CVE-2026-15779: Samba-winbind: samba: pam_winbind mkhomedir chowns critical system paths without validation

A flaw was found in samba's pamwinbind module. When pamwinbind.conf sets mkhomedir = yes, the module's pammkhomedir()/pamchownhomedir() functions call chown() on the target account's home directory (pwdir, taken directly from getpwnam()) without validating that the path is not a sensitive system directory. On RHEL, several stock system accounts (e.g. nobody, sssd, nscd) ship with / as their home directory by default in /etc/passwd. Opening a PAM session for such an account causes the root filesystem (/) to be chown'd to that account's uid/gid, breaking ownership checks relied on by sshd (StrictModes), sudo, and package tooling. Reproduced directly in an isolated sandbox using the exact affected NVRs: the trigger is not limited to root explicitly running su - nobody (the originally reported PoC) -- a non-root user holding only a narrow, common sudo delegation (e.g. 'someuser ALL=(nobody) NOPASSWD: ...') reaches the identical code path via the shared system-auth PAM stack included by sudo. Also confirmed: / ships with mode 0555 on RHEL, so the resulting ownership change does not grant the new owner write access to / (no arbitrary file creation/rename/deletion capability is gained); impact is denial of service via broken ownership checks, not privilege escalation via filesystem write access. Per the reporting bug (RHEL-178261), this was discovered during a customer penetration test (SFDC case 04452638). This flaw affects samba versions using the older pamchownhomedir() structure (samba 4.19.x/4.23.x, confirmed via source diff and live reproduction). A fix is gated and confirmed queued for RHEL 8.10.z (RHEL-178261, via RHBA-2026:169544, publishing 2026-07-28), RHEL 9.8.z (RHEL-180756, via RHBA-2026:168497, publishing 2026-08-04), and RHEL 10.2.z (RHEL-180758, via RHBA-2026:168496, publishing 2026-08-04) -- all three are non-security bug-fix errata. RHEL 9.9 and RHEL 10.3 ship samba 4.24.3, which refactored the affected code path upstream (chown() only fires when mkdir() creates a genuinely new directory, not when the target already exists) -- confirmed via live reproduction in isolated sandboxes using the exact NVRs those releases ship (samba-winbind-4.24.3-1.el9 and samba-winbind-4.24.3-100.el10) that neither su nor sudo-delegation triggers the chown. RHEL 9.9/10.3 are NOT AFFECTED.