Harden os password （terminal windows and headless linux) anti brute force (#14985)

* fix(windows): terminal, preauth bruteforce

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix(linux): headless, preauth bruteforce

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix(linux): headless, OS login, minimal fix

Signed-off-by: fufesou <linlong1266@gmail.com>

* Terminal session, click-only

Signed-off-by: fufesou <linlong1266@gmail.com>

* Simple refactor, logs

Signed-off-by: fufesou <linlong1266@gmail.com>

* harden os password, better scoped failure set

Signed-off-by: fufesou <linlong1266@gmail.com>

* harden os password, ip failure count

Signed-off-by: fufesou <linlong1266@gmail.com>

* Check prelogin before starting cm

Signed-off-by: fufesou <linlong1266@gmail.com>

* Isolate terminal OS login failure tracking

Terminal OS login no longer reads or updates the default RustDesk
per-IP failure bucket. It now uses only the OS credential policy, while
RustDesk password attempts keep using the existing LOGIN_FAILURES[0]
bucket.

Signed-off-by: fufesou <linlong1266@gmail.com>

---------

Signed-off-by: fufesou <linlong1266@gmail.com>

src/platform/linux_desktop_manager.rs

@@ -2,7 +2,7 @@ use super::{linux::*, ResultType};
 use crate::client::{
     LOGIN_MSG_DESKTOP_NO_DESKTOP, LOGIN_MSG_DESKTOP_SESSION_ANOTHER_USER,
     LOGIN_MSG_DESKTOP_SESSION_NOT_READY, LOGIN_MSG_DESKTOP_XORG_NOT_FOUND,
-    LOGIN_MSG_DESKTOP_XSESSION_FAILED,
+    LOGIN_MSG_DESKTOP_XSESSION_FAILED, LOGIN_MSG_PASSWORD_WRONG,
 };
 use hbb_common::{
     allow_err, bail, log,
@@ -94,6 +94,49 @@ fn detect_headless() -> Option<&'static str> {
     None
 }
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+enum XSessionStartErrorKind {
+    Auth,
+    Env,
+}
+
+const XSESSION_AUTH_FAILURE_DETAIL: &str = "authentication failed";
+
+#[derive(Debug)]
+struct XSessionStartError {
+    kind: XSessionStartErrorKind,
+    detail: String,
+}
+
+impl XSessionStartError {
+    fn auth(detail: String) -> Self {
+        Self {
+            kind: XSessionStartErrorKind::Auth,
+            detail,
+        }
+    }
+
+    fn env(detail: String) -> Self {
+        Self {
+            kind: XSessionStartErrorKind::Env,
+            detail,
+        }
+    }
+}
+
+impl std::fmt::Display for XSessionStartError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.detail)
+    }
+}
+
+fn map_xsession_start_error_to_login_msg(kind: XSessionStartErrorKind) -> &'static str {
+    match kind {
+        XSessionStartErrorKind::Auth => LOGIN_MSG_PASSWORD_WRONG,
+        XSessionStartErrorKind::Env => LOGIN_MSG_DESKTOP_XSESSION_FAILED,
+    }
+}
+
 pub fn try_start_desktop(_username: &str, _passsword: &str) -> String {
     debug_assert!(crate::is_server());
     if _username.is_empty() {
@@ -136,14 +179,21 @@ pub fn try_start_desktop(_username: &str, _passsword: &str) -> String {
                 }
             }
             Err(e) => {
-                log::error!("Failed to start xsession {}", e);
+                match e.kind {
-                LOGIN_MSG_DESKTOP_XSESSION_FAILED.to_owned()
+                    XSessionStartErrorKind::Auth => {
+                        log::warn!("Failed to authenticate xsession user {}", e);
+                    }
+                    XSessionStartErrorKind::Env => {
+                        log::error!("Failed to start xsession {}", e);
+                    }
+                }
+                map_xsession_start_error_to_login_msg(e.kind).to_owned()
             }
         }
     }
 }
 
-fn try_start_x_session(username: &str, password: &str) -> ResultType<(String, bool)> {
+fn try_start_x_session(username: &str, password: &str) -> Result<(String, bool), XSessionStartError> {
     let mut desktop_manager = DESKTOP_MANAGER.lock().unwrap();
     if let Some(desktop_manager) = &mut (*desktop_manager) {
         if let Some(seat0_username) = desktop_manager.get_supported_display_seat0_username() {
@@ -161,7 +211,9 @@ fn try_start_x_session(username: &str, password: &str) -> ResultType<(String, bo
             desktop_manager.is_running(),
         ))
     } else {
-        bail!(crate::client::LOGIN_MSG_DESKTOP_NOT_INITED);
+        Err(XSessionStartError::env(
+            crate::client::LOGIN_MSG_DESKTOP_NOT_INITED.to_owned(),
+        ))
     }
 }
 
