Ubuntu 26.04 LTS, codename Resolute in Ubuntu’s notice, ships PHP 8.5. The listed fix is php8.5 8.5.4-0ubuntu1.5 under standard support. Upstream’s fix is 8.5.12. As on 24.04, the Ubuntu package does not move the upstream version string to the php.net tag.
Install the revision, then restart php8.5-fpm or Apache. PHP 8.5 is also the branch where fatal_error_backtraces=0 is the temporary control for one confirmed producer. If the package install is delayed, set that and session.upload_progress.enabled=0, restart workers, and still install 8.5.4-0ubuntu1.5 or a newer Ubuntu revision.
Ubuntu 26.10 is a different release in the same notice, fixed in 8.5.9-0ubuntu2.1. Do not apply the 26.04 version string to 26.10 or the reverse.
This page treats 26.04 as released because Ubuntu’s 21 October 2026 advisory includes it as a supported release with a package version. If your mirror has not synced, wait for that version rather than pulling a development archive.