The New York Times have just published what they claim to be part of Donald Trump's tax return. Assuming their claim is true and that Trump and anyone who had legal representation rights for him did not authorize any release of the documents did the New York Times break any laws in either publishing or acquiring these documents? Or would it be safe to assume that whoever released these documents broke a law?
In Bartnicki v. Vopper 532 U.S. 514, SCOTUS ruled in a manner applicable to this case. In that case they assumed (did not decide, but operated from that initial position) that the information was illegally obtained, respondent knew that, but did not play any role in obtaining the information. They say that
In New York Times Co. v. United States, 403 U. S. 713, this Court upheld the press' right to publish information of great public concern obtained from documents stolen by a third party
and held that
privacy concerns give way when balanced against the interest in publishing matters of public importance. One of the costs associated with participation in public affairs is an attendant loss of privacy.
if a newspaper lawfully obtains truthful information about a matter of public significance, then state officials may not constitutionally punish publication of the information, absent a need to further a state interest of the highest order.
In other words, this is a matter where any restriction would be subject to strict scrutiny. Similar results are found in Boehner v. McDermott 332 F. Supp.2d 149. However, in Peavy v. WFAA-TV Inc, the TV station had a more active role in violating the law, so they were not protected by the First Amendment.
As for the initial source, it is said to be unknown (to NYT) where the documents came from, though they were verified by Jack Mitnick. There are federal laws that would prohibit government agents from disclosing a person's tax return, but there is no general law against disclosing a tax return. Without more information on the source, it is impossible to determine whether that act was legal.
NYT addresses this directly in an article titled "Is It Illegal to Publish a President’s Tax Returns?"
The article is in response to NBC's recent publication of another portion of the President's tax returns, with the reporter claiming he received them unsolicited in his mailbox, and broke no laws in obtaining them.
If it was a government employee who sent the documents and they were sent without authorization, that person may have broken the law. However, the NYT article says the law cited in this answer below the horizontal line "is almost certainly unconstitutional." They cite Bartnicki v. Vopper (US Supreme Court 2001):
the Supreme Court has said that journalists are free to publish truthful information on matters of public concern notwithstanding laws to the contrary as long as they did nothing illegal in obtaining the information.
See specifically 26 U.S. Code §7213(a)(3) - Unauthorized disclosure of information
(emphasis and link added):
It shall be unlawful for any person to whom any return or return information (as defined in section 6103(b)) is disclosed in a manner unauthorized by this title thereafter willfully to print or publish in any manner not provided by law any such return or return information. Any violation of this paragraph shall be a felony punishable by a fine in any amount not exceeding $5,000, or imprisonment of not more than 5 years, or both, together with the costs of prosecution.
Earlier answer based on legal citation without "almost certainly unconstitutional" article and Supreme Court citation:
It seems pretty clear that the Times "willfully" published the story, only several days after the executive editor publicly said he'd be willing to risk jail time to do so.
Yesterday, Robert Barnes published a blog post giving a "yes" answer to this question, and NPR published a piece suggesting that the likelihood of practical prosecution is extremely small, especially because Trump wouldn't want to have to answer potentially sensitive questions about his finances in any public venue under oath.
It probably comes down to how the documents were obtained, and the level of truth behind Trump's assertion that they were illegally obtained (e.g. if someone broke into an office and stole them from a locked filing cabinet, that would indicate laws were broken). The Times claims it obtained the documents just by opening mail that was properly addressed and delivered to it, and if pressed in court would probably claim the mail was unsolicited. Opponents would argue it was at least generally solicited.
Also of interest is subsection (4) of the same law quoted above:
It shall be unlawful for any person willfully to offer any item of material value in exchange for any return or return information (as defined in section 6103(b)) and to receive as a result of such solicitation any such return or return information. Any violation of this paragraph shall be a felony punishable by a fine in any amount not exceeding $5,000, or imprisonment of not more than 5 years, or both, together with the costs of prosecution.
Thus, if someone like Secretary Clinton or Reince Priebus were to successfully offer Trump (or anyone else with access to Trump's returns) any "item of material value" in exchange for his tax returns, they might be in violation of this section.
The constitutionality of restricting the press from publishing documents of public importance such as these is a different question. It may be that Times lawyers are banking on either a lack of prosecution or a first amendment defense, and/or are willing to accept the consequences in order to promote their preferred candidate and diminish the probability of electing a man they strongly feel should not be President, and/or are taking a page from Trump's own controversy-courting playbook to boost sales and pageviews. The boost they gained from this story was probably more than enough to cover a $5K+ fine. Finally, the negativity is so high in this election that there are probably folks on both sides who would rather be in jail while their preferred candidate occupies the White House than free under the main opponent's administration.