Campaigners say the avatar of a 21-year-old researcher was sexually assaulted in Meta's virtual reality platform Horizon Worlds.
Corporate accountability group SumOfUs, for whom the researcher works, says it shows Meta needs better plans to mitigate harms in the metaverse.
Meta's annual shareholder meeting takes place on Wednesday.
The company told the BBC there were safety tools in Horizon Worlds to help people have a "positive experience".
Noting that Meta had not yet seen the full SumOfUs report, a spokesperson told the BBC the firm wanted everyone in Horizon Worlds to access the safety procedures "and help us investigate and take action".
Horizon Worlds is currently only available to users in the US and Canada. Avatars in the platform have a simplified, cartoonish appearance.
SumOfUs says that virtual assaults can be intensely traumatic.
"It still counts, it still has a real impact on users," the group's campaigns director Vicky Wyatt told the BBC Tech Tent podcast.