Many foreign companies use outsourcing workers and do not pay social security. Is this legal?

What you said should belong to labor dispatch, which is allowed, but the labor dispatch unit should send employees to participate in social security and sign contracts for them, which is stipulated in the special chapter of labor contract law.

It is naturally a violation of the social insurance law not to insure the parties. Workers can report to the local labor security supervision brigade, but they should report to the labor dispatch company, not to the actual employer.