hacktricks/network-services-pentesting/pentesting-web/nginx.md

# Nginx

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{% hint style="danger" %}
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## Missing root location <a href="#missing-root-location" id="missing-root-location"></a>

```
server {
        root /etc/nginx;

        location /hello.txt {
                try_files $uri $uri/ =404;
                proxy_pass http://127.0.0.1:8080/;
        }
}
```

The root directive specifies the root folder for Nginx. In the above example, the root folder is `/etc/nginx` which means that we can reach files within that folder. The above configuration does not have a location for `/ (location / {...})`, only for `/hello.txt`. Because of this, the `root` directive will be globally set, meaning that requests to `/` will take you to the local path `/etc/nginx`.

A request as simple as `GET /nginx.conf` would reveal the contents of the Nginx configuration file stored in `/etc/nginx/nginx.conf`. If the root is set to `/etc`, a `GET` request to `/nginx/nginx.conf` would reveal the configuration file. In some cases it is possible to reach other configuration files, access-logs and even encrypted credentials for HTTP basic authentication.

## Alias LFI Misconfiguration <a href="#alias-lfi-misconfiguration" id="alias-lfi-misconfiguration"></a>

Inside the Nginx configuration look the "location" statements, if someone looks like:

```
location /imgs { 
    alias /path/images/ 
}
```

There is a LFI vulnerability because:

```
/imgs../flag.txt
```

Transforms to:

```
/path/images/../flag.txt
```

The correct configuration will be:

```
location /imgs/ { 
    alias /path/images/ 
}
```

**So, if you find some Nginx server you should check for this vulnerability. Also, you can discover it if you find that the files/directories brute force is behaving weird.**

More info: [https://www.acunetix.com/vulnerabilities/web/path-traversal-via-misconfigured-nginx-alias/](https://www.acunetix.com/vulnerabilities/web/path-traversal-via-misconfigured-nginx-alias/)

Accunetix tests:

```
alias../ => HTTP status code 403
alias.../ => HTTP status code 404
alias../../ => HTTP status code 403
alias../../../../../../../../../../../ => HTTP status code 400
alias../ => HTTP status code 403
```

## Unsafe variable use <a href="#unsafe-variable-use" id="unsafe-variable-use"></a>

An example of a vulnerable Nginx configuration is:

```
location / {
  return 302 https://example.com$uri;
}
```

The new line characters for HTTP requests are \r (Carriage Return) and \n (Line Feed). URL-encoding the new line characters results in the following representation of the characters `%0d%0a`. When these characters are included in a request like `http://localhost/%0d%0aDetectify:%20clrf` to a server with the misconfiguration, the server will respond with a new header named `Detectify` since the $uri variable contains the URL-decoded new line characters.

```
HTTP/1.1 302 Moved Temporarily
Server: nginx/1.19.3
Content-Type: text/html
Content-Length: 145
Connection: keep-alive
Location: https://example.com/
Detectify: clrf
```

Learn more about the risks of CRLF injection and response splitting at [https://blog.detectify.com/2019/06/14/http-response-splitting-exploitations-and-mitigations/](https://blog.detectify.com/2019/06/14/http-response-splitting-exploitations-and-mitigations/).

### Any variable

In some cases, user-supplied data can be treated as an Nginx variable. It’s unclear why this may be happening, but it’s not that uncommon or easy to test for as seen in this [H1 report](https://hackerone.com/reports/370094). If we search for the error message, we can see that it is found in the [SSI filter module](https://github.com/nginx/nginx/blob/2187586207e1465d289ae64cedc829719a048a39/src/http/modules/ngx\_http\_ssi\_filter\_module.c#L365), thus revealing that this is due to SSI.

One way to test for this is to set a referer header value:

```
$ curl -H ‘Referer: bar’ http://localhost/foo$http_referer | grep ‘foobar’
```

We scanned for this misconfiguration and found several instances where a user could print the value of Nginx variables. The number of found vulnerable instances has declined which could indicate that this was patched.

## Raw backend response reading

With Nginx’s `proxy_pass`, there’s the possibility to intercept errors and HTTP headers created by the backend. This is very useful if you want to hide internal error messages and headers so they are instead handled by Nginx. Nginx will automatically serve a custom error page if the backend answers with one. But what if Nginx does not understand that it’s an HTTP response?

If a client sends an invalid HTTP request to Nginx, that request will be forwarded as-is to the backend, and the backend will answer with its raw content. Then, Nginx won’t understand the invalid HTTP response and just forward it to the client. Imagine a uWSGI application like this:

```python
def application(environ, start_response):
   start_response('500 Error', [('Content-Type',
'text/html'),('Secret-Header','secret-info')])
   return [b"Secret info, should not be visible!"]
```

And with the following directives in Nginx:

```
http {
   error_page 500 /html/error.html;
   proxy_intercept_errors on;
   proxy_hide_header Secret-Header;
}
```

[proxy\_intercept\_errors](http://nginx.org/en/docs/http/ngx\_http\_proxy\_module.html#proxy\_intercept\_errors) will serve a custom response if the backend has a response status greater than 300. In our uWSGI application above, we will send a `500 Error` which would be intercepted by Nginx.

