lib/thin/connection.rb in Thin web server before 1.2.4 relies on the X-Forwarded-For header to determine the IP address of the client, which allows remote attackers to spoof the IP address and hide activities via a modified X-Forwarded-For header.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.