The Chained Quiz plugin for WordPress is vulnerable to Insecure Direct Object Reference in version 1.3.4 and below via the quiz submission and completion mechanisms due to missing validation on a user controlled key. This makes it possible for unauthenticated attackers to hijack and modify other users' quiz attempts by manipulating the chained_completion_id cookie value, allowing them to alter quiz answers, scores, and results of any user. The vulnerability was partially patched in versions 1.3.4 and 1.3.5.
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