Under both the ABA Model Rule and Louisiana Rule of Professional Conduct 1.9(c)(1), a lawyer may not use information relating to the representation of a former client to the former client’s disadvantage without informed consent, (or except as otherwise permitted or required by the Rules of Professional Conduct), unless the information has become “generally known”.  The ABA’s Standing Committee on Ethics and Professional Responsibility issued a Formal Opinion in order to clarify what is meant by “generally known”.  In sum:

“The ‘generally known’ exception to the duty of former-client confidentiality is limited. It applies (1) only to the use, and not the disclosure or revelation, of former-client information; and (2) only if the information has become (a) widely recognized by members of the public in the relevant geographic area; or (b) widely recognized in the former client’s industry, profession, or trade. Information is not ‘generally known’ simply because it has been discussed in open court, or is available in court records, in libraries, or in other public repositories of information.”

More specifically:

Rule 1.6(a) prohibits a lawyer from revealing information related to a client’s representation unless the client gives informed consent, the disclosure is impliedly authorized to carry out the representation, or the disclosure is permitted by Rule 1.6(b).  Rule 1.9 extends a lawyer’s duty of confidentiality to former clients.

Rules 1.9(a) and (b) govern situations in which a lawyer’s knowledge of a former client’s confidential information would create a conflict of interest in a subsequent representation.

Rule 1.9(c) separately regulates the use and disclosure of confidential information, regardless of whether or not a subsequent representation is involved.  Rule 1.9(c)(2) prevents a lawyer from revealing information relating to the representation, except as the Rules would permit or require.  Rule 1.9(c)(1), by contrast, provides that a lawyer shall not use information relating to a former client’s representation to the disadvantage of the former client, except as Rules would permit or require, or when the information has become generally known.

“Information may become widely recognized and thus generally known as a result of publicity through traditional media sources, such as newspapers, magazines, radio, or television; through publication on internet web sites; or through social media. With respect to category (b), information should be treated as generally known if it is announced, discussed, or identified in what reasonable members of the industry, profession, or trade would consider a leading print or online publication or other resource in the particular field. Information may be widely recognized within a former client’s industry, profession, or trade without being widely recognized by the public. For example, if a former client is in the insurance industry, information about the former client that is widely recognized by others in the insurance industry should be considered generally known within the meaning of Model Rule 1.9(c)(1) even if the public at large is unaware of the information.

“Unless information has become widely recognized by the public (for example by having achieved public notoriety), or within the former client’s industry, profession, or trade, the fact that the information may have been discussed in open court, or may be available in court records, in public libraries, or in other public repositories does not, standing alone, mean that the information is generally known for Model Rule 1.9(c)(1) purposes.19 Information that is publicly available is not necessarily generally known. Certainly, if information is publicly available but requires specialized knowledge or expertise to locate, it is not generally known within the meaning of Model Rule 1.9(c)(1).”

ABA Formal Opinion No. 479 (Dec. 15, 2017).