mypassword.c

#define _XOPEN_SOURCE
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <crypt.h>

void change_password(char *user) {
    // Check EUID to ensure the program has Root privileges (SetUID)
    if (geteuid() != 0) {
        fprintf(stderr, "Error: SetUID root required to write /etc/shadow\n");
        return;
    }

    FILE *fs = fopen("/etc/shadow", "r");
    FILE *ft = fopen("/etc/shadow.tmp", "w");
    if (!fs || !ft) { perror("File error"); return; }

    char line[512], tmp_line[512], stored_hash[256];
    int found = 0;

    // Verify Old Password
    while (fgets(line, sizeof(line), fs)) {
        strcpy(tmp_line, line);
        char *name = strtok(tmp_line, ":");
        if (name && strcmp(name, user) == 0) {
            strcpy(stored_hash, strtok(NULL, ":"));
            found = 1; break;
        }
    }

    if (!found) { printf("User not found.\n"); fclose(fs); fclose(ft); return; }

    char *old_pw = getpass("Old password: ");
    if (strcmp(crypt(old_pw, stored_hash), stored_hash) != 0) {
        printf("Incorrect password.\n"); fclose(fs); fclose(ft); return;
    }

    // Generate New Hash
    char *new_pw = getpass("New password: ");
    char *new_hash = crypt(new_pw, "$6$saltsalt"); // Using SHA-512 with a salt

    // Rewrite shadow file
    rewind(fs);
    while (fgets(line, sizeof(line), fs)) {
        strcpy(tmp_line, line);
        char *name = strtok(tmp_line, ":");
        if (name && strcmp(name, user) == 0) {
            // Reconstruct the shadow line: name:new_hash:rest_of_fields
            char *rest = strtok(NULL, ""); // Get everything after username
            rest = strchr(rest, ':');      // Move pointer to the first ':' after hash
            fprintf(ft, "%s:%s%s", user, new_hash, rest ? rest : ":::");
        } else {
            fputs(line, ft);
        }
    }

    fclose(fs); fclose(ft);
    
    // Swap files and set permissions
    if (rename("/etc/shadow.tmp", "/etc/shadow") == 0) {
        chmod("/etc/shadow", 0640);
        printf("Password updated successfully.\n");
    } else {
        perror("Rename failed");
    }
}

int main() {
    char user[50];
    printf("Username: ");
    scanf("%49s", user);
    change_password(user);
    return 0;
}