"Metadata" entered the lexicon thanks to the Edward Snowden revelations, prompting explainers on what the heck it is. The prefix "meta" is self-referential; metadata is data about data. To put it in a less confusing way, metadata is extraneous information about data.
You've got a document, and the content of the document -- the words -- are the data. But the document also contains other information about the data, like who authored the document and when it was created, and what parts of the text are underlined. This hidden data can present an entirely separate set of ethical problems.
An Ethical Minefield
By now, lawyers are familiar with the "clawback" provisions of discovery rules that allow inadvertently disclosed privileged information to get sent back as long as it isn't seen (or at least read thoroughly). Metadata is like a bomb that doesn't go off: Even if it is a problem, it might never be discovered. It just sits there, waiting.
Recognizing the hidden and sometimes inadvertent nature of metadata, the ABA and 18 states have drafted formal ethics opinions governing metadata. Unfortunately, they all cover varying degrees of conduct.
In Alabama and Arizona, for example, lawyers are prohibited from "mining" a document, which is the practice of deeply inspecting a document beyond just the text in order to extract any metadata it might contain. Alabama calls metadata mining "a knowing and deliberate attempt by the recipient attorney to acquire confidential and privileged information in order to obtain an unfair advantage against an opposing party."
Other states place no limitation on metadata mining, taking the position that opposing counsel's duty is to ensure that there's no metadata in files he or she sends. If the transmitting lawyer accidentally includes privileged information, that's his or her own fault for not being thorough and the data are fair game for the receiving lawyer, who doesn't have to alert opposing counsel that he found privileged information.
What Is Your Duty?
Most states don't place special ethical duties on lawyers when it comes to metadata, instead folding that into the normal duty to "take reasonable precautions to prevent the disclosure of confidential information, including metadata, to unintended recipients," in the language of the North Carolina State Bar Ethics Committee.
The ABA's rules on metadata recommend -- but don't require -- a lawyer to send files to opposing counsel in a format that removes metadata. This is a good idea, and good practice, too. As Lawyerist recommends, never send anyone a Microsoft Word document unless it's a draft being sent to a colleague for review. Word documents should always be converted to PDFs, which can be protected and secured and have all the Word document's metadata removed.