Server-Side Request Forgery (SSRF) is a vulnerability where an attacker manipulates a server into making HTTP requests to arbitrary destinations, including internal services and resources that are not directly accessible from the internet. The server acts as a proxy for the attacker, using its trusted network position to reach targets behind firewalls, load balancers, and network segmentation boundaries.
How It Works
SSRF occurs when an application accepts a URL or network address as user input and fetches the resource on the server side. Common features susceptible to SSRF include URL preview generators, webhook handlers, file importers that accept URLs, PDF generators that render remote content, and any functionality that fetches data from user-specified locations. An attacker replaces the intended URL with an internal address like http://169.254.169.254/latest/meta-data/ to access cloud instance metadata, or http://localhost:8080/admin to reach internal administration panels.
Bypassing SSRF protections is a well-developed area of security research. Applications that blocklist internal IP ranges can be circumvented using DNS rebinding, IPv6 representations, decimal IP notation, URL redirects, or alternative URL schemes. For example, http://0x7f000001 is the hexadecimal representation of 127.0.0.1, and many filters do not recognize it as a local address. DNS rebinding attacks use a domain that alternates between resolving to an allowed external IP and a forbidden internal IP, passing the validation check but ultimately reaching the internal target.
The impact of SSRF ranges from information disclosure to complete infrastructure compromise. In cloud environments, accessing the metadata endpoint can yield temporary credentials that provide full access to the cloud account's resources. Internal services that lack authentication because they are assumed to be unreachable from the internet become fully accessible. When SSRF allows arbitrary protocol handlers or response data to be returned to the attacker, the impact escalates further.
Why It Matters
SSRF has become increasingly critical as organizations migrate to cloud infrastructure where metadata services provide powerful credentials. A single SSRF vulnerability can bridge the gap between external and internal networks, bypassing years of network security investment. Security assessments test every feature that fetches external resources for SSRF because the vulnerability's impact in modern cloud environments is consistently severe.
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