@@ -247,10 +299,15 @@ impl DesktopManager {
         self.is_child_running.load(Ordering::SeqCst)
     }
 
-    fn try_start_x_session(&mut self, username: &str, password: &str) -> ResultType<()> {
+    fn try_start_x_session(
+        &mut self,
+        username: &str,
+        password: &str,
+    ) -> Result<(), XSessionStartError> {
         match get_user_by_name(username) {
             Some(userinfo) => {
-                let mut client = pam::Client::with_password(&pam_get_service_name())?;
+                let mut client = pam::Client::with_password(&pam_get_service_name())
+                    .map_err(|e| XSessionStartError::env(format!("failed to init pam client, {}", e)))?;
                 client
                     .conversation_mut()
                     .set_credentials(username, password);
@@ -267,17 +324,24 @@ impl DesktopManager {
                                 Ok(())
                             }
                             Err(e) => {
-                                bail!("failed to start x session, {}", e);
+                                Err(XSessionStartError::env(format!(
+                                    "failed to start x session, {}",
+                                    e
+                                )))
                             }
                         }
                     }
-                    Err(e) => {
+                    Err(_e) => {
-                        bail!("failed to check user pass for {}, {}", username, e);
+                        Err(XSessionStartError::auth(
+                            XSESSION_AUTH_FAILURE_DETAIL.to_owned(),
+                        ))
                     }
                 }
             }
             None => {
-                bail!("failed to get userinfo of {}", username);
+                Err(XSessionStartError::auth(
+                    XSESSION_AUTH_FAILURE_DETAIL.to_owned(),
+                ))
             }
         }
     }

src/server.rs

@@ -67,6 +67,7 @@ pub mod input_service {
 }
 
 mod connection;
+mod login_failure_check;
 pub mod display_service;
 #[cfg(windows)]
 pub mod portable_service;

src/server/connection.rs

@@ -1,3 +1,8 @@
+#[cfg(target_os = "windows")]
+use super::login_failure_check::try_acquire_os_credential_login_gate;
+use super::login_failure_check::{
+    evaluate_os_credential_policy, record_os_credential_failure, FailureScope,
+};
 use super::{input_service::*, *};
 #[cfg(feature = "unix-file-copy-paste")]
 use crate::clipboard::try_empty_clipboard_files;
@@ -82,6 +87,9 @@ lazy_static::lazy_static! {
     static ref PENDING_SWITCH_SIDES_UUID: Arc::<Mutex<HashMap<String, (Instant, uuid::Uuid)>>> = Default::default();
 }
 
+#[cfg(target_os = "windows")]
+const TERMINAL_OS_LOGIN_FAILED_MSG: &str = "Incorrect username or password.";
+
 fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
     if a.len() != b.len() {
         return false;
@@ -94,6 +102,32 @@ fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
     x == 0
 }
 
