When Pete Thomas spots a meeting on his calendar that he doesn't remember scheduling, the startup executive launches a mini investigation. He searches his inbox for the sender's name, and if that fails, he tries LinkedIn. If Thomas still comes up empty or senses tomfoolery, he'll scan the invitation's contents for odd URLs or other red flags before hitting delete. He's received several in recent months that failed those checks.

"It's a distraction," said Thomas, CTO of Kapwork, a fintech firm in Los Angeles. "You always have to look for context."

As if email and text message scams weren't enough, workers are contending with a growing wave of phishing attempts arriving through their calendars. In August, calendar-led attacks surged more than tenfold from July, according to a recent study from email security firm Sublime, while a midyear report from Microsoft found that such attacks nearly quadrupled in June from May.

Cybersecurity pros say calendars offer attackers advantages over more traditional avenues, and that AI is helping to make the schemes more convincing. They also warn that AI agents could give attackers a leg up by accepting fraudulent invitations on workers' behalf.

"You suddenly have new and interesting opportunities because of these personal agents," said Hugh Thompson, program chair of RSAC, an annual cybersecurity conference, and former chief technology officer of software firm Symantec.

The goal of most calendar scammers, digital safety experts say, is to lure victims to phishing sites or deploy social engineering attacks that trick people into revealing sensitive information. The tactic is especially problematic for workers in jobs that rely on regularly meeting with strangers online, such as sales, recruiting, consulting, and other client-facing roles.

Aman Gairola, cofounder of Indian design agency Say Design, thought he'd landed a potential client in June when someone booked a meeting on his calendar through his company's website. The invitation included a link to what appeared to be a Google Drive folder containing a project brief. Gairola said he clicked the link and landed on a page that looked like Google Drive, but something felt off. He checked the URL, saw the site was fake, and stopped short of opening any files.

"If I have to rate this, how convincing it is, I would rate it a 9.5 out of 10," said Gairola, adding that Say Design is considering removing the option to book intro calls directly through its website and switching to a form-based approach. "It has definitely added a layer of scrutiny to the invites that we get."

Calendar invitations are attractive vehicles for scammers in part because they create a sense of urgency, said Mark Beare, an executive at cybersecurity firm Malwarebytes. Workers are common targets since many are used to receiving meeting invitations from colleagues, clients, and partners, he added.

Another allure is that calendar invites are processed differently from standard email attachments, according to the Microsoft report. Malicious links can land on users' calendars without requiring them to explicitly open an attachment, the software giant said.

Calendar invitation scams often take one of two forms. In one, an invitation includes a compromised meeting link, or a form designed to collect credentials or prompt someone to download malware, said Ron Kerbs, CEO of Kidas, which develops tools to detect scams and deepfakes in voice calls.

In the other, he said, a victim joins what appears to be a legit video call, only to encounter a scammer posing as someone they have reason to trust, such as an employee at a real company. The impersonator might use the name of an actual staffer, but a fake email address, as well as AI-generated audio or video to make the ruse more convincing, he said.

"They're using deepfake technology to look the same as the guy from IT," Kerbs said.

Sometimes, the sender really is someone you know. Scores of Jenn Drummond's contacts recently received an unexpected calendar invitation from her email address, warning that they had been served with a court summons. The invitation, which was sent after Drummond's email account was hacked while she was boarding a flight, linked to a fake legal notice requesting their financial information.

Drummond, a world-record-setting mountaineer and keynote speaker, said that even if the majority of recipients knew it was a scam, she still feels embarrassed that some people might think she's suing them or that she was hacked in the first place.

"What are people going to think? I'm not trustworthy — and your brand is what you build trust on," Drummond said. "I was sick to my stomach."

Cybersecurity veterans say there are limits to what people can reasonably expect to catch. Matthew Woodyard, director of threat intelligence at identity and access management company Okta, said that the burden of securing users mostly falls on the businesses that operate the services.

"The strongest mitigation isn't user education about 'spotting a fake invite,' since the invites increasingly look identical to legitimate ones," he said. "It's giving users something that can't be phished in the first place."

Wooyard said that passkeys can help solve the problem. Because they're created for a specific page, they only work on the authentic website, rather than a mock-up. That removes the need for people to make a judgment call when logging in, Woodyard said.

"Until phishing-resistant factors like passkeys become the default rather than an opt-in afterthought, phishing attacks will keep working," Woodyard said.