Harden Windows control socket rendezvous (#42326)

## Why

Windows control sockets need a rendezvous directory that cannot be accessed or
replaced by another user while the listener is active.

## What changed

- Create socket directories with a protected, inheritable, current-user-only
  DACL, and reject existing directories with broader permissions.
- Validate socket paths without following a junction at the rendezvous
  directory, and pin the directory through listener cleanup to prevent
  replacement after validation.
- Add a Windows peer check that verifies the kernel-reported peer process uses
  the current user's non-elevated token.
- Document the Windows requirements for custom control socket paths.

## Testing

Add Windows coverage for ACL inheritance and rejection, junction and volume-root
rejection, extended-length paths, directory pinning, and elevated peer rejection.

GitOrigin-RevId: cc8dd76e5732c288f6346cfd6138051188533bfa

codex-rs/Cargo.lock

@@ -4622,6 +4622,7 @@ dependencies = [
  "tokio",
  "tokio-util",
  "uds_windows",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]

codex-rs/app-server-transport/src/transport/unix_socket.rs

@@ -1,3 +1,5 @@
+//! Control socket startup, guarded rendezvous paths, and WebSocket acceptance.
+
 use std::fs::OpenOptions;
 use std::io::ErrorKind;
 use std::io::Result as IoResult;
@@ -26,9 +28,21 @@ pub async fn start_control_socket_acceptor(
     transport_event_tx: mpsc::Sender<TransportEvent>,
     shutdown_token: CancellationToken,
 ) -> IoResult<JoinHandle<()>> {
+    #[cfg(windows)]
+    let (socket_path, directory_guard) = {
+        if let Some(parent) = socket_path.as_path().parent() {
+            codex_uds::prepare_private_socket_directory(parent).await?;
+        }
+        let (path, guard) = codex_uds::validate_private_socket_path(socket_path.as_path())?;
+        (AbsolutePathBuf::from_absolute_path_checked(path)?, guard)
+    };
     prepare_control_socket_path(socket_path.as_path()).await?;
     let listener = UnixListener::bind(socket_path.as_path()).await?;
-    let socket_guard = ControlSocketFileGuard { socket_path };
+    let socket_guard = ControlSocketFileGuard {
+        socket_path,
+        #[cfg(windows)]
+        _directory_guard: directory_guard,
+    };
     set_control_socket_permissions(socket_guard.socket_path.as_path()).await?;
     info!(
         socket_path = %socket_guard.socket_path.display(),
@@ -95,6 +109,13 @@ pub async fn prepare_control_socket_path(socket_path: &Path) -> IoResult<()> {
         codex_uds::prepare_private_socket_directory(parent).await?;
     }
 
+    #[cfg(windows)]
+    let (socket_path, _directory_guard) = codex_uds::validate_private_socket_path(socket_path)?;
+    #[cfg(windows)]
+    let socket_path = AbsolutePathBuf::from_absolute_path_checked(socket_path)?;
+    #[cfg(windows)]
+    let socket_path = socket_path.as_path();
+
     match UnixStream::connect(socket_path).await {
         Ok(_stream) => {
             return Err(std::io::Error::new(
@@ -173,6 +194,9 @@ async fn set_control_socket_permissions(_socket_path: &Path) -> IoResult<()> {
 
 struct ControlSocketFileGuard {
     socket_path: AbsolutePathBuf,
+    // Keep the directory pinned until after the socket file is removed in Drop.
+    #[cfg(windows)]
+    _directory_guard: std::os::windows::io::OwnedHandle,
 }
 
 impl Drop for ControlSocketFileGuard {

codex-rs/app-server-transport/src/transport/unix_socket_tests.rs

@@ -190,6 +190,24 @@ async fn control_socket_file_is_private_after_bind() {
     accept_handle.await.expect("acceptor should join");
 }
 
