Udall has been an opponent of the National Security Agency's (NSA) mass surveillance programs.[37] Following the revelation of the NSA's mass surveillance of Americans, Udall has been an advocate for reform.[38][39] Udall, along with Senators Ron Wyden and Rand Paul, published an Op-ed in the Los Angeles Times expressing their collective desire to "end the dragnet — and to affirm that we can keep our nation secure without trampling on and abandoning Americans' constitutional rights."[40] Udall has expressed his support for Edward Snowden to return to America to "make his case."[41] After reports that the Central Intelligence Agency improperly spied on U.S. Senators, Udall called for the resignation of Agency Director John O. Brennan.[42]

After the September 11 attacks, the one-term Representative Udall was one of 66 House members to vote against the Patriot Act.[3] In 2011, Udall voted against reauthorizing the Patriot Act.[43] In July 2014, Udall voted against the CISPA bill, a proposed law which would allow for the sharing of Internet traffic information between the U.S. government and technology and manufacturing companies, voicing his concerns that it "lacks adequate protections for the privacy rights."[44]

During the 2011 debate over the National Defense Authorization Act for Fiscal Year 2012 (NDAA), Udall introduced an amendment to end the practice of military detention of American citizens indefinitely and without trial. In response to the amendment's introduction, the Obama administration threatened to veto the bill.[45]The amendment was rejected by a vote of 60–38 (with 2 abstaining).[46] Udall subsequently voted for the Act in the joint session of Congress that passed it, and though he remained "extremely troubled" by the detainee provisions, he promised to "push Congress to conduct the maximum amount of oversight possible".[47]

Udall has supported PRISM, a clandestine anti-terrorism mass electronic surveillance data mining program launched in 2007 by the National Security Agency (NSA); however, he has also expressed support for introduction of measures to reform and limit the scope of the Patriot Act; 'The Patriot Act should be reformed so Americans' phone records do not get indiscriminately swept up in a federal government database.'[48]