diff --git a/classes/api.php b/classes/api.php
index 43cefef8f..7e4691b32 100755
--- a/classes/api.php
+++ b/classes/api.php
@@ -59,7 +59,7 @@ class API extends Handler {
 
 		if (SINGLE_USER_MODE) $login = "admin";
 
-		$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE login = ?");
+		$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE LOWER(login) = LOWER(?)");
 		$sth->execute([$login]);
 
 		if ($row = $sth->fetch()) {
diff --git a/classes/auth/base.php b/classes/auth/base.php
index 1b9015fe3..1d68ae537 100644
--- a/classes/auth/base.php
+++ b/classes/auth/base.php
@@ -27,7 +27,7 @@ abstract class Auth_Base extends Plugin implements IAuthModule {
 
 				$sth = $this->pdo->prepare("INSERT INTO ttrss_users
 						(login,access_level,last_login,created,pwd_hash,salt)
-						VALUES (?, 0, null, NOW(), ?,?)");
+						VALUES (LOWER(?), 0, null, NOW(), ?,?)");
 				$sth->execute([$login, $pwd_hash, $salt]);
 
 				return $this->find_user_by_login($login);
@@ -42,7 +42,7 @@ abstract class Auth_Base extends Plugin implements IAuthModule {
 
 	function find_user_by_login($login) {
 		$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE
-			login = ?");
+			LOWER(login) = LOWER(?)");
 		$sth->execute([$login]);
 
 		if ($row = $sth->fetch()) {
diff --git a/classes/handler/public.php b/classes/handler/public.php
index 13a6af4b1..a1ed667be 100755
--- a/classes/handler/public.php
+++ b/classes/handler/public.php
@@ -248,7 +248,7 @@ class Handler_Public extends Handler {
 		$login = clean($_REQUEST["login"]);
 		$fresh = clean($_REQUEST["fresh"]) == "1";
 
-		$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE login = ?");
+		$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE LOWER(login) = LOWER(?)");
 		$sth->execute([$login]);
 
 		if ($row = $sth->fetch()) {
@@ -272,7 +272,7 @@ class Handler_Public extends Handler {
 
 		if ($login) {
 			$sth = $this->pdo->prepare("SELECT ttrss_settings_profiles.* FROM ttrss_settings_profiles,ttrss_users
-			WHERE ttrss_users.id = ttrss_settings_profiles.owner_uid AND login = ? ORDER BY title");
+			WHERE ttrss_users.id = ttrss_settings_profiles.owner_uid AND LOWER(login) = LOWER(?) ORDER BY title");
 			$sth->execute([$login]);
 
 			$rv = [ [ "value" => 0, "label" => __("Default profile") ] ];
@@ -941,7 +941,7 @@ class Handler_Public extends Handler {
 
 			if ($login) {
 				$sth = $this->pdo->prepare("SELECT id, resetpass_token FROM ttrss_users
-					WHERE login = ?");
+					WHERE LOWER(login) = LOWER(?)");
 				$sth->execute([$login]);
 
 				if ($row = $sth->fetch()) {
@@ -1026,7 +1026,7 @@ class Handler_Public extends Handler {
 				$_SESSION["pwdreset:testvalue2"] = rand(1, 1000);
 
 				$sth = $this->pdo->prepare("SELECT id FROM ttrss_users
-					WHERE login = ? AND email = ?");
+					WHERE LOWER(login) = LOWER(?) AND email = ?");
 				$sth->execute([$login, $email]);
 
 				if ($row = $sth->fetch()) {
@@ -1066,7 +1066,7 @@ class Handler_Public extends Handler {
 
 						$sth = $this->pdo->prepare("UPDATE ttrss_users
 							SET resetpass_token = ?
-							WHERE login = ? AND email = ?");
+							WHERE LOWER(login) = LOWER(?) AND email = ?");
 
 						$sth->execute([$resetpass_token_full, $login, $email]);
 
diff --git a/classes/pref/users.php b/classes/pref/users.php
index f6acc0d20..45c4b82b8 100644
--- a/classes/pref/users.php
+++ b/classes/pref/users.php
@@ -206,7 +206,7 @@ class Pref_Users extends Handler_Protected {
 				$pass_query_part = "";
 			}
 