+#[cfg(windows)]
+#[tokio::test]
+async fn control_socket_pins_directory_until_shutdown() {
+    let temp_dir = tempfile::TempDir::new().expect("temp dir");
+    let socket_path = test_socket_path(temp_dir.path());
+    let directory = socket_path.as_path().parent().unwrap();
+    let moved = temp_dir.path().join("moved");
+    let (tx, _rx) = mpsc::channel::<TransportEvent>(CHANNEL_CAPACITY);
+    let shutdown = CancellationToken::new();
+    let acceptor = start_control_socket_acceptor(socket_path.clone(), tx, shutdown.clone())
+        .await
+        .expect("acceptor");
+    assert!(std::fs::rename(directory, &moved).is_err());
+    shutdown.cancel();
+    acceptor.await.expect("shutdown");
+    std::fs::rename(directory, moved).expect("directory unpinned after cleanup");
+}
+
 fn absolute_path(path: &str) -> AbsolutePathBuf {
     AbsolutePathBuf::from_absolute_path(path).expect("absolute path")
 }

codex-rs/app-server/README.md

@@ -43,6 +43,11 @@ opens exactly one raw stream connection to `$CODEX_HOME/app-server-control/app-s
 by default, or to `--sock PATH` when provided, and proxies bytes between that socket and stdin/stdout.
 The proxied stream carries the websocket HTTP Upgrade handshake followed by websocket frames.
 
+On Windows, the socket directory is created with a protected current-user-only DACL. Existing
+directories must already have that owner and DACL; startup rejects broader permissions rather
+than attempting to repair previously exposed state. Custom sockets should use a new dedicated
+subdirectory. The listener pins the validated directory until socket cleanup completes.
+
 Tracing/log output:
 
 - `RUST_LOG` controls log filtering/verbosity.

codex-rs/app-server/tests/suite/v2/remote_control.rs

@@ -213,7 +213,10 @@ async fn explicit_remote_control_startup_fails_when_disabled_by_requirements() -
         "allow_remote_control = false\n",
     )?;
     let managed_config_path = codex_home.path().join("managed_config.toml");
-    let socket_path = codex_home.path().join("app-server.sock");
+    let socket_path = codex_home
+        .path()
+        .join("app-server-control")
+        .join("app-server.sock");
     let transport =
         AppServerTransport::from_listen_url(&format!("unix://{}", socket_path.display()))?;
     let _codex_home_guard = EnvVarGuard::set("CODEX_HOME", codex_home.path().as_os_str());

codex-rs/uds/Cargo.toml

@@ -19,6 +19,7 @@ tokio = { workspace = true, features = ["fs", "net", "rt"] }
 async-io = { workspace = true }
 tokio-util = { workspace = true, features = ["compat"] }
 uds_windows = { workspace = true }
+windows-sys = { version = "0.52", features = ["Win32_Foundation", "Win32_Security", "Win32_Security_Authorization", "Win32_Storage_FileSystem", "Win32_System_Threading", "Win32_System_SystemServices", "Win32_Networking_WinSock", "Win32_System_IO"] }
 
 [dev-dependencies]
 pretty_assertions = { workspace = true }

codex-rs/uds/src/lib.rs

@@ -10,8 +10,10 @@ use tokio::io::AsyncRead;
 use tokio::io::AsyncWrite;
 use tokio::io::ReadBuf;
 
-/// Creates `socket_dir` if needed and restricts it to the current user where
-/// the platform exposes Unix permissions.
+/// Creates `socket_dir` if needed and restricts it to the current user as
+/// supported by the platform (0700 on Unix, a protected user-only DACL on Windows).
+/// Windows rejects existing directories unless they already have that owner and DACL;
+/// changing permissions cannot revoke access held through preexisting handles.
 pub async fn prepare_private_socket_directory(socket_dir: impl AsRef<Path>) -> IoResult<()> {
     platform::prepare_private_socket_directory(socket_dir.as_ref()).await
 }
@@ -50,6 +52,13 @@ pub struct UnixStream {
 }
 
 impl UnixStream {
+    /// Requires the Windows peer to belong to the current user, with neither
+    /// process elevated. Call before sending any application data.
+    #[cfg(windows)]
+    pub fn ensure_non_elevated_peer(&self) -> IoResult<()> {
+        self.inner.ensure_non_elevated_peer()
+    }
+
     /// Connects to `socket_path`.
     pub async fn connect(socket_path: impl AsRef<Path>) -> IoResult<Self> {
         platform::connect_stream(socket_path.as_ref())
@@ -184,8 +193,18 @@ mod platform {
 
     pub(super) struct Stream(Compat<Async<WindowsUnixStream>>);
 
