Listener stall via forged retry backoff header

Improper input validation in Spring for Apache Kafka allows remote authenticated users to stall Kafka listeners with forged retry headers. The retry topic infrastructure treats an out-of-range retry_topic-attempts value as a valid sequence position and accepts retry_topic_backoff-timestamp without bounds checking. The attacker must be able to produce records to a consumed Kafka topic; a crafted timestamp can pause the listener arbitrarily long, while a crafted attempt count disrupts retry routing.