+#[cfg(target_os = "linux")]
+fn should_check_linux_headless_os_auth_before_desktop_start(
+    is_headless_allowed: bool,
+    username: &str,
+) -> bool {
+    is_headless_allowed
+        && !username.trim().is_empty()
+        && linux_desktop_manager::get_username().is_empty()
+}
+
+#[cfg(target_os = "linux")]
+fn should_record_linux_headless_os_auth_failure(
+    is_headless_allowed: bool,
+    username: &str,
+    err_msg: &str,
+) -> bool {
+    is_headless_allowed
+        && !username.trim().is_empty()
+        && err_msg == crate::client::LOGIN_MSG_PASSWORD_WRONG
+}
+
+#[cfg(not(any(target_os = "android", target_os = "ios")))]
+fn should_use_terminal_os_login_scope(is_terminal: bool, os_login_username: &str) -> bool {
+    cfg!(target_os = "windows") && is_terminal && !os_login_username.trim().is_empty()
+}
+
 #[cfg(any(target_os = "windows", target_os = "linux"))]
 lazy_static::lazy_static! {
     static ref WALLPAPER_REMOVER: Arc<Mutex<Option<WallPaperRemover>>> = Default::default();
@@ -1497,6 +1531,9 @@ impl Connection {
             // Keep the connection alive so the client can continue with 2FA.
             return true;
         }
+        if let Some(keep_alive) = self.prepare_terminal_login_for_authorization().await {
+            return keep_alive;
+        }
         if !self.connect_port_forward_if_needed().await {
             return false;
         }
@@ -2376,33 +2413,6 @@ impl Connection {
                             o.terminal_persistent.enum_value() == Ok(BoolOption::Yes);
                     }
                     self.terminal_service_id = terminal.service_id;
-                    #[cfg(not(any(target_os = "android", target_os = "ios")))]
-                    if let Some(msg) =
-                        self.fill_terminal_user_token(&lr.os_login.username, &lr.os_login.password)
-                    {
-                        self.send_login_error(msg).await;
-                        sleep(1.).await;
-                        return false;
-                    }
-
-                    #[cfg(not(any(target_os = "android", target_os = "ios")))]
-                    if let Some(is_user) =
-                        terminal_service::is_service_specified_user(&self.terminal_service_id)
-                    {
-                        if let Some(user_token) = &self.terminal_user_token {
-                            let has_service_token =
-                                user_token.to_terminal_service_token().is_some();
-                            if is_user != has_service_token {
-                                // This occurs when the service id (in the configuration) is manually changed by the user, causing a mismatch in validation.
-                                log::error!("Terminal service user mismatch detected. The service ID may have been manually changed in the configuration, causing validation to fail.");
-                                // No need to translate the following message, because it is in an abnormal case.
-                                self.send_login_error("Terminal service user mismatch detected.")
-                                    .await;
-                                sleep(1.).await;
-                                return false;
-                            }
-                        }
-                    }
                 }
                 Some(login_request::Union::PortForward(mut pf)) => {
                     if !Self::permission(keys::OPTION_ENABLE_TUNNEL, &self.control_permissions) {
@@ -2420,8 +2430,43 @@ impl Connection {
                 }
             }
 
+            if !hbb_common::is_ip_str(&lr.username)
+                && !hbb_common::is_domain_port_str(&lr.username)
+                && lr.username != Config::get_id()
+            {
+                self.send_login_error(crate::client::LOGIN_MSG_OFFLINE)
+                    .await;
+                return false;
+            }
+
+            #[cfg(target_os = "windows")]
+            if self.terminal
+                && lr.os_login.username.trim().is_empty()
+                && crate::platform::is_prelogin()
+            {
+                self.send_login_error(
+                    "No active console user logged on, please connect and logon first.",
+                )
+                .await;
+                sleep(1.).await;
+                return false;
+            }
+
             #[cfg(not(any(target_os = "android", target_os = "ios")))]
-            self.try_start_cm_ipc();
+            if !should_use_terminal_os_login_scope(self.terminal, &lr.os_login.username) {
+                self.try_start_cm_ipc();
+            }
+
+            #[cfg(target_os = "linux")]
+            if should_check_linux_headless_os_auth_before_desktop_start(
+                self.linux_headless_handle.is_headless_allowed,
+                &lr.os_login.username,
+            ) {
+                let (_failure, res) = self.check_failure(0).await;
+                if !res {
+                    return true;
+                }
+            }
 
             #[cfg(not(target_os = "linux"))]
             let err_msg = "".to_owned();
@@ -2433,6 +2478,18 @@ impl Connection {
             // If err is LOGIN_MSG_DESKTOP_SESSION_NOT_READY, just keep this msg and go on checking password.
             if !err_msg.is_empty() && err_msg != crate::client::LOGIN_MSG_DESKTOP_SESSION_NOT_READY
             {
+                #[cfg(target_os = "linux")]
+                if should_record_linux_headless_os_auth_failure(
+                    self.linux_headless_handle.is_headless_allowed,
+                    &lr.os_login.username,
+                    &err_msg,
+                ) {
+                    let (failure, res) = self.check_failure(0).await;
+                    if !res {
+                        return true;
+                    }
+                    self.update_failure(failure, false, 0);
+                }
                 self.send_login_error(err_msg).await;
                 return true;
             }
@@ -2461,17 +2518,16 @@ impl Connection {
                 crate::get_builtin_option(keys::OPTION_ALLOW_LOGON_SCREEN_PASSWORD) == "Y"
                     && is_logon();
 