+    impl Stream {
+        pub(super) fn ensure_non_elevated_peer(&self) -> IoResult<()> {
+            crate::windows_peer::ensure_non_elevated_peer(
+                self.0.get_ref().get_ref().as_raw_socket(),
+            )
+        }
+    }
+
     pub(super) async fn prepare_private_socket_directory(socket_dir: &Path) -> IoResult<()> {
-        tokio::fs::create_dir_all(socket_dir).await
+        let socket_dir = socket_dir.to_path_buf();
+        spawn_blocking_io(move || crate::windows_security::prepare_private_directory(&socket_dir))
+            .await
     }
 
     pub(super) struct Listener(Async<WindowsUnixListener>);
@@ -329,3 +348,14 @@ mod platform {
 
 #[cfg(test)]
 mod lib_tests;
+
+#[cfg(windows)]
+mod windows_security;
+
+#[cfg(windows)]
+mod windows_socket_validation;
+#[cfg(windows)]
+pub use windows_socket_validation::validate_private_socket_path;
+
+#[cfg(windows)]
+mod windows_peer;

codex-rs/uds/src/lib_tests.rs

@@ -6,6 +6,168 @@ use tokio::io::AsyncWriteExt;
 
 use super::*;
 
+#[cfg(windows)]
+#[tokio::test]
+async fn private_directory_rejects_volume_roots() {
+    // A nonexistent volume keeps this regression safe even if root validation breaks.
+    let root = Path::new(r"\\?\Volume{00000000-0000-0000-0000-000000000000}\");
+    assert_eq!(
+        prepare_private_socket_directory(root)
+            .await
+            .unwrap_err()
+            .kind(),
+        ErrorKind::InvalidInput,
+    );
+}
+
+#[cfg(windows)]
+#[tokio::test]
+async fn private_directory_rejects_junctions() {
+    let temp = tempfile::TempDir::new().expect("temp directory");
+    let target = temp.path().join("target");
+    let junction = temp.path().join("junction");
+    std::fs::create_dir(&target).expect("target");
+    let output = std::process::Command::new("powershell.exe")
+        .args(["-NoProfile", "-NonInteractive", "-Command", "$ErrorActionPreference = 'Stop'; New-Item -ItemType Junction -Path $env:CODEX_TEST_LINK -Target $env:CODEX_TEST_TARGET | Out-Null"])
+        .env("CODEX_TEST_LINK", &junction)
+        .env("CODEX_TEST_TARGET", &target)
+        .output().expect("create junction");
+    assert!(
+        output.status.success(),
+        "{}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    assert_eq!(
+        prepare_private_socket_directory(&junction)
+            .await
+            .unwrap_err()
+            .kind(),
+        ErrorKind::PermissionDenied,
+    );
+    assert_eq!(
+        validate_private_socket_path(&junction.join("socket"))
+            .unwrap_err()
+            .kind(),
+        ErrorKind::PermissionDenied,
+    );
+}
+
+#[cfg(windows)]
+#[tokio::test]
+async fn socket_validation_pins_private_directory_without_creating_missing_paths() {
+    let temp = tempfile::TempDir::new().expect("temp directory");
+    let directory = temp.path().join("private");
+    let socket_path = directory.join("socket");
+    assert!(validate_private_socket_path(&socket_path).is_err());
+    assert!(!directory.exists());
+    prepare_private_socket_directory(&directory).await.unwrap();
+    let (_validated_path, _guard) = validate_private_socket_path(&socket_path).unwrap();
+    assert!(std::fs::rename(&directory, temp.path().join("moved")).is_err());
+}
+
+#[cfg(windows)]
+#[tokio::test]
+async fn socket_validation_rejects_broad_acl_without_repairing_it() {
+    let temp = tempfile::TempDir::new().expect("temp directory");
+    let directory = temp.path().join("private");
+    prepare_private_socket_directory(&directory).await.unwrap();
+    let inspect = |script: &str| {
+        let output = std::process::Command::new("powershell.exe")
+            .args(["-NoProfile", "-NonInteractive", "-Command", script])
+            .env("CODEX_TEST_DIRECTORY", &directory)
+            .output()
+            .unwrap();
