A “critical flaw” in macOS Sonoma’s firewall system can break Mullvad VPN and potentially other software dependent on packet filtering, according to the VPN provider.

On Wednesday, Mullvad warned users about the problem, which has been popping up in the early versions of macOS Sonoma ahead of its full release on Sept. 26. “The macOS 14 Sonoma betas and release candidate contain a bug that causes the firewall to not filter traffic correctly. As a result, our app does not work,” the Swedish VPN provider said.

However, Mullvad is signaling the bug can undermine any app, such as other VPN services, that rely on macOS firewall settings. Due to the problem, “firewall rules do not get applied properly to network traffic, and traffic that is not supposed to be allowed is allowed,” the company said, noting the bug can result in leaks when local network sharing is enabled.

“We deem this to be a critical flaw in the firewall, anyone relying on PF [packet filter] filtering, or apps using it in the background on their macOS devices should be cautious about upgrading to macOS 14,” Mullvad added.

Apple did not immediately respond to a request for comment. In the meantime, Mullvad says it reported the flaw to the company after macOS Sonoma beta 6 was released last month. “Unfortunately the bug is still present in later macOS 14 betas and the release candidate,” it said.

The company has been trying to implement a workaround, but according to Mullvad: “We believe the firewall bugs must be fixed by Apple.” In response, Mullvad is recommending its users avoid upgrading to macOS Sonoma until the problem is fixed.

We’ve reached out to Mullvad and other VPN providers for more information, and we’ll update the story if we hear back.