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.