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checkpw.c
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checkpw.c
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/**
* checkpw is a program that checks the validity of a users password on a
* Linux/PAM-based system.
*
* Author: Johannes Findeisen <[email protected]> - 2024
* License: MIT (see LICENSE)
*/
#include <pwd.h> // For struct passwd and getpwnam
#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <termios.h> // For terminal input settings
#include <unistd.h> // For getopt and access to user info
#define MAX_USERNAME_LEN 32
#define MAX_PASSWORD_LEN 256
#ifndef MAX_UID
#define MAX_UID 1000
#endif
#ifndef MIN_UID
#define MIN_UID 1000
#endif
#define VERSION 1_0_3
// Custom data structure to hold user-entered password
struct pam_credentials
{
const char *password;
};
// PAM conversation function to supply the password
int pam_conversation(int num_msg, const struct pam_message **msg,
struct pam_response **resp, void *appdata_ptr)
{
struct pam_response *response = NULL;
struct pam_credentials *credentials = (struct pam_credentials *)appdata_ptr;
int i;
response = (struct pam_response *)malloc(sizeof(struct pam_response) * num_msg);
if (response == NULL)
return PAM_CONV_ERR;
for (i = 0; i < num_msg; i++) {
switch (msg[i]->msg_style) {
case PAM_PROMPT_ECHO_ON:
case PAM_PROMPT_ECHO_OFF:
response[i].resp = strdup(credentials->password);
response[i].resp_retcode = 0;
break;
default:
free(response);
return PAM_CONV_ERR;
}
}
*resp = response;
return PAM_SUCCESS;
}
int authenticate(const char *username, const char *password, int verbose)
{
int retval;
pam_handle_t *pamh = NULL;
struct pam_credentials credentials = { password };
struct pam_conv conv = { pam_conversation, &credentials };
if (verbose)
printf("Starting PAM authentication for user '%s'.\n", username);
retval = pam_start("login", username, &conv, &pamh);
if (retval == PAM_SUCCESS) {
if (verbose)
printf("PAM authentication initialized.\n");
retval = pam_authenticate(pamh, 0); // Attempt to authenticate
} else {
if (verbose)
printf("pam_start failed: %s\n", pam_strerror(pamh, retval));
}
if (retval == PAM_SUCCESS) {
if (verbose)
printf("User '%s' authenticated successfully.\n", username);
retval = pam_acct_mgmt(pamh, 0); // Check account validity
if (retval != PAM_SUCCESS && verbose)
printf("pam_acct_mgmt failed: %s\n", pam_strerror(pamh, retval));
} else {
if (verbose)
printf("pam_authenticate failed: %s\n", pam_strerror(pamh, retval));
}
if (pam_end(pamh, retval) != PAM_SUCCESS) {
pamh = NULL;
fprintf(stderr, "Failed to release PAM authenticator\n");
exit(1);
}
if (retval != PAM_SUCCESS && verbose) {
printf("Authentication failed for user '%s'.\n", username);
}
return (retval == PAM_SUCCESS ? 0 : 1); // 0 for success, 1 for failure
}
// Function to prompt user for input, optionally hiding input
void prompt_for_input(char *buffer, size_t size, const char *prompt,
int hide_input)
{
printf("%s", prompt);
fflush(stdout);
if (hide_input) {
struct termios oldt, newt;
if (tcgetattr(STDIN_FILENO, &oldt) != 0) {
perror("tcgetattr");
exit(1);
}
newt = oldt;
newt.c_lflag &= ~(ECHO);
if (tcsetattr(STDIN_FILENO, TCSANOW, &newt) != 0) {
perror("tcsetattr");
exit(1);
}
}
if (fgets(buffer, size, stdin) == NULL) {
fprintf(stderr, "\nError reading input.\n");
if (hide_input) {
// Restore terminal settings
struct termios oldt;
tcgetattr(STDIN_FILENO, &oldt);
