FWIW, this behavior is documented:

https://docs.python.org/3/using/cmdline.html#cmdoption-m
"As with the -c option, the current directory will be added to the start of sys.path."

With the -c option, at least you could easily remove the sys.path element yourself:

   python -c 'import sys; sys.path.remove(""); ...'

(This works, because sys is always a builtin module, so it won't be imported from cwd.)

I don't see any obvious way to make "python -m foo" secure in untrusted cwd, though.
The best I could come up with is:

   python -c 'import sys; sys.path.remove(""); import runpy; runpy._run_module_as_main("foo")'

which is quite insane.