diff --git a/dashboard/auth_server.py b/dashboard/auth_server.py
index 825ab93..0546d58 100644
--- a/dashboard/auth_server.py
+++ b/dashboard/auth_server.py
@@ -104,6 +104,8 @@ async def login(request: Request) -> Response:
             keep_blank_values=True,
             strict_parsing=True,
             max_num_fields=3,
+            encoding="utf-8",
+            errors="strict",
         )
     except (UnicodeDecodeError, ValueError):
         authentication.log_auth_event("login_request_rejected", correlation_id)
diff --git a/dashboard/authentication.py b/dashboard/authentication.py
index fe09bcd..006d3c9 100644
--- a/dashboard/authentication.py
+++ b/dashboard/authentication.py
@@ -113,8 +113,12 @@ def verify_credentials(
 ) -> bool:
     if not isinstance(submitted_id, str) or not isinstance(submitted_password, str):
         return False
-    id_matches = hmac.compare_digest(submitted_id, credentials.admin_id)
-    password_matches = hmac.compare_digest(submitted_password, credentials.password)
+    id_matches = hmac.compare_digest(
+        submitted_id.encode("utf-8"), credentials.admin_id.encode("utf-8")
+    )
+    password_matches = hmac.compare_digest(
+        submitted_password.encode("utf-8"), credentials.password.encode("utf-8")
+    )
     return id_matches and password_matches
 
 
diff --git a/tests/test_dashboard_auth_server.py b/tests/test_dashboard_auth_server.py
index c68ebec..0508f54 100644
--- a/tests/test_dashboard_auth_server.py
+++ b/tests/test_dashboard_auth_server.py
@@ -87,6 +87,50 @@ def test_remember_me_sets_30_day_cookie(client: TestClient) -> None:
     )
 
 
+def test_unicode_credentials_can_login_through_adapter(client: TestClient, monkeypatch) -> None:
+    unicode_id = "管理员"
+    unicode_password = "päss🔐"
+    monkeypatch.setenv(authentication.ADMIN_ID_ENV, unicode_id)
+    monkeypatch.setenv(authentication.ADMIN_PASSWORD_ENV, unicode_password)
+
+    response = client.post(
+        "/api/auth/login",
+        data={"admin_id": unicode_id, "admin_password": unicode_password},
+        headers={"origin": "https://testserver"},
+        follow_redirects=False,
+    )
+
+    assert response.status_code == 303
+    assert response.headers["location"] == "/"
+    assert f"{authentication.AUTH_COOKIE_NAME}=" in response.headers["set-cookie"]
+
+
+def test_unicode_credential_mismatch_is_generic_and_delayed(
+    client: TestClient, monkeypatch
+) -> None:
+    import time as real_time
+
+    unicode_id = "管理员"
+    unicode_password = "päss🔐"
+    monkeypatch.setenv(authentication.ADMIN_ID_ENV, unicode_id)
+    monkeypatch.setenv(authentication.ADMIN_PASSWORD_ENV, unicode_password)
+    monkeypatch.setattr(authentication.time, "sleep", real_time.sleep)
+
+    started = real_time.monotonic()
+    response = client.post(
+        "/api/auth/login",
+        data={"admin_id": unicode_id, "admin_password": "неверный🔒"},
+        headers={"origin": "https://testserver"},
+        follow_redirects=False,
+    )
+    elapsed = real_time.monotonic() - started
+
+    assert response.status_code == 303
+    assert response.headers["location"] == "/?auth_error=invalid"
+    assert "set-cookie" not in response.headers
+    assert elapsed >= authentication.MINIMUM_FAILURE_DELAY_SECONDS * 0.9
+
+
 def test_invalid_credentials_do_not_set_cookie_and_use_general_error(
     client: TestClient,
 ) -> None:
@@ -104,6 +148,23 @@ def test_invalid_credentials_do_not_set_cookie_and_use_general_error(
     assert ADMIN_ID not in response.headers["location"]
 
 
+def test_login_rejects_malformed_percent_encoded_utf8_without_cookie(
+    client: TestClient,
+) -> None:
+    response = client.post(
+        "/api/auth/login",
+        content=b"admin_id=%FF&admin_password=wrong",
+        headers={
+            "origin": "https://testserver",
+            "content-type": "application/x-www-form-urlencoded",
+        },
+        follow_redirects=False,
+    )
+
+    assert response.status_code == 400
+    assert "set-cookie" not in response.headers
+
+
 def test_missing_configuration_fails_closed(client: TestClient, monkeypatch) -> None:
     monkeypatch.delenv(authentication.SIGNING_SECRET_ENV)
 
diff --git a/tests/test_dashboard_authentication.py b/tests/test_dashboard_authentication.py
index 47e85ee..4d16765 100644
--- a/tests/test_dashboard_authentication.py
+++ b/tests/test_dashboard_authentication.py
@@ -63,20 +63,26 @@ def test_load_admin_credentials_fails_closed_when_environment_cannot_be_read():
 
 def test_verify_credentials_uses_constant_time_comparison_for_both_fields(monkeypatch):
     credentials = authentication.AdminCredentials(ADMIN_ID, ADMIN_PASSWORD)
-    comparisons: list[tuple[str, str]] = []
+    comparisons: list[tuple[bytes, bytes]] = []
 
-    def record_compare(left: str, right: str) -> bool:
+    def record_compare(left: bytes, right: bytes) -> bool:
         comparisons.append((left, right))
         return left == right
 
     monkeypatch.setattr(authentication.hmac, "compare_digest", record_compare)
 
     assert authentication.verify_credentials(credentials, ADMIN_ID, ADMIN_PASSWORD) is True
-    assert comparisons == [(ADMIN_ID, ADMIN_ID), (ADMIN_PASSWORD, ADMIN_PASSWORD)]
+    assert comparisons == [
+        (ADMIN_ID.encode("utf-8"), ADMIN_ID.encode("utf-8")),
+        (ADMIN_PASSWORD.encode("utf-8"), ADMIN_PASSWORD.encode("utf-8")),
+    ]
 
     comparisons.clear()
     assert authentication.verify_credentials(credentials, "wrong-id", "wrong-password") is False
-    assert comparisons == [("wrong-id", ADMIN_ID), ("wrong-password", ADMIN_PASSWORD)]
+    assert comparisons == [
+        (b"wrong-id", ADMIN_ID.encode("utf-8")),
+        (b"wrong-password", ADMIN_PASSWORD.encode("utf-8")),
+    ]
 
 
 def test_failed_login_delay_waits_only_for_remaining_minimum_duration():