Server-Side Request Forgery (SSRF)
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Server-Side Request Forgery (SSRF) is a web security vulnerability that allows an attacker to induce the server-side application to make requests to an unintended location.
In a typical SSRF attack, the attacker might cause the server to make a connection to internal-only services within the organization's infrastructure. In other cases, they may be able to force the server to connect to arbitrary external systems, potentially leaking sensitive data such as authorization credentials.
A successful SSRF attack can often result in unauthorized actions or access to data within the organization, either in the vulnerable application itself or on other back-end systems that the application can communicate with. In some situations, the SSRF vulnerability might allow an attacker to perform arbitrary command execution.
An SSRF exploit that causes connections to external third-party systems might result in malicious onward attacks that appear to originate from the organization hosting the vulnerable application.
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 itself, or in relation to other back-end systems within the same organization.
It is common to see applications containing SSRF behavior together with defenses aimed at preventing malicious exploitation. Often, these defenses can be circumvented.
Many SSRF vulnerabilities are relatively easy to spot, because the application's normal traffic involves request parameters containing full URLs. Other examples of SSRF are harder to locate:
Partial URLs in requests
Sometimes, an application places only a hostname or part of a URL path into request parameters. The value submitted is then incorporated server-side into a full URL that is requested. If the value is readily recognized as a hostname or URL path, then the potential attack surface might be obvious. However, exploitability as full SSRF might be limited since you do not control the entire URL that gets requested.
URLs within data formats
Some applications transmit data in formats whose specification allows the inclusion of URLs that might get requested by the data parser for the format. An obvious example of this is the XML data format, which has been widely used in web applications to transmit structured data from the client to the server. When an application accepts data in XML format and parses it, it might be vulnerable to XXE injection, and in turn be vulnerable to SSRF via XXE. We'll cover this in more detail when we look at XXE injection vulnerabilities.
SSRF via the Referer header
Some applications employ server-side analytics software that tracks visitors. This software often logs the Referer header in requests, since this is of particular interest for tracking incoming links. Often the analytics software will actually visit any third-party URL that appears in the Referer header. This is typically done to analyze the contents of referring sites, including the anchor text that is used in the incoming links. As a result, the Referer header often represents fruitful attack surface for SSRF vulnerabilities.
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