Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-787 (Out-of-bounds Write) vulnerability exists in the code because the value of `cipher_len - 2` is assigned to `*out_len` without checking if `out_len` is a valid pointer or if it points to a buffer with enough space to hold the value of `cipher_len - 2`. If `out_len` is a null pointer or points to a buffer with insufficient space, writing `cipher_len - 2` to `*out_len` can result in an out-of-bounds write, which is CWE-787.