URL like https://attackerserver.com/redirect.aspx?target=http://localhost/test.
- It is essential to identify all endpoints where the application responds with an 'access denied' (403) error. These URLs can then be used in SSRF to compare differences in responses.
- By identifying the platform or components used in an application, it becomes possible to exploit platform-specific vulnerabilities through SSRF. For example, if the application relies on WordPress, its admin or configuration internal URLs can be targeted. Platform-specific details can be found at https://github.com/assetnote/blind-ssrf-chains, which assists in exploiting Blind/Time-based SSRF.
- DNS Rebinding attack: This type of attack occurs when an attacker-controlled DNS server initially responds to a DNS query with a valid IP address with very low TTL value, but subsequently returns internal, local, or restricted IP addresses. The application may allow these restricted IP addresses in later requests while restricting them in the first request. DNS Rebinding attacks can be valuable when the application imposes domain/IP-level restrictions.
- Cloud metadata exploitation: Cloud metadata URLs operate on specific IP addresses and control the configuration of cloud infrastructures. These endpoints are typically accessible only from the local environment. If an application is hosted on a cloud infrastructure and is susceptible to SSRF, these endpoints can be exploited to gain access to the cloud machine.
Exploiting external network
- If an application makes backend API calls and an attacker is aware of the backend API domains, they can exploit SSRF to abuse the application by targeting those backend APIs. Since the application is already authenticated with the domain of the backend API, it provides an avenue for the attacker to manipulate the requests.
- Furthermore, an attacker can utilize a vulnerable application as a proxy to launch attacks on third-party servers. By leveraging SSRF, they can make requests to external servers through the compromised application, potentially bypassing security measures in place.
- SSRF can be combined with other vulnerabilities such as XSS (Cross-Site Scripting), XXE (XML External Entity), Open redirect, and Request Smuggling to amplify the impact and severity of the overall vulnerability. This combination of vulnerabilities can lead to more advanced attacks and potentially result in unauthorized access, data leakage, or server-side compromise.
In the next section of this blog, we will delve into various strategies and techniques for preventing and mitigating SSRF attacks in different application scenarios.
Article by Amish Shah