| Author | Published |
|---|---|
| Jon Marien | February 13, 2026 |
Description
SSRF attacks often exploit trust relationships to escalate an attack from the vulnerable application and perform unauthorized actions. These trust relationships might exist in relation to the server, or in relation to other back-end systems within the same organization.
Attacks Against the Server
In an SSRF attack against the server, the attacker causes the application to make an HTTP request back to the server that is hosting the application, via its loopback network interface. This typically involves supplying a URL with a hostname like 127.0.0.1 (a reserved IP address that points to the loopback adapter) or localhost (a commonly used name for the same adapter).
For example, imagine a shopping application that lets the user view whether an item is in stock in a particular store. To provide the stock information, the application must query various back-end REST APIs. It does this by passing the URL to the relevant back-end API endpoint via a front-end HTTP request. When a user views the stock status for an item, their browser makes the following request:
POST /product/stock HTTP/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 118
stockApi=http://stock.weliketoshop.net:8080/product/stock/check%3FproductId%3D6%26storeId%3D1This causes the server to make a request to the specified URL, retrieve the stock status, and return this to the user.
In this example, an attacker can modify the request to specify a URL local to the server:
POST /product/stock HTTP/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 118
stockApi=http://localhost/adminThe server fetches the contents of the /admin URL and returns it to the user.
An attacker can visit the /admin URL, but the administrative functionality is normally only accessible to authenticated users. This means an attacker won’t see anything of interest. However, if the request to the /admin URL comes from the local machine, the normal access controls are bypassed. The application grants full access to the administrative functionality, because the request appears to originate from a trusted location.
Why do applications behave in this way, and implicitly trust requests that come from the local machine? This can arise for various reasons:
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The access control check might be implemented in a different component that sits in front of the application server. When a connection is made back to the server, the check is bypassed.
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For disaster recovery purposes, the application might allow administrative access without logging in, to any user coming from the local machine. This provides a way for an administrator to recover the system if they lose their credentials. This assumes that only a fully trusted user would come directly from the server.
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The administrative interface might listen on a different port number to the main application, and might not be reachable directly by users.
These kind of trust relationships, where requests originating from the local machine are handled differently than ordinary requests, often make SSRF into a critical vulnerability.
SSRF Attacks Against Back-End Systems
In some cases, the application server is able to interact with back-end systems that are not directly reachable by users. These systems often have non-routable private IP addresses. The back-end systems are normally protected by the network topology, so they often have a weaker security posture. In many cases, internal back-end systems contain sensitive functionality that can be accessed without authentication by anyone who is able to interact with the systems.
Imagine there is an administrative interface at the back-end URL https://192.168.0.68/admin. An attacker can submit the following request to exploit the SSRF vulnerability, and access the administrative interface:
POST /product/stock HTTP/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 118
stockApi=http://192.168.0.68/admin