linux-pam (aka Linux PAM) before 1.6.0 allows attackers to cause a denial of service (blocked login process) via mkfifo because the openat call (for protect_dir) lacks O_DIRECTORY. A vulnerability was found in Linux PAM. An unprivileged user that is not yet in a corresponding mount namespace with ~/tmp mounted as a polyinstantiated dir can place a FIFO there, and a subsequent attempt to login as this user with `pam_namespace` configured will cause the `openat()` in `protect_dir()` to block the attempt, causing a local denial of service.
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– Scott Cheney, Manager of Information Security, Sierra View Medical Center