It's a plausibly-deniable backdoor on top of LD_LIBRARY_PATH, just to clarify. As in - oops, I added the colon at the end, or if the LD_LIBRARY_PATH was programmatically generated, a sloppy loop can clearly do that...and now you're stuck with unexpected behavior which, while not directly unsafe, can be coopted in a nefarious manner. The $PWD behavior is niche, unexpected and likely to be missed in an audit. If you have software that relies of LD_LIBRARY_PATH and you distribute the OS, I'd change ld.so to avoid the $PWD behavior.