[proxy\_hide\_header](http://nginx.org/en/docs/http/ngx\_http\_proxy\_module.html#proxy\_hide\_header) is pretty much self explanatory; it will hide any specified HTTP header from the client.

If we send a normal `GET` request, Nginx will return:

```
HTTP/1.1 500 Internal Server Error
Server: nginx/1.10.3
Content-Type: text/html
Content-Length: 34
Connection: close
```

But if we send an invalid HTTP request, such as:

```
GET /? XTTP/1.1
Host: 127.0.0.1
Connection: close
```

We will get the following response:

```
XTTP/1.1 500 Error
Content-Type: text/html
Secret-Header: secret-info

Secret info, should not be visible!
```

## merge\_slashes set to off

The [merge\_slashes](http://nginx.org/en/docs/http/ngx\_http\_core\_module.html#merge\_slashes) directive is set to “on” by default which is a mechanism to compress two or more forward slashes into one, so `///` would become `/`. If Nginx is used as a reverse-proxy and the application that’s being proxied is vulnerable to local file inclusion, using extra slashes in the request could leave room for exploit it. This is described in detail by [Danny Robinson and Rotem Bar](https://medium.com/appsflyer/nginx-may-be-protecting-your-applications-from-traversal-attacks-without-you-even-knowing-b08f882fd43d).

We found 33 Nginx configuration files with `merge_slashes` set to “off”.

## default is not specified for map directive

It looks like common case when **`map` is used for some kind of authorization control**. Simplified example could look like:

```
http {
...
    map $uri $mappocallow {
        /map-poc/private 0;
        /map-poc/secret 0;
        /map-poc/public 1;
    }
...
}
```

```
server {
...
    location /map-poc {
        if ($mappocallow = 0) {return 403;}
        return 200 "Hello. It is private area: $mappocallow";
    }
...
}
```

[According to the manual](https://nginx.org/en/docs/http/ngx\_http\_map\_module.html):

> default value\
> sets the resulting value if the source value matches none of the specified variants. When default is not specified, the default\
> resulting value will be an empty string.

It is easy to forget about `default` value. So **malefactor can bypass this "authorization control"** simply accessing a **non existent case inside `/map-poc`** like `https://targethost.com/map-poc/another-private-area`.

## DNS Spoofing Nginx

According to this post: [http://blog.zorinaq.com/nginx-resol**ver-vulns/**](http://blog.zorinaq.com/nginx-resolver-vulns/) **It might be possible to spoof DNS records** to Nginx if you **know the DNS server Nginx** is using (and you can intercept somehow the communication, so this is **not valid if 127.0.0.1** is used) and the **domain it's asking**.

Nginx can specify a DNS server to use with:

```
resolver     8.8.8.8;
```

## `proxy_pass` and `internal` directives

The **`proxy_pass`** directive can be used to **redirect internally requests to other servers** internal or external.\
The **`internal`** directive is used to make it clear to Nginx that the **location can only be accessed internally**.

The use of these directives **isn't a vulnerability but you should check how are them configured**.

## proxy\_set\_header Upgrade & Connection

If the nginx server is configured to pass the Upgrade and Connection headers an [**h2c Smuggling attack**](../../pentesting-web/h2c-smuggling.md) could be performed to access protected/internal endpoints.

{% hint style="danger" %}
This vulnerability would allow an attacker to **stablish a direct connection with the `proxy_pass` endpoint** (`http://backend:9999` in this case) that whose content is not going to be checked by nginx.
{% endhint %}

Example of vulnerable configuration to steal `/flag` from [here](https://bishopfox.com/blog/h2c-smuggling-request):

```
server {
    listen       443 ssl;
    server_name  localhost;

    ssl_certificate       /usr/local/nginx/conf/cert.pem;
    ssl_certificate_key   /usr/local/nginx/conf/privkey.pem;

    location / {
     proxy_pass http://backend:9999;
     proxy_http_version 1.1;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection $http_connection;
    }

    location /flag {
     deny all;
    }
```

{% hint style="warning" %}
Note that even if the `proxy_pass` was pointing to a specific **path** such as `http://backend:9999/socket.io` the connection will be stablished with `http://backend:9999` so you can **contact any other path inside that internal endpoint. So it doesn't matter if a path is specified in the URL of proxy\_pass.**
{% endhint %}

## Try it yourself

Detectify has created a GitHub repository where you can use Docker to set up your own vulnerable Nginx test server with some of the misconfigurations discussed in this article and try finding them yourself!

[https://github.com/detectify/vulnerable-nginx](https://github.com/detectify/vulnerable-nginx)

## Static Analyzer tools

### [GIXY](https://github.com/yandex/gixy)

Gixy is a tool to analyze Nginx configuration. The main goal of Gixy is to prevent security misconfiguration and automate flaw detection.

## References

* [**https://blog.detectify.com/2020/11/10/common-nginx-misconfigurations/**](https://blog.detectify.com/2020/11/10/common-nginx-misconfigurations/)
* [**http://blog.zorinaq.com/nginx-resolver-vulns/**](http://blog.zorinaq.com/nginx-resolver-vulns/)
* [**https://github.com/yandex/gixy/issues/115**](https://github.com/yandex/gixy/issues/115)

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{% embed url="https://securityboat.in/contact-us" %}
{% endhint %}

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