Nuclease that is responsible for viral evasion of host cGAS-STING innate immunity. Cleaves 2',3'-cGAMP which is produced by host cGAS following recognition of cytosolic DNA and blocks the subsequent 2',3'-cGAMP-mediated activation of TMEM173/STING, which normally spreads to adjacent cells and activates the interferon and NF-kappa-B immune responses. 2',3'-cGAMP + H2O = Gp(2'-5')Ap(3') + H(+) Homodimer. The substrate binding site is formed by the N-terminus of a monomer and the C-terminus of the opposite monomer. The sequence shown comes from the strain chorioallantois vaccinia virus Ankara (CVA), which is the ancestor strain of the modified vaccinia virus Ankara (MVA). Belongs to the poxin family. The truncated poxin of the Modified Virus Ankara (MVA) is probably non-functional, since part of its active site is missing.