A lawyer shall not reveal information relating to the representation of a client unless the client gives informed consent, the disclosure is impliedly authorized in order to carry out the representation or the disclosure is permitted by paragraph (b).
While usually only followed if there is a possibility of harming the client, the letter of the rule basically prevents sharing anything ever at all.
Comment 4 also says that it prohibits revealing information that do not reveal protected information but could reasonably lead to the discovery of such information.
A lawyer may reveal information relating to the representation of a client to the extent the lawyer reasonably believes necessary:
to prevent reasonably certain death or substantial bodily harm;
to prevent the client from committing a crime or fraud that is reasonably certain to result in substantial injury to the financial interests or property of another and in furtherance of which the client has used or is using the lawyer's services;
to prevent, mitigate or rectify substantial injury to the financial interests or property of another that is reasonably certain to result or has resulted from the client's commission of a crime or fraud in furtherance of which the client has used the lawyer's services;
to secure legal advice about the lawyer's compliance with these Rules;
to establish a claim or defense on behalf of the lawyer in a controversy between the lawyer and the client, to establish a defense to a criminal charge or civil claim against the lawyer based upon conduct in which the client was involved, or to respond to allegations in any proceeding concerning the lawyer's representation of the client;
to comply with other law or a court order; or
to detect and resolve conflicts of interest arising from the lawyer’s change of employment or from changes in the composition or ownership of a firm, but only if the revealed information would not compromise the attorney-client privilege or otherwise prejudice the client.
A lawyer shall make reasonable efforts to prevent the inadvertent or unauthorized disclosure of, or unauthorized access to, information relating to the representation of a client.
If a client is currently engaged in a crime or fraud and using a lawyer's assistance therefor, Rule 4.1 requires disclosure, trumping Rule 1.6.