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Thread: 'authentication failed' and 'unknown module' errors preventing logon

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  1. #1
    Join Date
    Apr 2007
    Pompano Beach, FL, USA
    Ubuntu 13.10 Saucy Salamander

    Lightbulb 'authentication failed' and 'unknown module' errors preventing logon

    Effected Systems:
    I experienced this issue on Hardy but I found evidence of it occurring on earlier versions and on Debian.

    • You can't login using your normal credentials. Logging on in X (the graphical environment) raises an "authentication failed" error and the shell complains about an "unknown module".
    • You can access the system in recovery mode and you can confirm that the users and passwords are set correctly.
    • Attempting the su command from the shell raises "Unknown module: su" but you may be able to switch users (in single user mode)
    • You may also see "Segmentation fault" errors.
    • The system logs provide no help, there is no entry related to this failure.

    PAM (Pluggable Authentication Modules) is failing to authenticate you because it can't load its essential modules.

    **WARNING** Following these directions should be safe but you will run a potentially dangerous command. Take the proper precautions like backing up important data or the entire system. **

    If you haven't messed with PAM the easiest thing to do is to take it out and replace it with a working copy from the official repositories.

    Step 1: Start ubuntu using the recovery option (singe user mode)

    Step 2: Purge the pam runtime and modules packages and some related files (don't try using apt for this):
    dpkg --force-all --purge libpam-runtime
    dpkg --force-all --purge libpam-modules
    rm -rf /etc/pam.d
    rm -rf /lib/security
    Step 3: Reinstall those packages from the repositories:
    apt-get install libpam-runtime libpam-modules
    Step 4: Reboot. You should be able to login normally now:
    reboot now
    My main source for fixing my issue and deriving these instructions:

    The user in the following post had a similar issue but fixed it a different way, which did not work for me:
    Last edited by MikeEvans; May 21st, 2008 at 07:46 PM.

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