The only in-house legal team more stressed out than Yahoo Inc.’s right now might be the group at Verizon Communications Inc., as both companies decide what to do about the largest corporate data breach in history—and about how it will affect their pending merger.

“They have a few things going on right now,” says Timothy Newman, an associate at Haynes and Boone in Dallas who has managed corporate cyberbreach investigations and their legal fallout. “They’re likely continuing to investigate, and to coordinate with law enforcement, public relations professionals and outside counsel. They’re determining what to disclose, how much and to whom, and laying the groundwork to move forward.”