+        assert!(
+            output.status.success(),
+            "{}",
+            String::from_utf8_lossy(&output.stderr)
+        );
+        output.stdout
+    };
+    let before = inspect(
+        r#"
+$ErrorActionPreference = 'Stop'
+$acl = Get-Acl -LiteralPath $env:CODEX_TEST_DIRECTORY
+$everyone = [System.Security.Principal.SecurityIdentifier]::new('S-1-1-0')
+$rule = [System.Security.AccessControl.FileSystemAccessRule]::new($everyone, 'FullControl', 'ContainerInherit,ObjectInherit', 'None', 'Allow')
+$acl.AddAccessRule($rule)
+Set-Acl -LiteralPath $env:CODEX_TEST_DIRECTORY -AclObject $acl
+(Get-Acl -LiteralPath $env:CODEX_TEST_DIRECTORY).Sddl
+"#,
+    );
+    assert_eq!(
+        validate_private_socket_path(&directory.join("socket"))
+            .unwrap_err()
+            .kind(),
+        ErrorKind::PermissionDenied,
+    );
+    assert_eq!(
+        prepare_private_socket_directory(&directory)
+            .await
+            .unwrap_err()
+            .kind(),
+        ErrorKind::PermissionDenied,
+    );
+    assert_eq!(
+        before,
+        inspect(
+            "$ErrorActionPreference = 'Stop'; (Get-Acl -LiteralPath $env:CODEX_TEST_DIRECTORY).Sddl"
+        )
+    );
+}
+
+#[cfg(windows)]
+#[tokio::test]
+async fn private_directory_supports_extended_length_paths() {
+    let temp = tempfile::TempDir::new().expect("temp directory");
+    let directory = temp.path().join("a".repeat(150)).join("b".repeat(150));
+    prepare_private_socket_directory(&directory)
+        .await
+        .expect("create long private directory");
+    prepare_private_socket_directory(&directory)
+        .await
+        .expect("validate long private directory");
+    assert!(directory.is_dir());
+}
+
+#[cfg(windows)]
+#[tokio::test]
+async fn private_directory_acl_is_user_only_and_inherited_by_files() {
+    let temp = tempfile::TempDir::new().expect("temp directory");
+    let directory = temp.path().join("private");
+    // Cover reusing an existing private directory as well as inheriting its DACL.
+    prepare_private_socket_directory(&directory)
+        .await
+        .expect("create user-owned directory");
+    prepare_private_socket_directory(&directory)
+        .await
+        .expect("private directory");
+    std::fs::write(directory.join("child"), b"").expect("child file");
+    let _listener = UnixListener::bind(directory.join("socket"))
+        .await
+        .expect("private socket");
+    let output = std::process::Command::new("powershell.exe")
+        .args(["-NoProfile", "-NonInteractive", "-Command", r#"
+$ErrorActionPreference = 'Stop'
+$sid = [System.Security.Principal.WindowsIdentity]::GetCurrent().User.Value
+$directory = Get-Acl -LiteralPath $env:CODEX_TEST_PRIVATE_DIRECTORY
+if (-not $directory.AreAccessRulesProtected) { throw 'directory inherits broad access' }
+foreach ($acl in @($directory, (Get-Acl -LiteralPath (Join-Path $env:CODEX_TEST_PRIVATE_DIRECTORY 'child')), (Get-Acl -LiteralPath (Join-Path $env:CODEX_TEST_PRIVATE_DIRECTORY 'socket')))) {
+    $rules = @($acl.Access)
+    if ($rules.Count -ne 1) { throw 'unexpected access rules' }
+    if ($rules[0].IdentityReference.Translate([System.Security.Principal.SecurityIdentifier]).Value -ne $sid) { throw 'wrong user' }
+    if ($rules[0].AccessControlType -ne 'Allow') { throw 'user is denied access' }
+}
+"#])
+        .env("CODEX_TEST_PRIVATE_DIRECTORY", &directory)
+        .output().expect("inspect ACLs");
+    assert!(
+        output.status.success(),
+        "{}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+}
+
 #[tokio::test]
 async fn prepare_private_socket_directory_creates_directory() {
     let temp_dir = tempfile::TempDir::new().expect("temp dir");
@@ -119,3 +281,27 @@ async fn stream_round_trips_data_between_listener_and_client() {
 
