SSRF

Server-Side Request Forgery (SSRF) is a web security vulnerability that allows an attacker to coerce the server into making requests to arbitrary URLs.

Tools & Resources

• https://portswigger.net/web-security/ssrf/url-validation-bypass-cheat-sheet

• https://github.com/swisskyrepo/SSRFmap

• https://github.com/tarunkant/Gopherus

• https://app.interactsh.com/

• https://owasp.org/www-project-top-25-parameters

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Finding SSRF Vectors

If you are facing a BLIND SSRF, use Burp Collaborator, interact.sh or similar tools to gain a ping back If you are facing a target which validates your input, check out the PortSwigger Bypass CheatSheet

To identify potential SSRF vectors, locate GET or POST parameters used by the web application to access other resources via explicit or implicit external calls.

Other than the standard http:// and https:// schemes, it is sometimes possible to leverage SSRF with other URL schemes such as:

• file:// - Allows reading files from the local file system

• gopher:// - Allows sending arbitrary bytes to other services, potentially causing remote code execution

The OWASP top 25 vulnerable parameters list, as of the time of writing, contains:

?dest={target}
?redirect={target}
?uri={target}
?path={target}
?continue={target}
?url={target}
?window={target}
?next={target}
?data={target}
?reference={target}
?site={target}
?html={target}
?val={target}
?validate={target}
?domain={target}
?callback={target}
?return={target}
?page={target}
?feed={target}
?host={target}
?port={target}
?to={target}
?out={target}
?view={target}
?dir={target}

Using Gopher to send POST data

There is no way to send a POST request with the HTTP URL scheme. Instead, we can use the gopher URL scheme to send arbitrary bytes to a TCP socket. This protocol enables us to create a POST request by building the HTTP request ourselves.

Suppose you want to send a POST request to login.php with username sfoffo and password admin. To send a POST request with that data, you need to URL-Encode all special characters to construct a valid gopher URL. In particular, spaces (%20) and newlines (%0D%0A) must be URL-encoded.

One great tool to generate gopher-based SSRF payload is gopherus

After that, prefix the data with the gopher URL scheme, the target host and port, and an underscore, resulting in the following gopher URL:

gopher://example.sfoffo:80/_POST%20/login.php%20HTTP%2F1.1%0D%0AHost:%20example.sfoffo%0D%0AContent-Length:%2013%0D%0AContent-Type:%20application/x-www-form-urlencoded%0D%0A%0D%0Ausername%3Dsfoffo%26password%3Dadmin

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