Updated A hacker is claiming to have breached the FBI's website security, dumping staffers' email addresses and SHA1-scrambled password hashes with salts online. Meanwhile, the makers of the site's publishing software say it's all a hoax.

A miscreant using the handle @cyberzeist claims to have infiltrated Plone CMS used by FBI.gov, using a zero day flaw allegedly for sale on an unnamed dark web site.

The Register has contacted the FBI to confirm the allegations. The agency was not immediately available for comment – although a staffer said they were aware of the alleged break-in.

Cyberzeist claims to have conducted the hack last month and has posted to Twitter what they claim are screen captures showing the FBI patching against the vulnerability, which appeared to permit public access.

The hacker dumped the 155 purported stolen credentials to online clipboard pastebin, claiming a vulnerability resides in a Plone Python module. They said the websites of the European Union Agency for Network and Information Security and the National Intellectual Property Rights Coordination Center are also vulnerable.

Cyberzeist also claimed the FBI contacted the hacker requesting a copy of the stolen credentials, which they declined to provide.

The hacker reckoned the CMS was hosted on a virtual machine running a custom FreeBSD. They said they will tweet the zero day flaw once it is no longer for sale.

The FBI is a confirmed user of the Plone CMS, as is Amnesty International.

The latter organisation acknowledged a warning from Cyberzeist that its CMS was exposed.

The hacker claimed the FBI's site was offline on New Year's Eve, but none of the dozen WayBackMachine site captures of the FBI's homepage on 31 December and 1 January indicated it was unavailable. ®

Updated to add at 11.49 GMT, January 5

Plone's security team has been in touch since the publication of this article with this statement: "Some users on Twitter are circulating rumours about about a zero day vulnerability in Plone being used to attack the FBI.

"The Plone Security Team believes that these claims are a hoax. As Plone is open source software, it is easy to fake a screenshot showing Plone’s code. Causing source code to be leaked to the end user is a common form of attack against PHP applications, but as Python applications don’t use the cgi-bin model of execution it has never been a marker of an attack against a Python site.

"The hashes [the 'hacker'] claims to have released have several warning signs that point to them being fake. Firstly, the email addresses used match other FBI emails that have been harvested over the years that are publicly available. The password hashes and salts he claims to have found are not consistent with values generated by Plone, indicating they were bulk generated elsewhere."