Allow States and Victims to Fight Online Sex Trafficking Act of 2017 (Sec. 2) This bill expresses the sense of Congress that section 230 of the Communications Act of 1934 was not intended to provide legal protection to websites that unlawfully promote and facilitate prostitution and websites that facilitate traffickers in advertising the sale of unlawful sex acts with sex trafficking victims. Section 230 limits the legal liability of interactive computer service providers or users for content they publish that was created by others. (Sec. 3) The bill amends the federal criminal code to add a new section that imposes penalties—a fine, a prison term of up to 10 years, or both—on a person who, using a facility or means of interstate or foreign commerce, owns, manages, or operates an interactive computer service (or attempts or conspires to do so) to promote or facilitate the prostitution of another person. Additionally, it establishes enhanced penalties—a fine, a prison term of up to 25 years, or both—for a person who commits the offense in one of the following aggravating circumstances: (1) promotes or facilitates the prostitution of five or more persons, or (2) acts with reckless disregard that such conduct contributes to sex trafficking. A person injured by an aggravated offense may recover damages and attorneys' fees in a federal civil action. A court must order mandatory restitution, in addition to other criminal or civil penalties, for an aggravated offense in which a person acts with reckless disregard that such conduct contributes to sex trafficking. A defendant may assert, as an affirmative defense, that the promotion or facilitation of prostitution is legal in the jurisdiction where it was targeted. (Sec. 4) The bill amends the Communications Act of 1934 to declare that section 230 does not limit: (1) a federal civil claim for conduct that constitutes sex trafficking, (2) a federal criminal charge for conduct that constitutes sex trafficking, or (3) a state criminal charge for conduct that promotes or facilitates prostitution in violation of this bill. The amendments apply regardless of whether alleged conduct occurs before, on, or after this bill's enactment. (Sec. 5) The bill amends the federal criminal code to define a phrase related to the prohibition on sex trafficking. Currently, it a crime to knowingly benefit from participation in a venture that engages in sex trafficking. This bill defines "participation in a venture" to mean knowingly assisting, supporting, or facilitating a sex trafficking violation. (Sec. 6) A state may file a federal civil action to enforce federal sex trafficking violations. (Sec. 7) This section states that this bill does not limit federal or state civil actions or criminal prosecutions that are not preempted by section 230 of the Communications Act of 1934. (Sec. 8) The Government Accountability Office must report to Congress on information related to damages and mandatory restitution for aggravated offenses under this bill.