Under strict scrutiny, a public library’s use of filtering software is permissible only if it is narrowly tailored to further a compelling government interest and no less restrictive alternative would serve that interest.  We acknowledge that use of filtering software furthers public libraries’ legitimate interests in preventing patrons from accessing visual depictions of obscenity, child pornography, or in the case of minors, material harmful to minors.  Moreover, use of filters also helps prevent patrons from being unwillingly exposed to patently offensive, sexually explicit content on the Internet.

We are sympathetic to the position of the government, believing that it would be desirable if there were a means to ensure that public library patrons could share in the informational bonanza of the Internet while being insulated from materials that meet CIPA’s definitions, that is, visual depictions that are obscene, child pornography, or in the case of minors, harmful to minors.  Unfortunately this outcome, devoutly to be wished, is not available in this less than best of all possible worlds.  No category definition used by the blocking programs is identical to the legal definitions of obscenity, child pornography, or material harmful to minors, and, at all events, filtering programs fail to block access to a substantial amount of content on the Internet that falls into the categories defined by CIPA.  As will appear, we credit the testimony of plaintiffs’ expert Dr. Geoffrey Nunberg that the blocking software is (at least for the foreseeable future) incapable of effectively blocking the majority of materials in the categories defined by CIPA without overblocking a substantial amount of materials.  Nunberg’s analysis was supported by extensive record evidence.  As noted above, this inability to prevent both substantial amounts of underblocking and overblocking stems from several sources, including limitations on the technology that software filtering companies use to gather and review Web pages, limitations on resources for human review of Web pages, and the necessary error that results from human review processes.

Because the filtering software mandated by CIPA will block access to substantial amounts of constitutionally protected speech whose suppression serves no legitimate government interest, we are persuaded that a public library’s use of software filters is not narrowly tailored to further any of these interests.  Moreover, less restrictive alternatives exist that further the government’s legitimate interest in preventing the dissemination of obscenity, child pornography, and material harmful to minors, and in preventing patrons from being unwillingly exposed to patently offensive, sexually explicit content.  To prevent patrons from accessing visual depictions that are obscene and child pornography, public libraries may enforce Internet use policies that make clear to patrons that the library’s Internet terminals