Canvas's default feedback option allows students to see the correct answers for all questions both as soon as they submit the assessment and at any point after that. This default option makes your exam questions extremely insecure; it compromises the integrity of your exam questions for both the current semester and use in future semesters.
If you use Canvas™, you can prevent cheating by using Respondus® LockDown Browser® or Respondus Monitor®. Create a Canvas quiz that requires Respondus and use it to link to WebAssign. Your students will continue to be monitored by Respondus while they complete their exam in WebAssign.