-            if !hbb_common::is_ip_str(&lr.username)
+            if (password::approve_mode() == ApproveMode::Click && !allow_logon_screen_password)
-                && !hbb_common::is_domain_port_str(&lr.username)
-                && lr.username != Config::get_id()
-            {
-                self.send_login_error(crate::client::LOGIN_MSG_OFFLINE)
-                    .await;
-                return false;
-            } else if (password::approve_mode() == ApproveMode::Click
-                && !allow_logon_screen_password)
                 || password::approve_mode() == ApproveMode::Both && !password::has_valid_password()
             {
+                #[cfg(not(any(target_os = "android", target_os = "ios")))]
+                if should_use_terminal_os_login_scope(self.terminal, &lr.os_login.username) {
+                    if let Some(keep_alive) = self.prepare_terminal_login_for_authorization().await
+                    {
+                        return keep_alive;
+                    }
+                }
                 self.try_start_cm(lr.my_id, lr.my_name, false);
                 if hbb_common::get_version_number(&lr.version)
                     >= hbb_common::get_version_number("1.2.0")
@@ -2493,6 +2549,14 @@ impl Connection {
                 }
             } else if lr.password.is_empty() {
                 if err_msg.is_empty() {
+                    #[cfg(not(any(target_os = "android", target_os = "ios")))]
+                    if should_use_terminal_os_login_scope(self.terminal, &lr.os_login.username) {
+                        if let Some(keep_alive) =
+                            self.prepare_terminal_login_for_authorization().await
+                        {
+                            return keep_alive;
+                        }
+                    }
                     self.try_start_cm(lr.my_id, lr.my_name, false);
                 } else {
                     self.send_login_error(
@@ -2506,7 +2570,7 @@ impl Connection {
                     return true;
                 }
                 if !self.validate_password(allow_logon_screen_password) {
-                    self.update_failure(failure, false, 0);
+                    self.update_failure_with_scope(failure, false, 0, FailureScope::Default);
                     self.check_update_temporary_password(false);
                     if err_msg.is_empty() {
                         self.send_login_error(crate::client::LOGIN_MSG_PASSWORD_WRONG)
@@ -2519,7 +2583,7 @@ impl Connection {
                         .await;
                     }
                 } else {
-                    self.update_failure(failure, true, 0);
+                    self.update_failure_with_scope(failure, true, 0, FailureScope::Default);
                     if err_msg.is_empty() {
                         #[cfg(target_os = "linux")]
                         self.linux_headless_handle.wait_desktop_cm_ready().await;
@@ -3484,16 +3548,16 @@ impl Connection {
                     self.terminal_user_token = Some(TerminalUserToken::SelfUser);
                     None
                 } else {
-                    Some("The user is not an administrator.")
+                    Some(TERMINAL_OS_LOGIN_FAILED_MSG)
                 }
             }
             Ok(Err(e)) => {
                 log::error!("Failed to check if the user is an administrator: {}", e);
-                Some("Failed to check if the user is an administrator.")
+                Some(TERMINAL_OS_LOGIN_FAILED_MSG)
             }
             Err(e) => {
                 log::error!("Failed to get logon user token: {}", e);
-                Some("Incorrect username or password.")
+                Some(TERMINAL_OS_LOGIN_FAILED_MSG)
             }
         }
     }
@@ -3529,6 +3593,146 @@ impl Connection {
         }
     }
 
