[Bug 1485719] [NEW] Uninitialized struct field in the fix for CVE-2015-5600 causes random auth failures

[Benn Sundsrud]

Public bug reported:

In Ubuntu 12.04, the fix for CVE-2015-5600[1] just hit upstream in
package openssh-server_5.9p1-5ubuntu1.6, breaking authentication
mechanisms that rely on the keyboard-interactive method.  This patch
introduces the field 'devices_done' to the KbdintAuthctxt struct, but
does not initialize the field in the kbdint_alloc() function.  On Linux,
this ends up filling that field with junk data.  The attached patch
against adds the initialization of the `devices_done` field alongside
the existing initialization code.  This has also been reported upstream.

Reproducing:

Install openssh-server_5.9p1-5ubuntu1.6
Add an authentication mechanism that uses the keyboard-interactive method (like libpam-google-authenticator)
Attempt to log in via the above mechanism.  Instead of consistently prompting the user for input, it will sometimes fall straight through to password auth because the devices_done bit field is initialized with garbage data.

Downgrading to openssh-server_5.9p1-5ubuntu1.4 solves the issue.

[1]: http://cvsweb.openbsd.org/cgi-
bin/cvsweb/src/usr.bin/ssh/auth2-chall.c.diff?r1=1.42&r2=1.43&f=h

** Affects: openssh (Ubuntu)
     Importance: Undecided
         Status: New

** Patch added: "CVE-2015-5600_initialize_struct.patch"
   https://bugs.launchpad.net/bugs/1485719/+attachment/4446088/+files/CVE-2015-5600_initialize_struct.patch

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https://bugs.launchpad.net/bugs/1485719

Title:
  Uninitialized struct field in the fix for CVE-2015-5600 causes random
  auth failures

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