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  1. MariaDB Server
  2. MDEV-19882

pam v2: auth_pam_tool truncates passwords that are not null-terminated



    • Bug
    • Status: Closed (View Workflow)
    • Major
    • Resolution: Fixed
    • 10.4.6, 10.4.8
    • 10.4.9
    • Plugin - pam
    • None


      In MariaDB 10.4, version 2 of the pam plugin is provided. This plugin forks a new process and executes the auth_pam_tool utility that is now bundled with the server.

      Unfortunately, it seems to truncate passwords that are not null-terminated, because it always seems to assume that the last character is the NULL terminator.

      This is a problem because some implementations of mysql_clear_password don't seem to null-terminate passwords.

      The problem is fairly easy to reproduce.

      Configuring PAM

      We can configure PAM using the steps from MDEV-19877.

      Create a Unix user account and set a password for the user:

      sudo useradd alice
      sudo passwd alice

      Create the PAM service configuration:

      sudo tee /etc/pam.d/mariadb <<EOF
      auth required pam_unix.so audit
      account required pam_unix.so audit

      And then you might need to execute some commands to work around MDEV-19876:

      sudo chmod 0755 /usr/lib64/mysql/plugin/auth_pam_tool_dir/
      sudo chmod 4755 /usr/lib64/mysql/plugin/auth_pam_tool_dir/auth_pam_tool

      Configuring MariaDB

      We can configure PAM to use PAM authentication for this user account using the steps from MDEV-19877.

      Let's install the pam plugin:

      INSTALL SONAME 'auth_pam';

      And let's create the relevant user:

      CREATE USER 'alice'@'localhost' IDENTIFIED VIA pam USING 'mariadb'

      Constructing the Input

      We can construct the input for the auth_pam_tool tool using the information from MDEV-19877.

      Let's assume that the alice user's password is uGBXHxID3dJRALw2.

      Let's create input with a null-terminated password:

      echo -n -e '0\0\x05alice\0\x07mariadb\0\x11uGBXHxID3dJRALw2\0' > good_input.txt

      And let's also create input with a password that is not null-terminated:

      echo -n -e '0\0\x05alice\0\x07mariadb\0\x10uGBXHxID3dJRALw2' > bad_input.txt

      And then confirm the contents of each file:

      $ hexdump -c good_input.txt
      0000000   0  \0 005   a   l   i   c   e  \0  \a   m   a   r   i   a   d
      0000010   b  \0 021   u   G   B   X   H   x   I   D   3   d   J   R   A
      0000020   L   w   2  \0
      $ hexdump -c bad_input.txt
      0000000   0  \0 005   a   l   i   c   e  \0  \a   m   a   r   i   a   d
      0000010   b  \0 020   u   G   B   X   H   x   I   D   3   d   J   R   A
      0000020   L   w   2

      Running the Tool

      Next, we can run the auth_pam_tool tool and redirect the tool's stdin to the input files that we constructed.

      We can also attach strace to the process, so we can passwords the tool is reading and writing.

      First, run the tool with the good input:

      cat good_input.txt | sudo strace -o strace_good_input.out -f -ff /usr/lib64/mysql/plugin/auth_pam_tool_dir/auth_pam_tool

      The null-terminated password is properly read and passed to PAM:

      write(1, "C", 1)                        = 1
      write(1, "\0\v", 2)                     = 2
      write(1, "\4Password: ", 11)            = 11
      read(0, "\0\21", 2)                     = 2
      read(0, "uGBXHxID3dJRALw2\0", 17)       = 17
      write(4, "uGBXHxID3dJRALw2", 16)        = 16
      write(4, "\0", 1)                       = 1

      Next, run the tool with the bad input:

      cat bad_input.txt | sudo strace -o strace_bad_input.out -f -ff /usr/lib64/mysql/plugin/auth_pam_tool_dir/auth_pam_tool

      The non-null-terminated password is properly read, but its last character is truncated when it is passed to PAM:

      write(1, "C", 1)                        = 1
      write(1, "\0\v", 2)                     = 2
      write(1, "\4Password: ", 11)            = 11
      read(0, "\0\20", 2)                     = 2
      read(0, "uGBXHxID3dJRALw2", 16)         = 16
      write(4, "uGBXHxID3dJRALw", 15)         = 15
      write(4, "\0", 1)                       = 1

      Checking Syslog

      The syslog (i.e. /var/log/secure on RHEL or /var/log/auth.log on Debian/Ubuntu) also shows an authentication failure, since the auth_pam_tool
      tool provided a truncated password to PAM:

      Oct 16 02:24:14 ip-172-30-0-123 unix_chkpwd[11398]: password check failed for user (alice)
      Oct 16 02:24:14 ip-172-30-0-123 auth_pam_tool: pam_unix(mariadb:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=  user=alice


        Issue Links



              serg Sergei Golubchik
              GeoffMontee Geoff Montee (Inactive)
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