An issue was discovered in Ratpack before 1.7.5. Due to a misuse of the Netty library class DefaultHttpHeaders, there is no validation that headers lack HTTP control characters. Thus, if untrusted data is used to construct HTTP headers with Ratpack, HTTP Response Splitting can occur.

Affected: Maven: io.ratpack:ratpack-core

## CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')

Versions of Ratpack 0.9.1 through and including 1.7.4 are vulnerable to [HTTP Response Splitting](https://www.owasp.org/index.php/HTTP_Response_Splitting), if untrusted and unsanitized data is used to populate the headers of an HTTP response. An attacker can utilize this vulnerability to have the server issue any HTTP response they specify.

If your application uses arbitrary user input as the value of a response header it is vulnerable. If your application does not use arbitrary values as response header values, it is not vulnerable.

Previously, Ratpack did not validate response header values. Now, adding a header value that contains the header value termination characters (CRLF) produces a runtime exception. Since there is no mechanism for escaping or encoding the termination characters in a String, a runtime exception is necessary.

As potentially dangerous values now cause runtime exceptions, it is a good idea to continue to validate and sanitize any user-supplied values being used as response headers.

We would like to thank [Jonathan Leitschuh](https://github.com/JLLeitschuh) for reporting this vulnerability.

### Vulnerable Example

The following example server uses a query parameter value as a response header, without validating or sanitizing it. ```java RatpackServer startedServer = RatpackServer.start(server -> { server.handlers(chain -> chain.all(ctx -> {…