     server_task.await.expect("server task should join");
 }
+
+#[cfg(windows)]
+#[tokio::test]
+async fn implicit_peer_validation_rejects_elevated_listener() {
+    let temp = tempfile::TempDir::new().unwrap();
+    let path = temp.path().join("socket");
+    let mut listener = UnixListener::bind(&path).await.unwrap();
+    let client = UnixStream::connect(&path).await.unwrap();
+    let _server = listener.accept().await.unwrap();
+    // PowerShell inherits our token and supplies an independent elevation oracle.
+    let output = std::process::Command::new("powershell.exe")
+        .args(["-NoProfile", "-NonInteractive", "-Command", "([Security.Principal.WindowsPrincipal] [Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)"])
+        .output().unwrap();
+    assert!(output.status.success());
+    let expected = match String::from_utf8(output.stdout).unwrap().trim() {
+        "True" => Err(ErrorKind::PermissionDenied),
+        "False" => Ok(()),
+        other => panic!("unexpected elevation result: {other}"),
+    };
+    assert_eq!(
+        client.ensure_non_elevated_peer().map_err(|err| err.kind()),
+        expected
+    );
+}

codex-rs/uds/src/windows_peer.rs

@@ -0,0 +1,103 @@
+//! Authenticate implicitly discovered Windows socket peers before sending data.
+//! The socket's kernel-reported PID, rather than a PID file, identifies the token.
+
+use std::io;
+use std::os::windows::io::FromRawHandle;
+use std::os::windows::io::OwnedHandle;
+use std::os::windows::io::RawSocket;
+use std::ptr;
+
+use windows_sys::Win32::Networking::WinSock::SIO_AF_UNIX_GETPEERPID;
+use windows_sys::Win32::Networking::WinSock::SOCKET_ERROR;
+use windows_sys::Win32::Networking::WinSock::WSAGetLastError;
+use windows_sys::Win32::Networking::WinSock::WSAIoctl;
+use windows_sys::Win32::Security::EqualSid;
+use windows_sys::Win32::Security::GetTokenInformation;
+use windows_sys::Win32::Security::TOKEN_ELEVATION;
+use windows_sys::Win32::Security::TOKEN_QUERY;
+use windows_sys::Win32::Security::TOKEN_USER;
+use windows_sys::Win32::Security::TokenElevation;
+use windows_sys::Win32::System::Threading::GetCurrentProcess;
+use windows_sys::Win32::System::Threading::OpenProcess;
+use windows_sys::Win32::System::Threading::OpenProcessToken;
+use windows_sys::Win32::System::Threading::PROCESS_QUERY_LIMITED_INFORMATION;
+
+pub(super) fn ensure_non_elevated_peer(socket: RawSocket) -> io::Result<()> {
+    let mut pid = 0u32;
+    let mut returned = 0;
+    if unsafe {
+        WSAIoctl(
+            socket as _,
+            SIO_AF_UNIX_GETPEERPID,
+            ptr::null(),
+            /*cbinbuffer*/ 0,
+            ptr::addr_of_mut!(pid).cast(),
+            std::mem::size_of_val(&pid) as u32,
+            &mut returned,
+            ptr::null_mut(),
+            /*lpcompletionroutine*/ None,
+        )
+    } == SOCKET_ERROR
+    {
+        return Err(io::Error::from_raw_os_error(unsafe { WSAGetLastError() }));
+    }
+    // Older Windows versions can return a valid PID with zero bytes reported.
+    if pid == 0 {
+        return Err(io::Error::new(
+            io::ErrorKind::PermissionDenied,
+            "socket peer PID unavailable",
+        ));
+    }
+    let process = unsafe {
+        OpenProcess(
+            PROCESS_QUERY_LIMITED_INFORMATION,
+            /*binherithandle*/ 0,
+            pid,
+        )
+    };
+    if process == 0 {
+        return Err(io::Error::last_os_error());
+    }
+    let _process = unsafe { OwnedHandle::from_raw_handle(process as _) };
+    let mut token = 0;
+    if unsafe { OpenProcessToken(process, TOKEN_QUERY, &mut token) } == 0 {
+        return Err(io::Error::last_os_error());
+    }
+    let _token = unsafe { OwnedHandle::from_raw_handle(token as _) };
+    let mut current_token = 0;
+    if unsafe { OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &mut current_token) } == 0 {
+        return Err(io::Error::last_os_error());
+    }
+    let _current_token = unsafe { OwnedHandle::from_raw_handle(current_token as _) };
+    let current_user = crate::windows_security::token_user(current_token)?;
+    let peer_user = crate::windows_security::token_user(token)?;
+    let same_user = unsafe {
+        EqualSid(
+            (*(current_user.as_ptr().cast::<TOKEN_USER>())).User.Sid,
+            (*(peer_user.as_ptr().cast::<TOKEN_USER>())).User.Sid,
+        ) != 0
+    };
+    // Implicit reuse must not change either side's elevation context.
+    for token in [current_token, token] {
+        let mut elevation: TOKEN_ELEVATION = unsafe { std::mem::zeroed() };
+        if unsafe {
+            GetTokenInformation(
+                token,
+                TokenElevation,
+                ptr::addr_of_mut!(elevation).cast(),
+                std::mem::size_of_val(&elevation) as u32,
+                &mut returned,
+            )
+        } == 0
+        {
+            return Err(io::Error::last_os_error());
+        }
+        if !same_user || elevation.TokenIsElevated != 0 {
+            return Err(io::Error::new(
+                io::ErrorKind::PermissionDenied,
+                "implicit daemon connection requires non-elevated current-user tokens",
+            ));
+        }
+    }
+    Ok(())
+}