+    #[cfg(not(any(target_os = "android", target_os = "ios")))]
+    async fn prepare_terminal_login_for_authorization(&mut self) -> Option<bool> {
+        if !self.terminal || self.terminal_user_token.is_some() {
+            return None;
+        }
+
+        #[derive(Copy, Clone)]
+        enum TerminalAuthorizationMode {
+            OsLogin {
+                failure: ((i32, i32, i32), i32),
+                scope: FailureScope,
+            },
+            SessionUser,
+        }
+
+        let normalized_username = self.lr.os_login.username.trim().to_owned();
+        let auth_mode = if should_use_terminal_os_login_scope(self.terminal, &normalized_username) {
+            // Check failure state
+            let failure_scope = FailureScope::TerminalOsLogin;
+            let (failure, res) = self.check_failure_with_scope(0, failure_scope).await;
+            if !res {
+                log::warn!(
+                    "OS credential login blocked by failure policy: ip={} conn_id={} scope={:?}",
+                    self.ip,
+                    self.inner.id(),
+                    failure_scope
+                );
+                // Terminal OS login is sensitive. Close this connection instead of keeping it
+                // alive for retries on the same socket after a rate-limit block.
+                return Some(false);
+            }
+            TerminalAuthorizationMode::OsLogin {
+                failure,
+                scope: failure_scope,
+            }
+        } else {
+            TerminalAuthorizationMode::SessionUser
+        };
+
+        let is_terminal_os_login = matches!(auth_mode, TerminalAuthorizationMode::OsLogin { .. });
+        let failure_scope = match auth_mode {
+            TerminalAuthorizationMode::OsLogin { scope, .. } => scope,
+            TerminalAuthorizationMode::SessionUser => FailureScope::Default,
+        };
+
+        let username = normalized_username;
+        let password = self.lr.os_login.password.clone();
+        let terminal_login_error = {
+            #[cfg(target_os = "windows")]
+            {
+                // Concurrency gate for terminal OS login with credentials, to prevent brute-force attacks.
+                let _os_login_concurrency_guard = if is_terminal_os_login {
+                    let guard = try_acquire_os_credential_login_gate();
+                    if guard.is_err() {
+                        log::warn!(
+                            "OS credential login blocked by concurrency gate: ip={} conn_id={} scope={:?}",
+                            self.ip,
+                            self.inner.id(),
+                            failure_scope
+                        );
+                        self.send_login_error("Please try 1 minute later").await;
+                        sleep(1.).await;
+                        Self::post_alarm_audit(
+                            AlarmAuditType::TerminalOsLoginConcurrency,
+                            json!({
+                                "ip": self.ip,
+                                "id": self.lr.my_id.clone(),
+                                "name": self.lr.my_name.clone(),
+                            }),
+                        );
+                        return Some(false);
+                    }
+                    guard.ok()
+                } else {
+                    None
+                };
+                self.fill_terminal_user_token(&username, &password)
+            }
+            #[cfg(not(target_os = "windows"))]
+            {
+                self.fill_terminal_user_token(&username, &password)
+            }
+        };
+        if let Some(msg) = terminal_login_error {
+            if let TerminalAuthorizationMode::OsLogin { failure, scope } = auth_mode {
+                self.update_failure_with_scope(failure, false, 0, scope);
+            }
+            let auth_context = if is_terminal_os_login {
+                "OS credential login verification"
+            } else {
+                "Terminal session-user authorization"
+            };
+            log::warn!(
+                "{} failed: ip={} conn_id={} scope={:?} msg='{}'",
+                auth_context,
+                self.ip,
+                self.inner.id(),
+                failure_scope,
+                msg
+            );
+            self.send_login_error(msg).await;
+            sleep(1.).await;
+            return Some(false);
+        }
+        if let TerminalAuthorizationMode::OsLogin { failure, scope } = auth_mode {
+            self.update_failure_with_scope(failure, true, 0, scope);
+        }
+
+        if let Some(is_user) =
+            terminal_service::is_service_specified_user(&self.terminal_service_id)
+        {
+            if let Some(user_token) = &self.terminal_user_token {
+                let has_service_token = user_token.to_terminal_service_token().is_some();
+                if is_user != has_service_token {
+                    log::error!(
+                        "Terminal service user mismatch: ip={} conn_id={} service_is_user={} has_service_token={}. The service ID may have been manually changed in the configuration, causing validation to fail.",
+                        self.ip,
+                        self.inner.id(),
+                        is_user,
+                        has_service_token
+                    );
+                    // No need to translate the following message, because it is in an abnormal case.
+                    self.send_login_error("Terminal service user mismatch detected.")
+                        .await;
+                    sleep(1.).await;
+                    return Some(false);
+                }
+            }
+        }
+        if is_terminal_os_login {
+            self.try_start_cm_ipc();
+        }
+        None
+    }
+
+    #[cfg(any(target_os = "android", target_os = "ios"))]
+    async fn prepare_terminal_login_for_authorization(&mut self) -> Option<bool> {
+        None
+    }
+
     // Try to parse connection IP as IPv6 address, returning /64, /56, and /48 prefixes.
     // Parsing an IPv4 address just returns None.
     // note: we specifically don't use hbb_common::is_ipv6_str to avoid divergence issues
@@ -3555,18 +3759,37 @@ impl Connection {
         Some((p64, p56, p48))
     }
 
