CryptoLib's Crypto_TC_Prep_AAD Has Buffer Overflow Due to Integer Underflow

CryptoLib provides a software-only solution using the CCSDS Space Data Link Security Protocol - Extended Procedures (SDLS-EP) to secure communications between a spacecraft running the core Flight System (cFS) and a ground station. A critical heap buffer overflow vulnerability was identified in the Crypto_TC_Prep_AAD function of CryptoLib versions 1.3.3 and prior. This vulnerability allows an attacker to trigger a Denial of Service (DoS) or potentially execute arbitrary code (RCE) by providing a maliciously crafted telecommand (TC) frame that causes an unsigned integer underflow. The vulnerability lies in the function Crypto_TC_Prep_AAD, specifically during the computation of tc_mac_start_index. The affected code incorrectly calculates the MAC start index without ensuring it remains within the bounds of the ingest buffer. When tc_mac_start_index underflows due to an incorrect length calculation, the function attempts to access an out-of-bounds memory location, leading to a segmentation fault. The vulnerability is still present in the repository as of commit d3cc420ace96d02a5b7e83d88cbd2e48010d5723.