If you publish your work online, you are already in the practice of using links to enhance your content. The Web's basic architecture relies heavily on the ability of webpages to link to other pages to allow natural navigation between related content. It is hard to imagine the smooth functioning (or even continued existence) of the Web without hypertext links that act as a reference system identifying and enabling quick access to other material. Fortunately, courts generally agree that linking to another website does not infringe the copyrights of that site, nor does it give rise to a likelihood of confusion necessary for a federal trademark infringement claim. However, different kinds of linking raise different legal issues, and the law is not entirely settled in all of these areas. Moreover, some linking activities may expose you to liability for contributory copyright infringement or trafficking in circumvention technology in violation of the Digital Millennium Copyright Act (DMCA).
Types of Links
Deep Linking: The most straightforward case is so-called "deep linking," which refers to placing a link on your site that leads to a particular page within another site (i.e., other than its homepage). No court has ever found that deep linking to another website constitutes copyright or trademark infringement. Therefore, you can link to other websites without serious concerns about legal liability for the link itself, with the exception of activities that might be contributory copyright infringement or trafficking in circumvention technology (discussed below).
Inline linking: Inline linking involves placing a line of HTML on your site that so that your webpage displays content directly from another site. We now commonly refer to this practice as embedding. For example, many bloggers embed videos from YouTube on their blogs to illustrate a point or initiate discussion. While there is some uncertainty on this point, a recent case from the Ninth Circuit Court of Appeals concluded that inline linking does not directly infringe copyright because no copy is made on the site providing the link; the link is just HTML code pointing to the image or other material. See Perfect 10, Inc. v. Google, Inc., 508 F.3d 1146 (2007). Other courts may or may not follow this reasoning. However, the Ninth Circuit's decision is consistent with the majority of copyright linking cases which have found that linking, whether simple, deep, or inline, does not give rise to liability for copyright infringement. For discussion of these cases, see The Internet Law Treatise. In addition, merely using an inline link should not create trademark liability, unless you do something affirmative to create the impression that you are somehow affiliated with or endorsed by the site to which you are linking. Thus, embedding media in your online work should not expose you to legal liability, with the possible exceptions discussed below.
Framing: Framing refers to the practice of dividing a web page into multiple sections that use HTML code to pull content from different sources. The law should treat framing much like inline linking for purposes of copyright infringement (see discussion immediately above), but no case has considered the issue of framing in the context of copyright law. Framing potentially raises trademark problems. Depending on how you design your page, a user might be confused into believing that all of the source material is yours. Some plaintiffs have sued websites for framing under trademark and related areas of law, but most cases have settled and the law remains unclear.
Linking to Infringing Works
The situation changes when you knowingly link to works that clearly infringe somebody's copyright, like pirated music files or video clips of commercially distributed movies and music videos. In this situation, you might be liable for what is known as "contributory copyright infringement." Contributory copyright infringement occurs by "intentionally inducing or encouraging direct infringement" of a copyrighted work. Metro-Goldwyn-Mayer Studios, Inc. v. Grokster, Ltd, 545 U.S. 913, (2005). As long as you do not know that a work infringes someone's copyright, then you cannot be held liable for contributory infringement for directing users to that work. On the other hand, it is not necessarily safe to simply claim that you "didn't know" when the circumstances make it clear the material you link to is infringing. Use your common sense. Fred vonLohman gives the following rules of thumb to help avoid contributory copyright infringement (specifically with reference to embedding videos):"(1) don't embed videos that are obviously infringing, and (2) consider removing embedded videos once you've been notified by a copyright owner that they are infringing." Relatedly, you may be able to protect yourself against claims of contributory copyright infringement by complying with the notice-and-takedown procedures of the DMCA. For details, see Notice-and-Takedown.
Linking to Circumvention Technology Proscribed by the DMCA
Linking also raises legal issues in connection with the anti-circumvention provisions of the DMCA. Section 1201 of the DMCA makes it illegal to traffic in technology that enables others to circumvent technological measures put in place by copyright holders to control access to or uses of their copyright work. 17 U.S.C. § 1201(a)(2), (b). "Trafficking" means making, selling, giving away, or otherwise offering these devices or tools to the public. You can "traffic" in circumvention tools simply by posting them on your website or linking to other websites that host them. For example, in 1999 a Norwegian teenager created a software program called "DeCSS" that allowed users to circumvent CSS, the encryption technology used by movie studios to stop unlicensed playing and copying of commercially distributed DVDs. A number of websites posted the source and object code for DeCSS on the Internet, and other websites linked to them. The Second Circuit held that hosting and linking to the DeCSS code violated the DMCA's anti-trafficking provisions, and that this application of the DMCA did not violate the First Amendment. See Universal City Studios, Inc. v. Corley, 273 F.3d 429 (2d Cir. 2001). This decision is controversial, and it is not clear that other courts would necessarily follow its reasoning. For example, one court has held that linking itself is not enough, and liability requires some more direct tie between the offending websites, such as receiving compensation in exchange for linking. See Comcast v. Hightech Elecs., Inc., 2004 WL 1718522 (N.D.Ill. July 29, 2004).
Given the uncertain state of the law, it is best not to knowingly link to sites hosting circumvention software. To be on the safe side, you should also remove user-generated content that links to such postings. This cautious approach may put websites that depend upon high levels of user interactivity in an uncomfortable position, as illustrated by the Digg.com user "revolt" in May 2007.