codex-rs/uds/src/windows_security.rs

@@ -0,0 +1,128 @@
+//! Protect Windows socket rendezvous directories with an inheritable user-only
+//! DACL at creation time. Reject unsafe existing directories instead of repairing them.
+
+use std::io;
+use std::os::windows::ffi::OsStrExt;
+use std::os::windows::io::FromRawHandle;
+use std::os::windows::io::OwnedHandle;
+use std::path::Path;
+use std::ptr;
+
+use windows_sys::Win32::Foundation::HANDLE;
+use windows_sys::Win32::Foundation::LocalFree;
+use windows_sys::Win32::Security::Authorization::ConvertSidToStringSidW;
+use windows_sys::Win32::Security::Authorization::ConvertStringSecurityDescriptorToSecurityDescriptorW;
+use windows_sys::Win32::Security::GetTokenInformation;
+use windows_sys::Win32::Security::SECURITY_ATTRIBUTES;
+use windows_sys::Win32::Security::TOKEN_QUERY;
+use windows_sys::Win32::Security::TOKEN_USER;
+use windows_sys::Win32::Security::TokenUser;
+use windows_sys::Win32::Storage::FileSystem::CreateDirectoryW;
+use windows_sys::Win32::System::Threading::GetCurrentProcess;
+use windows_sys::Win32::System::Threading::OpenProcessToken;
+
+pub(super) fn prepare_private_directory(path: &Path) -> io::Result<()> {
+    let user = current_user()?;
+    let user_sid = unsafe { (*(user.as_ptr().cast::<TOKEN_USER>())).User.Sid };
+    let mut sid_string = ptr::null_mut();
+    if unsafe { ConvertSidToStringSidW(user_sid, &mut sid_string) } == 0 {
+        return Err(io::Error::last_os_error());
+    }
+    let mut sid_length = 0;
+    while unsafe { *sid_string.add(sid_length) } != 0 {
+        sid_length += 1;
+    }
+    let sid =
+        String::from_utf16_lossy(unsafe { std::slice::from_raw_parts(sid_string, sid_length) });
+    unsafe { LocalFree(sid_string as _) };
+    let sddl: Vec<u16> = format!("O:{sid}D:P(A;OICI;FA;;;{sid})")
+        .encode_utf16()
+        .chain(Some(0))
+        .collect();
+    let mut descriptor = ptr::null_mut();
+    if unsafe {
+        ConvertStringSecurityDescriptorToSecurityDescriptorW(
+            sddl.as_ptr(),
+            /*stringsdrevision*/ 1,
+            &mut descriptor,
+            ptr::null_mut(),
+        )
+    } == 0
+    {
+        return Err(io::Error::last_os_error());
+    }
+    let result = (|| {
+        // Canonicalize only the parent: this supplies Win32's extended-length
+        // prefix without following a reparse point at the directory being secured.
+        let absolute = std::path::absolute(path)?;
+        let path = match (absolute.parent(), absolute.file_name()) {
+            (Some(parent), Some(name)) => {
+                std::fs::create_dir_all(parent)?;
+                std::fs::canonicalize(parent)?.join(name)
+            }
