diff --git a/server/db/create-admin-user.js b/server/db/create-admin-user.js
index a41fa5e2..4edc8f70 100644
--- a/server/db/create-admin-user.js
+++ b/server/db/create-admin-user.js
@@ -7,6 +7,8 @@
 /* eslint-disable no-console */
 
 const { read } = require('read');
+const validator = require('validator');
+const zxcvbn = require('zxcvbn');
 const initKnex = require('knex');
 
 const knexfile = require('./knexfile');
@@ -47,20 +49,86 @@ const input = async (fieldName, options = {}) => {
   try {
     await knex.migrate.latest();
 
-    process.env.DEFAULT_ADMIN_EMAIL = await input('Email', {
-      isRequired: true,
-    });
+    let isEmailValid = false;
+    while (!isEmailValid) {
+      process.env.DEFAULT_ADMIN_EMAIL = await input('Email', {
+        isRequired: true,
+      });
 
-    process.env.DEFAULT_ADMIN_PASSWORD = await input('Password', {
-      isRequired: true,
-      isPassword: true,
-    });
+      if (
+        !validator.isEmail(process.env.DEFAULT_ADMIN_EMAIL) ||
+        process.env.DEFAULT_ADMIN_EMAIL.length > 256
+      ) {
+        console.log('Email must be a valid e-mail address with no more than 256 characters.');
+      } else {
+        const existingUser = await knex('user_account')
+          .where('email', process.env.DEFAULT_ADMIN_EMAIL.toLowerCase())
+          .first();
+
+        if (existingUser) {
+          console.log('Email is already in use.');
+        } else {
+          isEmailValid = true;
+        }
+      }
+    }
+
+    let isPasswordValid = false;
+    while (!isPasswordValid) {
+      process.env.DEFAULT_ADMIN_PASSWORD = await input('Password', {
+        isRequired: true,
+        isPassword: true,
+      });
+
+      if (zxcvbn(process.env.DEFAULT_ADMIN_PASSWORD).score >= 2) {
+        isPasswordValid = true;
+      } else {
+        console.log('Password is too weak. Choose a stronger password.');
+      }
+    }
 
     process.env.DEFAULT_ADMIN_NAME = await input('Name', {
       isRequired: true,
     });
 
-    process.env.DEFAULT_ADMIN_USERNAME = await input('Username');
+    while (process.env.DEFAULT_ADMIN_NAME.length > 128) {
+      console.log('Name must be no more than 128 characters.');
+      process.env.DEFAULT_ADMIN_NAME = await input('Name', {
+        isRequired: true,
+      });
+    }
+
+    const USERNAME_REGEX = /^[a-zA-Z0-9]+((_|\.)?[a-zA-Z0-9])*$/;
+
+    let isUsernameValid = false;
+    while (!isUsernameValid) {
+      process.env.DEFAULT_ADMIN_USERNAME = await input('Username');
+
+      if (!process.env.DEFAULT_ADMIN_USERNAME) {
+        break;
+      }
+
+      const isValid =
+        process.env.DEFAULT_ADMIN_USERNAME.length >= 3 &&
+        process.env.DEFAULT_ADMIN_USERNAME.length <= 32 &&
+        USERNAME_REGEX.test(process.env.DEFAULT_ADMIN_USERNAME);
+
+      if (!isValid) {
+        console.log(
+          'Username must be 3-32 characters and contain only letters, digits, underscores, and dots (e.g., john_doe).',
+        );
+      } else {
+        const existingUser = await knex('user_account')
+          .where('username', process.env.DEFAULT_ADMIN_USERNAME.toLowerCase())
+          .first();
+
+        if (existingUser) {
+          console.log('Username is already in use.');
+        } else {
+          isUsernameValid = true;
+        }
+      }
+    }
 
     await knex.seed.run({
       specific: 'default.js',
diff --git a/server/db/seeds/default.js b/server/db/seeds/default.js
index 6a92ae90..82828cdf 100644
--- a/server/db/seeds/default.js
+++ b/server/db/seeds/default.js
@@ -3,7 +3,12 @@
  * Licensed under the Fair Use License: https://github.com/plankanban/planka/blob/master/LICENSE.md
  */
 
+/* eslint-disable no-console */
+
 const bcrypt = require('bcrypt');
+const validator = require('validator');
+
+const USERNAME_REGEX = /^[a-zA-Z0-9]+((_|\.)?[a-zA-Z0-9])*$/;
 
 const buildUserData = () => {
   const data = {
@@ -15,10 +20,23 @@ const buildUserData = () => {
     data.password = bcrypt.hashSync(process.env.DEFAULT_ADMIN_PASSWORD, 10);
   }
   if (process.env.DEFAULT_ADMIN_NAME) {
-    data.name = process.env.DEFAULT_ADMIN_NAME;
+    if (process.env.DEFAULT_ADMIN_NAME.length > 128) {
+      console.warn('Warning: DEFAULT_ADMIN_NAME exceeds 128 characters; truncating.');
+      data.name = process.env.DEFAULT_ADMIN_NAME.slice(0, 128);
+    } else {
+      data.name = process.env.DEFAULT_ADMIN_NAME;
+    }
   }
   if (process.env.DEFAULT_ADMIN_USERNAME) {
-    data.username = process.env.DEFAULT_ADMIN_USERNAME.toLowerCase();
+    const username = process.env.DEFAULT_ADMIN_USERNAME.toLowerCase();
+
+    if (username.length < 3 || username.length > 32 || !USERNAME_REGEX.test(username)) {
+      console.warn(
+        `Warning: DEFAULT_ADMIN_USERNAME "${process.env.DEFAULT_ADMIN_USERNAME}" is invalid; skipping.`,
+      );
+    } else {
+      data.username = username;
+    }
   }
 
   return data;
@@ -45,8 +63,36 @@ exports.seed = async (knex) => {
     process.env.DEFAULT_ADMIN_EMAIL && process.env.DEFAULT_ADMIN_EMAIL.toLowerCase();
 
   if (defaultAdminEmail) {
+    if (!validator.isEmail(defaultAdminEmail)) {
+      throw new Error(
+        `DEFAULT_ADMIN_EMAIL "${process.env.DEFAULT_ADMIN_EMAIL}" is not a valid e-mail address.`,
+      );
+    }
+
+    if (defaultAdminEmail.length > 256) {
+      throw new Error(
+        `DEFAULT_ADMIN_EMAIL "${process.env.DEFAULT_ADMIN_EMAIL}" exceeds 256 characters.`,
+      );
+    }
+
+    const existingEmailUser = await knex('user_account').where('email', defaultAdminEmail).first();
+
+    if (existingEmailUser) {
+      throw new Error(`User with DEFAULT_ADMIN_EMAIL "${defaultAdminEmail}" already exists.`);
+    }
+
     const userData = buildUserData();
 
+    if (userData.username) {
+      const existingUsernameUser = await knex('user_account')
+        .where('username', userData.username)
+        .first();
+
+      if (existingUsernameUser) {
+        throw new Error(`User with DEFAULT_ADMIN_USERNAME "${userData.username}" already exists.`);
+      }
+    }
+
     let userId;
     try {
       [{ id: userId }] = await knex('user_account').insert(