Looks like this could be a nasty one- be prepared to update your PHP installations (or ask your host to). If previous incidents are anything to go by, exploits will be in the wild not long after the patches are released.

The PHP security team would like to give advance notice of an upcoming security release.
It is scheduled for Wednesday, 21 October 2026 at 14:00 UTC.
It addresses two critical vulnerabilities, both rated CVSS v4.0 9.2.
All PHP users are encouraged to prepare now so they can upgrade as soon as the release is available.
We reserve the right to release earlier should details of either vulnerability become public, or should evidence of active exploitation appear in the wild.
Full details, affected versions, and the fixed versions will be published with the release.

We put a summary of what we know (so far) together for our customers, but it might be helpful here too- https://prostack.uk/blog/critical-php-security-update-october-2026/