What plant is that?

Our Mystery Plant this month is a non-native, low scrambling perennial herb reaching 2 to 3 feet. The leaves are entire, 1/2 to 1-1/8 inches long by about 1/8-inch wide, shiny dark green, hairy underneath, in whorls of 6-8 (to 12) leaves. Tiny yellow flowers with 4-petals and stamens are arranged in dense clusters in July and August. Introduced from Europe where it has a long history of use in cheesemaking and dyeing, this plant is considered a noxious weed in parts of North America.