http4k is a functional toolkit for Kotlin HTTP applications. Prior to version 6.50.0.0, there is a potential XXE (XML External Entity Injection) vulnerability when http4k handling malicious XML contents within requests, which might allow attackers to read local sensitive information on server, trigger Server-side Request Forgery and even execute code under some circumstances. The original fix shipped in v5.41.0.0 / v4.50.0.0 closed the documented external-entity attack class (SSRF, local-file disclosure, code execution) by setting `ACCESS_EXTERNAL_DTD=""`, `ACCESS_EXTERNAL_SCHEMA=""`, and `isExpandEntityReferences=false` on the default `DocumentBuilderFactory`. A residual gap remained: the parser still accepted documents containing `<!DOCTYPE>` declarations even though external entity resolution was blocked. This left open billion-laughs-style internal entity expansion DoS attacks against any application using `Body.xml()` or `Document.asXmlDocument()` on untrusted XML. v6.50.0.0 closes this residual by adding `disallow-doctype-decl=true` and `FEATURE_SECURE_PROCESSING=true` to `defaultXmlParsingConfig`. Any document containing a `<!DOCTYPE>` is now rejected at parse time.

Affected: Maven: org.http4k:http4k-format-xml

### Summary _Short summary of the problem. Make the impact and severity as clear as possible. For example: An unsafe deserialization vulnerability allows any unauthenticated user to execute arbitrary code on the server._

There is a potential XXE(XML External Entity Injection) vulnerability when http4k handling malicious XML contents within requests, which might allow attackers to read local sensitive information on server, trigger Server-side Request Forgery and even execute code under some circumstances.

### Details _Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer._ https://github.com/http4k/http4k/blob/25696dff2d90206cc1da42f42a1a8dbcdbcdf18c/core/format/xml/src/main/kotlin/org/http4k/format/Xml.kt#L42-L46 XML contents is parsed with DocumentBuilder without security settings on or external entity enabled

### PoC _Complete instructions, including specific configuration details, to reproduce the vulnerability._ #### Example Vulnerable server code: ``` import org.http4k.core.* import org.http4k.format.Xml.xml import org.http4k.server.Netty import org.http4k.server.asServer import org.w3c.dom.Document

fun main() {

val xmlLens = Body.xml().toLens()

// Create an HTTP handler val app: HttpHandler = { request -> try { // Parse the incoming XML payload to a Document object val xmlDocument: Document = xmlLens(request)

// Extract root element name or other…