CISPA Helmholtz Center for Information Security Historically, TCP as specified in RFC 793 was threatened by the blind data injection attack because of the loose SEG.ACK value validation, where the SEG.ACK value of a TCP segment is considered valid as long as it does not acknowledge data ahead of what has been sent. RFC 5961 improved the input validation by shrinking the range of acceptable SEG.ACK values in a TCP segment. Later, RFC 9293 incorporated the updates proposed by RFC 5961 as a TCP stack implementation option. However, an endpoint that follows the RFC 9293 specifications can still accept a TCP segment containing an SEG.ACK value acknowledging data that the endpoint has never sent. This document specifies small modifications to the way TCP verifies incoming TCP segments' SEG.ACK value to prevent TCP from accepting such invalid SEG.ACK values.