-    fn update_failure(&self, (failure, time): ((i32, i32, i32), i32), remove: bool, i: usize) {
+    fn bump_failure_entry(mut cur: (i32, i32, i32), time: i32) -> (i32, i32, i32) {
-        fn bump(mut cur: (i32, i32, i32), time: i32) -> (i32, i32, i32) {
+        if cur.0 == time {
-            if cur.0 == time {
+            cur.1 += 1;
-                cur.1 += 1;
+            cur.2 += 1;
-                cur.2 += 1;
+        } else {
-            } else {
+            cur.0 = time;
-                cur.0 = time;
+            cur.1 = 1;
-                cur.1 = 1;
+            cur.2 += 1;
-                cur.2 += 1;
-            }
-            cur
         }
+        cur
+    }
+
+    fn update_failure(&self, failure: ((i32, i32, i32), i32), remove: bool, i: usize) {
+        self.update_failure_with_scope(failure, remove, i, FailureScope::Default);
+    }
+
+    fn update_failure_with_scope(
+        &self,
+        (failure, time): ((i32, i32, i32), i32),
+        remove: bool,
+        i: usize,
+        scope: FailureScope,
+    ) {
+        let os_credential_scope = matches!(scope, FailureScope::TerminalOsLogin);
+        if os_credential_scope {
+            if !remove {
+                record_os_credential_failure(scope);
+            }
+            return;
+        }
+
         let map_mutex = &LOGIN_FAILURES[i];
         if remove {
             if failure.0 != 0 {
@@ -3587,14 +3810,15 @@ impl Connection {
             let mut m = map_mutex.lock().unwrap();
             for key in [p64, p56, p48] {
                 let cur = m.get(&key).copied().unwrap_or((0, 0, 0));
-                m.insert(key, bump(cur, time));
+                m.insert(key, Self::bump_failure_entry(cur, time));
             }
-            // Update full IP: bump from the *original* passed-in failure
+            let current_ip = m.get(&self.ip).copied().unwrap_or((0, 0, 0));
-            m.insert(self.ip.clone(), bump(failure, time));
+            m.insert(self.ip.clone(), Self::bump_failure_entry(current_ip, time));
         } else {
-            // Update full IP: bump from the *original* passed-in failure
+            // Re-read the full IP bucket in case another failed attempt updated it.
             let mut m = map_mutex.lock().unwrap();
-            m.insert(self.ip.clone(), bump(failure, time));
+            let current_ip = m.get(&self.ip).copied().unwrap_or((0, 0, 0));
+            m.insert(self.ip.clone(), Self::bump_failure_entry(current_ip, time));
         }
     }
 
@@ -3634,8 +3858,50 @@ impl Connection {
     }
 
     async fn check_failure(&mut self, i: usize) -> (((i32, i32, i32), i32), bool) {
+        self.check_failure_with_scope(i, FailureScope::Default)
+            .await
+    }
+
+    async fn check_failure_with_scope(
+        &mut self,
+        i: usize,
+        scope: FailureScope,
+    ) -> (((i32, i32, i32), i32), bool) {
         let time = (get_time() / 60_000) as i32;
 
+        if matches!(scope, FailureScope::TerminalOsLogin) {
+            let decision = evaluate_os_credential_policy(scope, get_time());
+            let res = if decision.allowed {
+                true
+            } else {
+                log::warn!(
+                    "OS credential login blocked by policy: ip={} conn_id={} i={} msg='{}'",
+                    self.ip,
+                    self.inner.id(),
+                    i,
+                    decision.login_error.as_deref().unwrap_or("")
+                );
+                if let Some(login_error) = decision.login_error {
+                    // Rare branch and currently temporary response copy; translation can be added later if needed.
+                    self.send_login_error(login_error).await;
+                }
+                if let Some(audit) = decision.audit {
+                    // For OS blocked/backoff events, we currently emit one alarm report per blocked attempt.
+                    // TODO: Add unified cumulative/aggregation fields across alarm producers.
+                    Self::post_alarm_audit(
+                        audit,
+                        json!({
+                                    "ip": self.ip,
+                                    "id": self.lr.my_id.clone(),
+                                    "name": self.lr.my_name.clone(),
+                        }),
+                    );
+                }
+                false
+            };
+            return (((0, 0, 0), time), res);
+        }
+
         // IPv6 addresses are cheap to make so we check prefix/netblock as well
         if let Some((p64, p56, p48)) = self.get_ipv6_prefixes() {
             if let Some(res) = self.check_failure_ipv6_prefix(i, time, &p64, 64, 60).await {
@@ -5219,6 +5485,8 @@ pub enum AlarmAuditType {
     // MultipleLoginsAttemptsWithinOneMinute = 4,
     // MultipleLoginsAttemptsWithinOneHour = 5,
     ExceedIPv6PrefixAttempts = 6,
+    TerminalOsLoginBackoff = 7,
+    TerminalOsLoginConcurrency = 8,
 }
 
