User execution of su fails with error "bash: /bin/su: Permission denied"

Issue

• After updating the system, no one (including root) can use su without getting bash: /bin/su: Permission denied errors, but ssh and terminal logins work as normal. SELinux is disabled and no new log entries are generated in /var/log/secure or /var/log/messages when root tries to run /bin/su, which has appropriate permissions, file size, and md5sum. The /etc/nsswitch.conf, /etc/pam.d/system-auth, and /etc/pam.d/su files have all been replaced with default versions and still the problem remains.

  [root@localhost ~]# ls -l /bin/su
  -rwsr-xr-x 1 root root 28336 May 11  2011 /bin/su
  [root@localhost ~]# /bin/su -
  bash: /bin/su: Permission denied
  [root@localhost ~]# strace -tvfs 2048 -o su_strace_root.log su -lc exit
  strace: exec: Permission denied
  <truncacted strace output:>
  28530 15:17:38 execve("/bin/su", ["su", "-lc", "exit"], ... "_=/usr/bin/strace"]) = -1 EACCES (Permission denied)