A recent policy that’s been causing headaches for employers that place H-1B skilled guestworkers at third-party work sites doesn’t have any legal backing, according to a lawsuit filed in federal district court in Washington.

The lawsuit filed Oct. 11 by the ITServe Alliance is the second to challenge the February policy from U.S. Citizenship and Immigration Services. The policy requires staffing companies that place their H-1B workers at third-party sites to list all sites the visa holder will work for...