How to authenticate username/password using PAM w/o root privileges

Question

I have program written in C. It takes 2 arguments username/password and try to authenticate this user using PAM. It works fine when I'm root. When I'm 'normal' user, it works for this user, but not for another one. I think, it's due to using shadow passwords..

As service I'm using:

retval = pam_start("test", username, &local_conversation, &local_auth_handle);

I add this to the /etc/pam.d/test

#%PAM-1.0
auth    required    pam_unix.so shadow nullok
account required    pam_unix.so
session required    pam_unix.so

Could you help me, please? Thanks a lot!

Al

I found one solution: chmod 444 /etc/shadow.. but: is it safe?? – Al

The obvious answer to your question about safety is: NO! – Detrusion

I resolved this issue. Check my site vkrejcirik.info/2012/06/09/… – Al

I imagine it's unsafe because PAM would allow unchecked brute-forcing of passwords. – Author

Answer 1

The application needed to be able to read /etc/shadow.

See my post for one way to do this.

EDIT: Add post from above link in case the link ever breaks

I wrote authentication module in C++ which alows to check username/password through PAM in Linux (I’m using Fedora Linux). I would like to share with you, what I made :-) . So, let’s go :-)

Prerequisities:

Install package pam-devel
(This step is necessary when you use shadow password) Create new Linux user and group. Set this group as default for this user. Then

follow with these steps: Go to /etc Log in as root (su) Change group to the new one for file shadow (chgrp new_group shadow) Set ‘read’ privilage for this group (chmod 0440 shadow)

Write this code: (authModule.c) view plaincopy to clipboardprint?

#include <stdio.h>  
#include <security/pam_appl.h>  
#include <unistd.h>  
#include <stdlib.h>  
#include <string.h>  

struct pam_response *reply;  

// //function used to get user input  
int function_conversation(int num_msg, const struct pam_message **msg, struct pam_response **resp, void *appdata_ptr)  
{  
    *resp = reply;  
        return PAM_SUCCESS;  
}  

int authenticate_system(const char *username, const char *password)   
{  
    const struct pam_conv local_conversation = { function_conversation, NULL };  
    pam_handle_t *local_auth_handle = NULL; // this gets set by pam_start  

    int retval;  
    retval = pam_start("su", username, &local_conversation, &local_auth_handle);  

    if (retval != PAM_SUCCESS)  
    {  
            printf("pam_start returned: %d\n ", retval);  
            return 0;  
    }  

    reply = (struct pam_response *)malloc(sizeof(struct pam_response));  

    reply[0].resp = strdup(password);  
    reply[0].resp_retcode = 0;  
    retval = pam_authenticate(local_auth_handle, 0);  

    if (retval != PAM_SUCCESS)  
    {  
            if (retval == PAM_AUTH_ERR)  
            {  
                    printf("Authentication failure.\n");  
            }  
            else  
            {  
                printf("pam_authenticate returned %d\n", retval);  
            }  
            return 0;  
    }  

    printf("Authenticated.\n");  
    retval = pam_end(local_auth_handle, retval);  

    if (retval != PAM_SUCCESS)  
    {  
            printf("pam_end returned\n");  
            return 0;  
    }  

    return 1;  
}  

int main(int argc, char** argv)  
{  
    char* login;  
    char* password;  

    printf("Authentication module\n");  

    if (argc != 3)  
    {  
        printf("Invalid count of arguments %d.\n", argc);  
        printf("./authModule <username> <password>");  
        return 1;  
    }  

    login = argv[1];  
    password = argv[2];  

    if (authenticate_system(login, password) == 1)  
    {  
        printf("Authenticate with %s - %s through system\n", login, password);  
        return 0;  
    }     

    printf("Authentication failed!\n");  
    return 1;  
}  

Compile code:

gcc -o authModule authModule.c -lpam  

Run code (as the new user!):

./authModule user password  

That’s all!! :-) Hope it helps!

Al

Doesn't work if run as root user. – Konya

Answer 2

I yet cannot comment on the post of Vladislav due to too low rep but I have to say something on his code.

This does ONLY work if you do not run this code as user root. When you run it as root EVERY password for a known user is accepted.

Does anyone know how to fix this?

EDIT: To solve this problem you should use the sudo pam service intead of the su service with pam_start()

Gastritis