SSRF

SSRF attack

SSRF attacks against the server itself

api=http://localhost/admin

SSRF attacks against other back-end systems

api=http://192.168.0.68/admin

Protocols

If you can control the protocol you can change it.

file://, sftp://, gopher://, etc.

Tip: with gopher in some case it is possibile to get a shell. E.g. interacting with mysql, redis PostgreSQL, etc. https://github.com/tarunkant/Gopherus

Blind

Fifficult to exploit because you will be able to exploit only well-known vulnerabilities.

Detection: Out-of-band techniques

Bypass SSRF defenses

Blacklist-based

  • Alternative IP representation of 127.0.0.1, such as 2130706433, 017700000001, or 127.1

  • Obfuscating blocked strings using URL encoding or Double encoding

  • Case variation admin -> aDmIn

  • Registering your own domain name that resolves to 127.0.0.1. You can use spoofed.burpcollaborator.net for this purpose

  • Providing a URL that you control, which subsequently redirects to the target URL. Try using different redirect codes, as well as different protocols for the target URL. For example, switching from an http to https

Whitelist-based

  • Add credentials: https://expected-host:fakepassword@evil-host

  • https://expected-host.evil-host

  • URL encode and URL double encode

Bypassing SSRF filters via open redirection

  1. Identify endpoint with open redirect

https://website.com/login?redirect=/my-account

  1. Have the server execute a request that performs a redirect

# Original
api=http://website.com/product?productId=6
# Exploit
api=http://website.com/login?redirect=http://192.168.0.68/admin

Finding hidden attack surface for SSRF

  • Partial URLs in requests

  • URLs within data formats (e.g. in XML)

  • SSRF via the Referer header (Some applications use server-side analytics software to tracks visitors)

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