K.184.108.40.206, p5 permits strncpy_s to "clobber" characters in the destination buffer past the terminating null:
All elements following the terminating null character (if any) written by strncpy_s in the array of s1max characters pointed to by s1 take unspecified values when strncpy_s returns. 420)Footnote 420 explains that the intent is to allow implementations to copy characters from s2 to s1 while simultaneously checking if any of those characters are null. Such an approach might write a character to every element of s1 before discovering that the first element should be set to the null character.
This intent is to allow efficient implementations to make a single pass over the source sequence that simultaneously copies characters and checks the runtime constraints. (Otherwise two passes would be required, one to compute the length of the source sequence and another to copy it.)
It has been pointed out that the implementation latitude granted by this text goes too far, since the function only might need to write past the null after a constraint violation. Otherwise, when all runtime constraints are satisfied, the function stops copying characters after either the first null is encountered or all n characters have been copied.
Since the mention of unspecified values tends to raise security concerns about information leakage, and since permitting the implementations to modify the contents of the destination buffer past the terminating null on success serves no useful purpose, the requirements on the function can and should be tightened up.
The proposed corrigendum below tightens up the requirements on the function so as to leave intact the contents of the destination buffer past the terminating null on success, while allowing it to clobber its contents on runtime constraint violation.
Modify K.220.127.116.11, p5 as indicated below:
All elements following the terminating null character (if any) written by strncpy_s in the array of s1max characters pointed to by s1 take unspecified values when strncpy_s returns. 420)