| Name |
Server Side Request Forgery |
|
| Likelyhood of attack |
Typical severity |
| High |
High |
|
| Summary |
An adversary exploits improper input validation by submitting maliciously crafted input to a target application running on a server, with the goal of forcing the server to make a request either to itself, to web services running in the server’s internal network, or to external third parties. If successful, the adversary’s request will be made with the server’s privilege level, bypassing its authentication controls. This ultimately allows the adversary to access sensitive data, execute commands on the server’s network, and make external requests with the stolen identity of the server. Server Side Request Forgery attacks differ from Cross Site Request Forgery attacks in that they target the server itself, whereas CSRF attacks exploit an insecure user authentication mechanism to perform unauthorized actions on the user's behalf. |
| Prerequisites |
Server must be running a web application that processes HTTP requests. |
| Execution Flow |
| Step |
Phase |
Description |
Techniques |
| 1 |
Explore |
[Find target application] Find target web application that accepts a user input and retrieves data from the server |
|
| 2 |
Experiment |
[Examine existing application requests] Examine HTTP/GET requests to view the URL query format. Adversaries test to see if this type of attack is possible through weaknesses in an application's protection to Server Side Request Forgery |
- Attempt manipulating the URL to retrieve an error response/code from the server to determine if URL/request validation is done.
- Use a list of XSS probe strings to specify as parameters to known URLs. If possible, use probe strings with unique identifiers.
- Create a GET request with a common server file path such as /etc/passwd as a parameter and examine output.
|
| 3 |
Exploit |
[Malicious request] Adversary crafts a malicious URL request that assumes the privilege level of the server to query internal or external network services and sends the request to the application |
|
|
| Solutions | Handling incoming requests securely is the first line of action to mitigate this vulnerability. This can be done through URL validation. Further down the process flow, examining the response and verifying that it is as expected before sending would be another way to secure the server. Allowlist the DNS name or IP address of every service the web application is required to access is another effective security measure. This ensures the server cannot make external requests to arbitrary services. Requiring authentication for local services adds another layer of security between the adversary and internal services running on the server. By enforcing local authentication, an adversary will not gain access to all internal services only with access to the server. Enforce the usage of relevant URL schemas. By limiting requests be made only through HTTP or HTTPS, for example, attacks made through insecure schemas such as file://, ftp://, etc. can be prevented. |
| Related Weaknesses |
|
CWE ID
|
Description
|
| CWE-20 |
Improper Input Validation |
| CWE-918 |
Server-Side Request Forgery (SSRF) |
|
| Related CAPECS |
|
CAPEC ID
|
Description
|
| CAPEC-115 |
An attacker gains access to application, service, or device with the privileges of an authorized or privileged user by evading or circumventing an authentication mechanism. The attacker is therefore able to access protected data without authentication ever having taken place. |
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