The simplistic understanding of the rulebook is "no cash contributions",
but it actually broader than that. For example:
"The Act and Commission regulations include a broad prohibition
on foreign national activity in connection with elections in the
United States. 52 U.S.C. § 30121 and generally, 11 CFR 110.20.
In general, foreign nationals are prohibited from the following activities:
•Making any contribution or donation of money or other thing of
value, or making any expenditure, independent expenditure, or
disbursement in connection with any federal, state or local
election in the United States;"
<
https://www.fec.gov/updates/foreign-nationals/>
Note the 'or other thing of value', as this includes the less
tangible lane of data/information, as has been amplified in
FEC rulings:
"The FEC also recently determined that a foreign national
could act as a volunteer in creating intellectual property
for a PAC where the PAC would assume joint ownership of the
intellectual property, even though, as a general matter,
foreign nationals may not make in-kind political contributions."
<
http://www.wileyrein.com/newsroom-newsletters-item-5297.html>
FWIW, what this loophole is saying is that a FN can't hand over
information directly to a candidate; it may become permissible if
it has been laundered through a PAC, although do note:
"PACs, campaign committees, and political party committees should
review their compliance policies and procedures to ensure that
they are properly screening for foreign contributions and
preventing foreign nationals from having any operational,
managerial, or decisionmaking role."
Which also illustrates that this is just the do's & don'ts from
the FN's side - - there's also rules for US Citizens in terms of
what they are allowed to accept from a FN.
-hh