The vulnerability stems from incomplete file validation in HTTP API handlers. Both REST/File and JSON:API modules contain file upload entry points (POST handlers) that were missing proper validation chaining. These methods are directly responsible for processing upload requests but failed to integrate with Drupal's full file validation stack, allowing dangerous files to bypass security checks. The high confidence comes from: 1) The vulnerability description specifically implicates these two modules' HTTP APIs 2) File upload resources in REST/JSON:API would logically be implemented through POST handler methods 3) The CWE mapping (CWE-434) directly correlates with file upload handling functions.