The vulnerability stemmed from two key points: 1) The Gist embed module explicitly allowed gist.github.com as a trusted host via SCRIPT_HOST constant. 2) This host was included in FilterScript's HOST_WHITE_LIST, disabling security sanitization for <script> tags from this domain. Attackers could craft malicious gist links that would execute arbitrary scripts due to this trust relationship. The fix removed both the Gist module reference and its whitelist entry, confirming these were the vulnerable components.