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Date: Wed, 5 Mar 2014 00:54:53 +0400
From: Solar Designer <>
Subject: Linux-PAM pam_unix/unix_chkpwd is fail-open


Just off Twitter, but relevant to this list:

<kragen> thoughts on #gotofail and how it's too easy for TLS software to "fail open".

<@solardiz> @kragen @tedunangst Re: BSD auth not relying on exit code, it's relevant that Linux-PAM's pam_unix/unix_chkpwd does:

<@solardiz> @kragen @tedunangst We avoided this in our pam_tcb/tcb_chkpwd since its initial version in 2002: lines 441-451

TCB_MAGIC is 0x0a00ff7fUL

This might not be viewed as a vulnerability in pam_unix/unix_chkpwd, but
an authentication service being fail-open is against best practices.

The issue is mitigated by the fact that unix_chkpwd is only used to
check the user's own password, when unlocking an X desktop or GNU screen
(when it's patched to use PAM).  Another "mitigation" is that X desktop
locking is generally fail-open anyway. ;-(

Someone might want to patch this issue in Linux-PAM.


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