Software cracks are illegal under the Digital Millennium Copyright Act (17 U.S.C. 1201), except under certain specific conditions, according to the Cornell Legal Information Institute. A crack is, by definition, a mechanism for the circumvention of a copyright protection system, which the Act prohibits.
The DMCA prohibits the manufacture, import and sale of any product, service or device that is primarily designed for the purpose of circumventing a technological measure that controls access to a copyrighted work, states the Cornell Legal Information Institute. The Act also prohibits the use of such tools for the purposes of copyright infringement.
The DMCA provides exemptions for nonprofit libraries and archives, security researchers and law enforcement agencies, according to the Cornell Legal Information Institute. These exemptions only apply for the specific purposes of research and the preservation of copyrighted works for posterity.