diff --git a/nativeauthenticator/handlers.py b/nativeauthenticator/handlers.py
index f9e21e8..7e454c1 100644
--- a/nativeauthenticator/handlers.py
+++ b/nativeauthenticator/handlers.py
@@ -70,6 +70,7 @@ def get_result_message(
         user,
         username_already_taken,
         confirmation_matches,
+        password_was_pwned,
         assume_user_is_human=True,
     ):
         """Helper function to discern exactly what message and alert level are
@@ -91,6 +92,13 @@ def get_result_message(
         elif not confirmation_matches:
             alert = "alert-danger"
             message = "Your password did not match the confirmation. Please try again."
+        elif password_was_pwned:
+            alert = "alert-danger"
+            message = (
+                "That password is known to be included in prominent leaks of user data "
+                "(via haveibeenpwned.com) and is therefore not considered secure. "
+                "Please use a different password."
+            )
         # Error if user creation was not successful.
         elif not user:
             alert = "alert-danger"
@@ -185,10 +193,15 @@ async def post(self):
             "signup_password_confirmation", strip=False
         )
         confirmation_matches = password == confirmation
+        password_was_pwned = self.authenticator.is_password_pwned(password)
 
         # Call helper function from above for precise alert-level and message.
         alert, message = self.get_result_message(
-            user, username_already_taken, confirmation_matches, assume_user_is_human
+            user,
+            username_already_taken,
+            confirmation_matches,
+            password_was_pwned,
+            assume_user_is_human,
         )
 
         otp_secret, user_2fa = "", ""
@@ -343,6 +356,7 @@ async def post(self):
         ).is_valid_password(old_password)
 
         new_password_matches_confirmation = new_password == confirmation
+        password_was_pwned = self.authenticator.is_password_pwned(new_password)
 
         if not correct_password_provided:
             alert = "alert-danger"
@@ -352,6 +366,13 @@ async def post(self):
             message = (
                 "Your new password didn't match the confirmation. Please try again."
             )
+        elif password_was_pwned:
+            alert = "alert-danger"
+            message = (
+                "That password is known to be included in prominent leaks of user data "
+                "(via haveibeenpwned.com) and is therefore not considered secure. "
+                "Please use a different password."
+            )
         else:
             success = self.authenticator.change_password(user.name, new_password)
             if success:
@@ -411,12 +432,20 @@ async def post(self, user_name):
         confirmation = self.get_body_argument("new_password_confirmation", strip=False)
 
         new_password_matches_confirmation = new_password == confirmation
+        password_was_pwned = self.authenticator.is_password_pwned(new_password)
 
         if not new_password_matches_confirmation:
             alert = "alert-danger"
             message = (
                 "The new password didn't match the confirmation. Please try again."
             )
+        elif password_was_pwned:
+            alert = "alert-danger"
+            message = (
+                "That password is known to be included in prominent leaks of user data "
+                "(via haveibeenpwned.com) and is therefore not considered secure. "
+                "Please use a different password."
+            )
         else:
             success = self.authenticator.change_password(user_name, new_password)
             if success:
diff --git a/nativeauthenticator/nativeauthenticator.py b/nativeauthenticator/nativeauthenticator.py
index f678bee..60db237 100644
--- a/nativeauthenticator/nativeauthenticator.py
+++ b/nativeauthenticator/nativeauthenticator.py
@@ -9,6 +9,7 @@
 from pathlib import Path
 
 import bcrypt
+import pwnedpasswords
 from jupyterhub.auth import Authenticator
 from sqlalchemy import inspect
 from tornado import web
@@ -269,6 +270,11 @@ def is_password_strong(self, password):
 
         return all(checks)
 
+    def is_password_pwned(self, password):
+        """Checks against HaveIBeenPwned.com's database if this
+        password appears in any prominent data leaks."""
+        return pwnedpasswords.check(password, plain_text=True) > 0
+
     def get_user(self, username):
         return UserInfo.find(self.db, self.normalize_username(username))
 
diff --git a/nativeauthenticator/tests/test_authenticator.py b/nativeauthenticator/tests/test_authenticator.py
index 72781a4..d95f2c5 100644
--- a/nativeauthenticator/tests/test_authenticator.py
+++ b/nativeauthenticator/tests/test_authenticator.py
@@ -139,6 +139,16 @@ async def test_create_user_with_strong_passwords(
     assert bool(user) == expected
 
 
+async def test_pwned_passwords(tmpcwd, app):
+    """Tests the library interface to HaveIBeenPwned.com"""
+    auth = NativeAuthenticator(db=app.db)
+
+    assert auth.is_password_pwned("password")
+    assert auth.is_password_pwned("Daenerys")
+    assert not auth.is_password_pwned("9n+VX7QgVaaqy7PU#S8Bm5GB")
+    assert not auth.is_password_pwned("?JWR%9_gEQ-t%c4eJ%%CAqq=")
+
+
 async def test_change_password(tmpcwd, app):
     auth = NativeAuthenticator(db=app.db)
     user = auth.create_user("johnsnow", "password")
diff --git a/setup.py b/setup.py
index feb8aa4..29b0c8a 100644
--- a/setup.py
+++ b/setup.py
@@ -15,6 +15,11 @@
     author_email="leportella@protonmail.com",
     license="3 Clause BSD",
     packages=find_packages(),
-    install_requires=["jupyterhub>=1.3", "bcrypt", "onetimepass"],
+    install_requires=[
+        "jupyterhub>=1.3",
+        "bcrypt",
+        "onetimepass",
+        "pwnedpasswords>=2.0",
+    ],
     include_package_data=True,
 )