I am working on a project in the area of IT security where we are analyzing a proprietary embedded system. While researching the system, we came across a confidential data sheet that was made publically available by a third party without the consent of the manufacturer of the system. The datasheet is marked confidential and usually only given out after an NDA is signed. It describes an outdated version of the system we are analyzing, but is partially relevant, as some parts of the system have not changed since then.
Is it ethical / acceptable to use and reference this resource in a paper?
"Related Work": this and this question ask about citing documents that aren't widely available (while my document can be found by anyone using Google, and none of the involved researchers have signed any NDAs).
Edit: To add some additional information that I added spread over a few comments:
The datasheet in question describes the security protocols used by an older version of a popular embedded security chip, current versions of which are being used in payment and access control systems
The manufacturer is aware that the data sheet has been leaked and is not happy about it, but has not gotten the datasheet removed from the third-party servers where it is located for the past two years. The reasons for this are unclear
The data is confidential (as in private-sector NDA-confidential), but not classified in a government sense.
The company has already stated that they would prefer if we did not publish based on this document, but that they would not be taking any legal action if we did, as long as all information was factually correct.