-			$sth = $this->pdo->prepare("UPDATE ttrss_users SET $pass_query_part login = ?,
+			$sth = $this->pdo->prepare("UPDATE ttrss_users SET $pass_query_part login = LOWER(?),
 				access_level = ?, email = ?, otp_enabled = false WHERE id = ?");
 			$sth->execute([$login, $access_level, $email, $uid]);
 
@@ -238,18 +238,18 @@ class Pref_Users extends Handler_Protected {
 			if (!$login) return; // no blank usernames
 
 			$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE
-				login = ?");
+				LOWER(login) = LOWER(?)");
 			$sth->execute([$login]);
 
 			if (!$sth->fetch()) {
 
 				$sth = $this->pdo->prepare("INSERT INTO ttrss_users
 					(login,pwd_hash,access_level,last_login,created, salt)
-					VALUES (?, ?, 0, null, NOW(), ?)");
+					VALUES (LOWER(?), ?, 0, null, NOW(), ?)");
 				$sth->execute([$login, $pwd_hash, $salt]);
 
 				$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE
-					login = ? AND pwd_hash = ?");
+					LOWER(login) = LOWER(?) AND pwd_hash = ?");
 				$sth->execute([$login, $pwd_hash]);
 
 				if ($row = $sth->fetch()) {
diff --git a/plugins/auth_internal/init.php b/plugins/auth_internal/init.php
index b31a23187..a69ea444c 100644
--- a/plugins/auth_internal/init.php
+++ b/plugins/auth_internal/init.php
@@ -26,7 +26,7 @@ class Auth_Internal extends Auth_Base {
 		if (get_schema_version() > 96) {
 
 			$sth = $this->pdo->prepare("SELECT otp_enabled,salt FROM ttrss_users WHERE
-				login = ?");
+				LOWER(login) = LOWER(?)");
 			$sth->execute([$login]);
 
 			if ($row = $sth->fetch()) {
@@ -104,7 +104,7 @@ class Auth_Internal extends Auth_Base {
 
 		if (get_schema_version() > 87) {
 
-			$sth = $this->pdo->prepare("SELECT salt FROM ttrss_users WHERE login = ?");
+			$sth = $this->pdo->prepare("SELECT salt FROM ttrss_users WHERE LOWER(login) = LOWER(?)");
 			$sth->execute([$login]);
 
 			if ($row = $sth->fetch()) {
@@ -113,7 +113,7 @@ class Auth_Internal extends Auth_Base {
 				if ($salt == "") {
 
 					$sth = $this->pdo->prepare("SELECT id FROM ttrss_users WHERE
-						login = ? AND (pwd_hash = ? OR pwd_hash = ?)");
+						LOWER(login) = LOWER(?) AND (pwd_hash = ? OR pwd_hash = ?)");
 
 					$sth->execute([$login, $pwd_hash1, $pwd_hash2]);
 
@@ -128,7 +128,7 @@ class Auth_Internal extends Auth_Base {
 						$pwd_hash = encrypt_password($password, $salt, true);
 
 						$sth = $this->pdo->prepare("UPDATE ttrss_users SET
-							pwd_hash = ?, salt = ? WHERE login = ?");
+							pwd_hash = ?, salt = ? WHERE LOWER(login) = LOWER(?)");
 
 						$sth->execute([$pwd_hash, $salt, $login]);
 
@@ -143,7 +143,7 @@ class Auth_Internal extends Auth_Base {
 
 					$sth = $this->pdo->prepare("SELECT id
 						  FROM ttrss_users WHERE
-						  login = ? AND pwd_hash = ?");
+						  LOWER(login) = LOWER(?) AND pwd_hash = ?");
 					$sth->execute([$login, $pwd_hash]);
 
 					if ($row = $sth->fetch()) {
@@ -154,7 +154,7 @@ class Auth_Internal extends Auth_Base {
 			} else {
 				$sth = $this->pdo->prepare("SELECT id
 					FROM ttrss_users WHERE
-					  login = ? AND (pwd_hash = ? OR pwd_hash = ?)");
+					  LOWER(login) = LOWER(?) AND (pwd_hash = ? OR pwd_hash = ?)");
 
