runc v1.4.0-rc.3 -- "その日、人類は思い出した。" This release contains fixes for three high-severity security vulnerabilities in runc (CVE-2025-31133, CVE-2025-52565, and CVE-2025-52881). All three vulnerabilities ultimately allow (through different methods) for full container breakouts by bypassing runc's restrictions for writing to arbitrary /proc files. Security: * CVE-2025-31133 exploits an issue with how masked paths are implemented in runc. When masking files, runc will bind-mount the container's /dev/null inode on top of the file. However, if an attacker can replace /dev/null with a symlink to some other procfs file, runc will instead bind-mount the symlink target read-write. This issue affected all known runc versions. <https://github.com/opencontainers/runc/security/advisories/GHSA-9493-h29p-rfm2> * CVE-2025-52565 is very similar in concept and application to CVE-2025-31133, except that it exploits a flaw in /dev/console bind-mounts. When creating the /dev/console bind-mount (to /dev/pts/$n), if an attacker replaces /dev/pts/$n with a symlink then runc will bind-mount the symlink target over /dev/console. This issue affected all versions of runc >= 1.0.0-rc3. <https://github.com/opencontainers/runc/security/advisories/GHSA-qw9x-cqr3-wc7r> * CVE-2025-52881 is a more sophisticated variant of CVE-2019-19921, which was a flaw that allowed an attacker to trick runc into writing the LSM process labels for a container process into a dummy tmpfs file and thus not apply the correct LSM labels to the container process. The mitigation we applied for CVE-2019-19921 was fairly limited and effectively only caused runc to verify that when we write LSM labels that those labels are actual procfs files. This issue affects all known runc versions. <https://github.com/opencontainers/runc/security/advisories/GHSA-cgrx-mc8f-2prm> Fixed: * Switched to (*CPUSet).Fill rather than our hacky optimisation when resetting the CPU affinity of runc. (#4926, #4927) * Correctly close child fds during (*setns).start if an error occurs. (#4930, #4936) Thanks to the following contributors for making this release possible: * Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp> * Aleksa Sarai <cyphar@cyphar.com> * Kir Kolyshkin <kolyshkin@gmail.com> * Lei Wang <ssst0n3@gmail.com> * Li Fubang <lifubang@acmcoder.com> * Rodrigo Campos <rodrigoca@microsoft.com> * Tõnis Tiigi <tonistiigi@gmail.com> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>