 pub enum FileAuditType {

src/server/login_failure_check.rs

@@ -0,0 +1,231 @@
+use crate::AlarmAuditType;
+use hbb_common::get_time;
+#[cfg(target_os = "windows")]
+use hbb_common::tokio::sync::{Mutex as TokioMutex, OwnedMutexGuard};
+use std::sync::Mutex;
+#[cfg(target_os = "windows")]
+use std::sync::Arc;
+
+const OS_CREDENTIAL_LOGIN_TOTAL_IDLE_RESET_MS: i64 = 120 * 60 * 1_000;
+const OS_CREDENTIAL_LOGIN_BACKOFF_BASE_SECONDS: i64 = 15;
+const OS_CREDENTIAL_LOGIN_BACKOFF_MAX_SECONDS: i64 = 30 * 60;
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub(crate) enum FailureScope {
+    Default,
+    TerminalOsLogin,
+}
+
+pub(crate) struct OsCredentialPolicyDecision {
+    pub allowed: bool,
+    pub login_error: Option<String>,
+    pub audit: Option<AlarmAuditType>,
+}
+
+#[derive(Copy, Clone, Debug, Default)]
+struct OsCredentialFailureState {
+    total_failures: i32,
+    backoff_until_ms: Option<i64>,
+    last_failure_ms: Option<i64>,
+}
+
+lazy_static::lazy_static! {
+    static ref OS_CREDENTIAL_LOGIN_FAILURE_STATE: Mutex<OsCredentialFailureState> =
+        Mutex::new(OsCredentialFailureState::default());
+}
+
+#[cfg(target_os = "windows")]
+lazy_static::lazy_static! {
+    static ref OS_CREDENTIAL_LOGIN_MUTEX: Arc<TokioMutex<()>> = Arc::new(TokioMutex::new(()));
+}
+
+fn is_os_credential_scope(scope: FailureScope) -> bool {
+    matches!(scope, FailureScope::TerminalOsLogin)
+}
+
+fn state_for_os_credential_scope(
+    scope: FailureScope,
+) -> Option<&'static Mutex<OsCredentialFailureState>> {
+    if is_os_credential_scope(scope) {
+        Some(&OS_CREDENTIAL_LOGIN_FAILURE_STATE)
+    } else {
+        None
+    }
+}
+
+fn backoff_audit_type_for_scope(scope: FailureScope) -> Option<AlarmAuditType> {
+    match scope {
+        FailureScope::TerminalOsLogin => Some(AlarmAuditType::TerminalOsLoginBackoff),
+        FailureScope::Default => None,
+    }
+}
+
+fn os_credential_login_backoff_seconds(total_failures: i32) -> i64 {
+    if total_failures <= 2 {
+        return 0;
+    }
+    let exp = (total_failures - 3).min(7);
+    let seconds = OS_CREDENTIAL_LOGIN_BACKOFF_BASE_SECONDS * (1_i64 << exp);
+    seconds.min(OS_CREDENTIAL_LOGIN_BACKOFF_MAX_SECONDS)
+}
+
+fn normalize_backoff(state: &mut OsCredentialFailureState, now_ms: i64) {
+    if let Some(until_ms) = state.backoff_until_ms {
+        if until_ms <= now_ms {
+            state.backoff_until_ms = None;
+        }
+    }
+}
+
+fn reset_totals_on_idle(state: &mut OsCredentialFailureState, now_ms: i64) {
+    if let Some(last_ms) = state.last_failure_ms {
+        if now_ms.saturating_sub(last_ms) >= OS_CREDENTIAL_LOGIN_TOTAL_IDLE_RESET_MS {
+            state.total_failures = 0;
+            state.backoff_until_ms = None;
+            state.last_failure_ms = None;
+        }
+    }
+}
+
+fn allow_decision() -> OsCredentialPolicyDecision {
+    OsCredentialPolicyDecision {
+        allowed: true,
+        login_error: None,
+        audit: None,
+    }
+}
+
+fn block_decision(
+    login_error: String,
+    alarm_type: Option<AlarmAuditType>,
+) -> OsCredentialPolicyDecision {
+    OsCredentialPolicyDecision {
+        allowed: false,
+        login_error: Some(login_error),
+        audit: alarm_type,
+    }
+}
+
+pub(crate) fn evaluate_os_credential_policy(
+    scope: FailureScope,
+    now_ms: i64,
+) -> OsCredentialPolicyDecision {
+    if !is_os_credential_scope(scope) {
+        return allow_decision();
+    }
+    let Some(state_mutex) = state_for_os_credential_scope(scope) else {
+        return allow_decision();
+    };
+    let mut state = state_mutex.lock().unwrap();
+    reset_totals_on_idle(&mut state, now_ms);
+    normalize_backoff(&mut state, now_ms);
+
+    if let Some(until_ms) = state.backoff_until_ms {
+        let remaining_ms = (until_ms - now_ms).max(0);
+        let remaining_seconds = ((remaining_ms + 999) / 1_000).max(1);