oldt.c_lflag |= ECHO;
tcsetattr(STDIN_FILENO, TCSANOW, &oldt);
}
exit(1);
}
// Remove trailing newline
buffer[strcspn(buffer, "\n")] = '\0';
if (hide_input) {
// Restore terminal settings
struct termios oldt;
tcgetattr(STDIN_FILENO, &oldt);
oldt.c_lflag |= ECHO;
tcsetattr(STDIN_FILENO, TCSANOW, &oldt);
printf("\n"); // Move to the next line after password input
}
}
void print_usage(const char *prog_name)
{
fprintf(stderr, "\n");
fprintf(stderr, "Usage: %s [-u <username>] [-p <password>] [-i] [-v] [-h]\n", prog_name);
fprintf(stderr, "\n");
fprintf(stderr, "Options:\n");
fprintf(stderr, " -u <username> Specify username.\n");
fprintf(stderr, " -p <password> Specify password.\n");
fprintf(stderr, " -i Enable interactive mode to prompt for missing username/password.\n");
fprintf(stderr, " -v Enable verbose mode.\n");
fprintf(stderr, " -V Show program version.\n");
fprintf(stderr, " -h Show this help.\n");
fprintf(stderr, "\n");
}
int main(int argc, char *argv[])
{
bool interactive = false;
bool verbose = false;
bool version = false;
char username[MAX_USERNAME_LEN] = {0};
char password[MAX_PASSWORD_LEN] = {0};
int opt;
// Parse command-line arguments
while ((opt = getopt(argc, argv, "u:p:hivV")) != -1) {
switch (opt) {
case 'u':
if (strlen(optarg) >= MAX_USERNAME_LEN) {
fprintf(stderr, "Error: Username is too long (maximum %d characters).\n", MAX_USERNAME_LEN);
exit(1);
}
strncpy(username, optarg, MAX_USERNAME_LEN - 1);
break;
case 'p':
if (strlen(optarg) >= MAX_PASSWORD_LEN) {
fprintf(stderr, "Error: Password is too long (maximum %d characters).\n", MAX_PASSWORD_LEN);
exit(1);
}
strncpy(password, optarg, MAX_PASSWORD_LEN - 1);
break;
case 'h':
print_usage(argv[0]);
exit(0);
case 'i':
interactive = true;
break;
case 'v':
verbose = true;
break;
case 'V':
version = true;
break;
default:
print_usage(argv[0]);
exit(1);
}
}
if (version) {
printf("1.0.3\n");
exit(0);
}
// If interactive mode is enabled, prompt for missing username and/or password
if (interactive) {
if (username[0] == '\0') {
prompt_for_input(username, sizeof(username), "Username: ", 0);
if (strlen(username) == 0) {
fprintf(stderr, "Error: Username cannot be empty.\n");
exit(1);
}
}
if (password[0] == '\0') {
prompt_for_input(password, sizeof(password), "Password: ", 1);
if (strlen(password) == 0) {
fprintf(stderr, "Error: Password cannot be empty.\n");
exit(1);
}
}
} else {
// If not in interactive mode, ensure username and password are provided
if (username[0] == '\0' || password[0] == '\0') {
fprintf(stderr, "Error: Username and password must be provided unless interactive mode is enabled.\n");
print_usage(argv[0]);
exit(1);
}
}
// Retrieve user information from the username
struct passwd *pwd = getpwnam(username);
if (pwd == NULL) {
fprintf(stderr, "Error: User '%s' not found.\n", username);
exit(1);
}
// Check if the user's UID is below the minimum allowed UID and not higher than maximum allowed UID
if (pwd->pw_uid < MIN_UID || pwd->pw_uid > MAX_UID) {
fprintf(stderr, "Error: User '%s' has a UID less than %d or higher than %d and is not allowed to authenticate.\n", username, MIN_UID, MAX_UID);
exit(1);
}
if (verbose)
printf("User '%s' passed UID check (UID: %d).\n", username, pwd->pw_uid);
// Authenticate the user
if (authenticate(username, password, verbose) == 0) {
printf("Authenticated successfully.\n");
return 0;
} else {
printf("Authentication failed.\n");
return 1;
}
}