Loose lips sink ships! Here're the US Army's employee guidelines for the keeping of secrets.
Conditions that could raise a security concern and may be disqualifying include:
Any service, whether compensated, volunteer, or employment with:
a. A foreign country;
b. Any foreign national;
c. A representative of any foreign interest;
d. Any foreign, domestic, or international organization or person engaged in analysis, discussion, or publication of material on intelligence, defense, foreign affairs, or protected technology.