+        let seconds_label = if remaining_seconds == 1 {
+            "second"
+        } else {
+            "seconds"
+        };
+        block_decision(
+            format!(
+                "Please try again in {} {}.",
+                remaining_seconds, seconds_label
+            ),
+            backoff_audit_type_for_scope(scope),
+        )
+    } else {
+        allow_decision()
+    }
+}
+
+pub(crate) fn record_os_credential_failure(scope: FailureScope) {
+    if !is_os_credential_scope(scope) {
+        return;
+    }
+    let Some(state_mutex) = state_for_os_credential_scope(scope) else {
+        return;
+    };
+    let mut state = state_mutex.lock().unwrap();
+    let now_ms = get_time();
+    reset_totals_on_idle(&mut state, now_ms);
+    normalize_backoff(&mut state, now_ms);
+    state.total_failures = state.total_failures.saturating_add(1);
+    state.last_failure_ms = Some(now_ms);
+    let backoff_seconds = os_credential_login_backoff_seconds(state.total_failures);
+    if backoff_seconds > 0 {
+        state.backoff_until_ms = Some(now_ms + backoff_seconds * 1_000);
+    }
+}
+
+#[cfg(target_os = "windows")]
+pub(crate) fn try_acquire_os_credential_login_gate() -> Result<OwnedMutexGuard<()>, ()> {
+    OS_CREDENTIAL_LOGIN_MUTEX
+        .clone()
+        .try_lock_owned()
+        .map_err(|_| ())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    static TEST_MUTEX: Mutex<()> = Mutex::new(());
+
+    fn clear_os_credential_failure_state(scope: FailureScope) {
+        if let Some(state_mutex) = state_for_os_credential_scope(scope) {
+            *state_mutex.lock().unwrap() = OsCredentialFailureState::default();
+        }
+    }
+
+    #[test]
+    fn os_credential_policy_prioritizes_backoff() {
+        let _guard = TEST_MUTEX.lock().unwrap();
+        clear_os_credential_failure_state(FailureScope::TerminalOsLogin);
+        let now_ms = get_time();
+        for _ in 0..3 {
+            record_os_credential_failure(FailureScope::TerminalOsLogin);
+        }
+        let decision = evaluate_os_credential_policy(FailureScope::TerminalOsLogin, now_ms);
+        assert!(!decision.allowed);
+        assert!(decision.login_error.is_some());
+        clear_os_credential_failure_state(FailureScope::TerminalOsLogin);
+    }
+
+    #[test]
+    fn os_credential_policy_idle_window_resets_total_counter() {
+        let _guard = TEST_MUTEX.lock().unwrap();
+        clear_os_credential_failure_state(FailureScope::TerminalOsLogin);
+        for _ in 0..13 {
+            record_os_credential_failure(FailureScope::TerminalOsLogin);
+        }
+        let blocked = evaluate_os_credential_policy(FailureScope::TerminalOsLogin, get_time());
+        assert!(!blocked.allowed);
+
+        let after_failures_ms = get_time();
+        let after_idle_ms = after_failures_ms + OS_CREDENTIAL_LOGIN_TOTAL_IDLE_RESET_MS + 1_000;
+        let allowed = evaluate_os_credential_policy(FailureScope::TerminalOsLogin, after_idle_ms);
+        assert!(allowed.allowed);
+        clear_os_credential_failure_state(FailureScope::TerminalOsLogin);
+    }
+
+    #[test]
+    fn os_credential_policy_audits_every_backoff_block() {
+        let _guard = TEST_MUTEX.lock().unwrap();
+        clear_os_credential_failure_state(FailureScope::TerminalOsLogin);
+
+        for _ in 0..3 {
+            record_os_credential_failure(FailureScope::TerminalOsLogin);
+        }
+        let now_ms = get_time();
+        let first = evaluate_os_credential_policy(FailureScope::TerminalOsLogin, now_ms);
+        let second = evaluate_os_credential_policy(FailureScope::TerminalOsLogin, now_ms + 1_000);
+        assert!(!first.allowed);
+        assert!(!second.allowed);
+        assert!(first.audit.is_some());
+        assert!(second.audit.is_some());
+
+        clear_os_credential_failure_state(FailureScope::TerminalOsLogin);
+    }
+}