The use of a pen register order to track IP address in cyberattack investigation governed by third-party doctrine and not Carpenter. United States v. Soybel, 19-1936 (7th Cir. Sept. 8, 2021):
This appeal presents a constitutional issue of first impression for our circuit: whether the use of a pen register to identify IP addresses visited by a criminal suspect is a Fourth Amendment “search” that requires a warrant. We hold that it is not. IP pen registers are analogous in all material respects to the telephone pen registers that the Supreme Court upheld against a Fourth Amendment challenge in Smith v. Maryland, 442 U.S. 735 (1979). The connection between Soybel’s IP address and external IP addresses was routed through a third party—here, an internet-service provider. Soybel has no expectation of privacy in the captured routing information, any more than the numbers he might dial from a landline telephone.
Soybel insists that this case is governed not by Smith but by Carpenter v. United States, 138 S. Ct. 2206 (2018). We disagree. Carpenter concerned historical cell-site location information (“CSLI”). The warrantless acquisition of that type of data implicates unique privacy interests that are absent here. Historical CSLI provides a detailed record of a person’s past movements, which is made possible so long as he carries a cell phone. In contrast, the IP pen register had no ability to track Soybel’s past movements. And Carpenter is also distinguishable based on the extent to which a person voluntarily conveys IP-address information to third parties. Accordingly, though our reasoning differs from the district judge’s, we hold that the suppression motion was properly denied.