hacktricks/pentesting-web/proxy-waf-protections-bypass.md

# Proxy / WAF Protections Bypass

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## Bypass Nginx ACL Rules with Pathname Manipulation <a href="#heading-pathname-manipulation-bypassing-reverse-proxies-and-load-balancers-security-rules" id="heading-pathname-manipulation-bypassing-reverse-proxies-and-load-balancers-security-rules"></a>

Techniques [from this research](https://rafa.hashnode.dev/exploiting-http-parsers-inconsistencies).

Nginx rule example:

```plaintext
location = /admin {
    deny all;
}

location = /admin/ {
    deny all;
}
```

In order to prevent bypasses Nginx performs path normalization before checking it. However, if the backend server performs a different normalization (removing characters that nginx doesn't remove) it might be possible to bypass this defense.

### **NodeJS - Express**

| 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**

| 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**

| 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**

Nginx FPM configuration:

```plaintext
location = /admin.php {
    deny all;
}

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

Nginx is configured to block access to `/admin.php` but it's possible to bypass this by accessing `/admin.php/index.php`.

### How to prevent

```plaintext
location ~* ^/admin {
    deny all;
}
```

## Bypass Mod Security Rules <a href="#heading-bypassing-aws-waf-acl" id="heading-bypassing-aws-waf-acl"></a>

### Path Confusion

[**In this post**](https://blog.sicuranext.com/modsecurity-path-confusion-bugs-bypass/) is explained that ModSecurity v3 (until 3.0.12), **improperly implemented the `REQUEST_FILENAME`** variable which was supposed to contain the accessed path (until the start of the parameters). This is because it performed an URL decode to get the path.\
Therefore, a request like `http://example.com/foo%3f';alert(1);foo=` in mod security will suppose that the path is just `/foo` because `%3f` is transformed into `?` ending the URL path, but actually the path that a server will receive will be `/foo%3f';alert(1);foo=`.

The variables `REQUEST_BASENAME` and `PATH_INFO` were also affected by this bug.

Something similar ocurred in version 2 of Mod Security that allowed to bypass a protection that prevented user accessing files with specific extensions related to backup files (such as `.bak`) simply by sending the dot URL encoded in `%2e`, for example: `https://example.com/backup%2ebak`.

## Bypass AWS WAF ACL <a href="#heading-bypassing-aws-waf-acl" id="heading-bypassing-aws-waf-acl"></a>

### Malformed Header

[This research](https://rafa.hashnode.dev/exploiting-http-parsers-inconsistencies) mentions that it was possible to bypass AWS WAF rules applied over HTTP headers by sending a "malformed" header that wasn't properly parsed by AWS but it was by the backend server.

For example, sending the following request with a SQL injection in the header X-Query:

```http
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
```

It was possible to bypass AWS WAF because it wouldn't understand that the next line is part of the value of the header while the NODEJS server did (this was fixed).

## References

* [https://rafa.hashnode.dev/exploiting-http-parsers-inconsistencies](https://rafa.hashnode.dev/exploiting-http-parsers-inconsistencies)
* [https://blog.sicuranext.com/modsecurity-path-confusion-bugs-bypass/](https://blog.sicuranext.com/modsecurity-path-confusion-bugs-bypass/)

<details>

<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>

Other ways to support HackTricks:

* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.

</details>