Endoribonuclease that cleaves single-stranded RNAs at 5' of uridylates and releases a product with a 2',3'-cyclic phosphate at the 3'-end. The UU and GU sites are more efficiently cleaved than CU and AU sites. ribonucleotidyl-uridine-RNA = a 3'-end 2',3'-cyclophospho-ribonucleotide-RNA + a 5'-end dephospho-uridine-RNA Monomer. Belongs to the ENDOU family.