 				$sth->execute([$login, $pwd_hash1, $pwd_hash2]);
 
@@ -165,7 +165,7 @@ class Auth_Internal extends Auth_Base {
 		} else {
 			$sth = $this->pdo->prepare("SELECT id
 					FROM ttrss_users WHERE
-					  login = ? AND (pwd_hash = ? OR pwd_hash = ?)");
+					  LOWER(login) = LOWER(?) AND (pwd_hash = ? OR pwd_hash = ?)");
 
 			$sth->execute([$login, $pwd_hash1, $pwd_hash2]);
 
@@ -266,7 +266,7 @@ class Auth_Internal extends Auth_Base {
 	private function check_app_password($login, $password, $service) {
 		$sth = $this->pdo->prepare("SELECT p.id, p.pwd_hash, u.id AS uid
 			FROM ttrss_app_passwords p, ttrss_users u
-			WHERE p.owner_uid = u.id AND u.login = ? AND service = ?");
+			WHERE p.owner_uid = u.id AND LOWER(u.login) = LOWER(?) AND service = ?");
 		$sth->execute([$login, $service]);
 
 		while ($row = $sth->fetch()) {
diff --git a/plugins/auth_remote/init.php b/plugins/auth_remote/init.php
index 18ec0e1a6..85be67d05 100644
--- a/plugins/auth_remote/init.php
+++ b/plugins/auth_remote/init.php
@@ -58,13 +58,13 @@ class Auth_Remote extends Auth_Base {
 				// LemonLDAP can send user informations via HTTP HEADER
 				if (defined('AUTH_AUTO_CREATE') && AUTH_AUTO_CREATE){
 					// update user name
-					$fullname = $_SERVER['HTTP_USER_NAME'] ? $_SERVER['HTTP_USER_NAME'] : $_SERVER['AUTHENTICATE_CN'];
+					$fullname = isset($_SERVER['HTTP_USER_NAME']) ? $_SERVER['HTTP_USER_NAME'] : ($_SERVER['AUTHENTICATE_CN'] ?? "");
 					if ($fullname){
 						$sth = $this->pdo->prepare("UPDATE ttrss_users SET full_name = ? WHERE id = ?");
 						$sth->execute([$fullname, $user_id]);
 					}
 					// update user mail
-					$email = $_SERVER['HTTP_USER_MAIL'] ? $_SERVER['HTTP_USER_MAIL'] : $_SERVER['AUTHENTICATE_MAIL'];
+					$email = isset($_SERVER['HTTP_USER_MAIL']) ? $_SERVER['HTTP_USER_MAIL'] : ($_SERVER['AUTHENTICATE_MAIL'] ?? "");
 					if ($email){
 						$sth = $this->pdo->prepare("UPDATE ttrss_users SET email = ? WHERE id = ?");
 						$sth->execute([$email, $user_id]);
diff --git a/register.php b/register.php
index 43ca89dea..be0f9d40f 100644
--- a/register.php
+++ b/register.php
@@ -277,7 +277,7 @@
 
 				db_query( "INSERT INTO ttrss_users
 					(login,pwd_hash,access_level,last_login, email, created, salt)
-					VALUES ('$login', '$pwd_hash', 0, null, '$email', NOW(), '$salt')");
+					VALUES (LOWER('$login'), '$pwd_hash', 0, null, '$email', NOW(), '$salt')");
 
 				$result = db_query( "SELECT id FROM ttrss_users WHERE
 					login = '$login' AND pwd_hash = '$pwd_hash'");
diff --git a/update.php b/update.php
index d8c648e69..0bf8f499f 100755
--- a/update.php
+++ b/update.php
@@ -502,7 +502,7 @@
 
 		Debug::log("Exporting feeds of user $user to $filename as OPML...");
 
-		$sth = $pdo->prepare("SELECT id FROM ttrss_users WHERE login = ?");
+		$sth = $pdo->prepare("SELECT id FROM ttrss_users WHERE LOWER(login) = LOWER(?)");
 		$sth->execute([$user]);
 
 		if ($res = $sth->fetch()) {