{{Short description|HTTP status code}}

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File:Status code 451 example.png website to a client in Pennsylvania, 30 July 2018.{{cite press release|url=https://www.attorneygeneral.gov/taking-action/press-releases/attorney-general-shapiro-governor-wolf-state-police-successfully-block-access-to-3d-downloadable-guns-in-pennsylvania/|title=Attorney General Shapiro, Governor Wolf, State Police Successfully Block Access to 3D Downloadable Guns in Pennsylvania|date=July 29, 2018|publisher=Pennsylvania Office of Attorney General}}|250x250px]]

{{HTTP}}

In computer networking, 451 Unavailable For Legal Reasons is an HTTP status code used when the user requests a resource which cannot be served for legal reasons, such as a web page censored by a government. The number 451 is a reference to Ray Bradbury's 1953 dystopian novel Fahrenheit 451, in which books are outlawed.{{cite news|url=https://www.theguardian.com/books/2012/jun/22/ray-bradbury-internet-error-message-451|title=Call for Ray Bradbury to be honoured with internet error message|last=Flood|first=Alison|date=22 June 2012|newspaper=The Guardian|access-date=22 June 2012}} 451 provides more information than HTTP 403, which is often used for the same purpose.{{cite web|url=https://nakedsecurity.sophos.com/2013/08/19/http-error-code-451-unavailable-for-legal-reasons/|title=HTTP error code 451: "Unavailable For Legal Reasons"|last=Ducklin|first=Paul|date=19 August 2013|website=Naked Security|publisher=Sophos}} This status code is currently a proposed standard in {{IETF RFC|7725}}, which updated the IANA HTTP Status Codes Registry to include 451.{{cite web|url=https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml|title=Hypertext Transfer Protocol (HTTP) Status Code Registry|date=13 November 2024|access-date=3 February 2025|website=IANA|publisher=ICANN}}

Examples of situations where an HTTP 451 error code could be displayed include web pages deemed a danger to national security, or web pages deemed to violate copyright, privacy, blasphemy laws, or any other law or court order.

After introduction of the GDPR in the EEA it became common practice for websites located outside the EEA to serve HTTP 451 errors to EEA visitors instead of trying to comply with this new privacy law. For instance, many regional U.S. news sites no longer serve web browsers from the EU.{{cite news|author=Matt Burgess|title=The tyranny of GDPR popups and the websites failing to adapt|url=https://www.wired.co.uk/article/gdpr-cookies-eprivacy-regulation-popups|magazine=WIRED|date=2018-08-29|access-date=2018-10-01}}{{Cite web |url=https://www.niemanlab.org/2018/08/more-than-1000-u-s-news-sites-are-still-unavailable-in-europe-two-months-after-gdpr-took-effect/|title = More than 1,000 U.S. News sites are still unavailable in Europe, two months after GDPR took effect|website=www.niemanlab.org}}{{Cite web|url=https://www.engadget.com/2018-08-09-us-news-sites-unavailable-europe-gdpr.html|title=Major US news sites are still blocking Europeans due to GDPR|website=Engadget|accessdate=27 July 2023}}

The RFC is specific that a 451 response does not indicate whether the resource exists but requests for it have been blocked, if the resource has been removed for legal reasons and no longer exists, or even if the resource has never existed, but any discussion of its topic has been legally forbidden (see injunction).{{cite ietf|rfc=7725|title=An HTTP Status Code to Report Legal Obstacles|date=February 2016|section=3|sectionname=451 Unavailable For Legal Reasons|first=Tim|last=Bray|author-link=Tim Bray}} Some sites have previously returned HTTP 404 (missing) or similar if they are not legally permitted to disclose that the resource has been removed. It is used in the United Kingdom by some Internet service providers utilising the Internet Watch Foundation blacklist, returning a 404 message or another error message instead of showing a message indicating the site is blocked.{{cite web|quote=If the request is for the blocked content then the proxy server will return a 404 error page to the customer|url=https://wiki.openrightsgroup.org/wiki/Cleanfeed|title=Cleanfeed|website=ORG Wiki|publisher=Open Rights Group}}{{cite web|quote=TCP Reset is sent back to the customer instead of content.|url=https://www.theguardian.com/technology/blog/2008/dec/08/internet-censorship-wikipedia-diagram|title=How the IWF blacklist stops you seeing the Scorpions' album cover|website=Technology blog, The Guardian|first=Charles|last=Arthur|date=8 December 2008}}

The status code was formally proposed in 2013 by Tim Bray, following earlier informal proposals by Chris Applegate{{cite web|url=http://www.qwghlm.co.uk/2008/12/09/there-is-no-http-code-for-censorship/|title=There is no HTTP code for censorship|first=Chris|last=Applegate|website=qwghlm.co.uk|date=9 December 2008|access-date=23 December 2015|archive-date=19 April 2021|archive-url=https://web.archive.org/web/20210419112431/http://www.qwghlm.co.uk/2008/12/09/there-is-no-http-code-for-censorship/|url-status=dead}} in 2008 and Terence Eden{{cite web|last=Byrne|first=Michael|date=21 December 2015|title=The HTTP 451 Error Code for Censorship Is Now an Internet Standard|url=https://www.vice.com/en/article/the-http-451-error-code-for-censorship-is-now-an-internet-standard/|access-date=2020-07-03|website=Vice}} in 2012. It was approved by the IETF on 18 December 2015.{{cite web|url=https://www.mnot.net/blog/2015/12/18/451|title=Why 451?|first=Mark|last=Nottingham|website=mnot’s blog|date=18 December 2015|access-date=20 December 2015}} It was published as in the Proposed Standard {{IETF RFC|7725}} in February 2016.

HTTP 451 was mentioned by the BBC's From Our Own Correspondent programme, as an indication of the effects of sanctions on Sudan and the inability to access Airbnb, the App Store, or other Western web services.{{Cite AV media|title=From Our Own Correspondent|type=radio|publisher=BBC Radio 4|author=Sally Hayden|date=28 September 2017 |author-link=Sally Hayden |url=http://www.bbc.co.uk/programmes/b095tcx0}}