Bug 2522497 (CVE-2026-78408)

Summary:	CVE-2026-78408 util-linux: util-linux: nsenter --join-cgroup leaks root cgroup migration authority
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security <prodsec-ir-bot>
Status:	NEW ---	QA Contact:
Severity:	high	Docs Contact:
Priority:	high
Version:	unspecified	CC:	akhatavk, aos-team-art-private, asdas, crizzo, dpaolell, gtanzill, jbuscemi, jdelft, jupierce, kshier, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, security-response-team, sghai, sidsharm, stcannon, suppawar, teagle, vlaad, watson-tool-maintainers, yguenane
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
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Doc Text:	The nsenter --join-cgroup option opens the target cgroup.procs file as root and leaves that file descriptor open across later namespace and credential changes and across execve(). Because the kernel checks later cgroup migrations using the credentials from the original open, a program run in an attacker-controlled target can inherit root's ability to move host processes between cgroups. After a privileged operator uses --join-cgroup against that target, an unprivileged user can migrate and terminate unrelated root processes.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-08-24 18:22:25 UTC

nsenter --join-cgroup (sys-utils/nsenter.c) opens the target cgroup v2 cgroup.procs while still running as root, writes nsenter's own PID to join that cgroup, and then leaves the descriptor open across later namespace/credential transitions and execve(). The FD is neither closed nor marked O_CLOEXEC. Linux performs later cgroup migration checks using the credentials captured in file->f_cred at open time, so a program executed inside an attacker-controlled target inherits root's cgroup-migration authority even after dropping to an unprivileged UID with no capabilities. A privileged operator must invoke the documented --join-cgroup operation against the attacker-controlled target (nsenter is not SUID on typical distros). After the FD is inherited, an unprivileged process can migrate an unrelated host root PID into an attacker-owned systemd user-service cgroup and terminate it via cgroup.kill. This is not an autonomous LPE. Introduced by b40650b71a74 (2023-06-28); first released in util-linux v2.40. Affects v2.40 through v2.42.2 and current master (confirmed on Debian 13 util-linux 2.41-5). No upstream fix as of 2026-08-24. Reported upstream by 4drez. GHSA-55fx-f4gg-cfhj.