I work for company A and depend on company B to pay social security, is this illegal?

This is theoretically not allowed and is a violation of the relevant regulations in fictitious labor relations. If you have established a labor relationship with Company A and also signed a labor contract, you can let the company to solve your social security problems, for example, by the company and the third-party human resources service agency to establish a cooperative relationship. This is just safer and secure and there is no possibility of breaking the law.

Social security is important, but some people sign a part-time contract with company a so that the company won't pay social security for its employees, and can appoint a company B to make social security payments for you. The fact that you have a real labor relationship with company a, but choose another company to pay your social security, shows that company a is very good to you and is a conscientious company. Because this is wrong, if you report it, Company a will be inspected by the relevant authorities and may be held liable. It is highly likely that there will be some mistakes during the audit. Therefore, the payment of social security is still to be carried out by the company, it is best not to hand over to other companies, to be responsible for their own, to be responsible for the company.

In fact, there is also a possibility that you signed a contract with company B, but company B dispatched your labor to company a. This way you would work for company a, but the social security is attached to company B. This is also possible, but labor dispatch is more unstable, and it is highly likely that you will be discriminated against when you look for a job again. You can change your job in company A, so you can transfer your social security relationship to company A. You don't need to be affiliated with company B. You can also become a regular employee of company A, and the treatment you enjoy is better.

The payment of social security is a very serious matter, and employees must pay attention to it. If the company did not pay for your social security, you can also labor arbitration, so you can also get some compensation, and then go to find a new job, be sure to ask the social security matter clearly.