The vulnerability stems from the poll_read implementation in DeflateOutput's AsyncRead trait. The advisory explicitly states that an uninitialized buffer was passed to AsyncRead::poll_read(), which is a user-provided trait function. The fix involved ensuring buffer initialization before passing it to poll_read, confirming the vulnerability was in this specific AsyncRead implementation. The struct name (DeflateOutput) and trait method (poll_read) are directly implicated in the vulnerability description and subsequent patch information.