+            _ => {
+                return Err(io::Error::new(
+                    io::ErrorKind::InvalidInput,
+                    "private directory must not be a filesystem root",
+                ));
+            }
+        };
+        let path_wide: Vec<u16> = path.as_os_str().encode_wide().chain(Some(0)).collect();
+        let attributes = SECURITY_ATTRIBUTES {
+            nLength: std::mem::size_of::<SECURITY_ATTRIBUTES>() as u32,
+            lpSecurityDescriptor: descriptor,
+            bInheritHandle: 0,
+        };
+        if unsafe { CreateDirectoryW(path_wide.as_ptr(), &attributes) } != 0 {
+            return Ok(());
+        }
+        let err = io::Error::last_os_error();
+        if err.kind() != io::ErrorKind::AlreadyExists {
+            return Err(err);
+        }
+        // Changing a DACL cannot revoke handles already held by another user.
+        // Existing directories must already meet our private-directory contract.
+        let _guard = crate::windows_socket_validation::validate_private_directory(&path)?;
+        Ok(())
+    })();
+    unsafe { LocalFree(descriptor as _) };
+    result
+}
+
+// Keep the pointer-aligned buffer alive while using its TOKEN_USER SID.
+pub(super) fn current_user() -> io::Result<Vec<usize>> {
+    // Token information contains pointers, so allocate pointer-aligned storage.
+    let mut token = 0;
+    if unsafe { OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &mut token) } == 0 {
+        return Err(io::Error::last_os_error());
+    }
+    let _token = unsafe { OwnedHandle::from_raw_handle(token as _) };
+    token_user(token)
+}
+
+pub(super) fn token_user(token: HANDLE) -> io::Result<Vec<usize>> {
+    let mut length = 0;
+    unsafe {
+        GetTokenInformation(
+            token,
+            TokenUser,
+            ptr::null_mut(),
+            /*tokeninformationlength*/ 0,
+            &mut length,
+        )
+    };
+    let mut user = vec![0usize; (length as usize).div_ceil(std::mem::size_of::<usize>())];
+    if unsafe {
+        GetTokenInformation(
+            token,
+            TokenUser,
+            user.as_mut_ptr().cast(),
+            length,
+            &mut length,
+        )
+    } == 0
+    {
+        return Err(io::Error::last_os_error());
+    }
+    Ok(user)
+}

codex-rs/uds/src/windows_socket_validation.rs

@@ -0,0 +1,138 @@
+//! Read-only validation of Windows daemon rendezvous directories. Pin the
+//! validated directory while connecting; never repair an untrusted listener's ACL.
+
+use std::io;
+use std::os::windows::ffi::OsStrExt;
+use std::os::windows::io::FromRawHandle;
+use std::os::windows::io::OwnedHandle;
+use std::path::Path;
+use std::path::PathBuf;
+use std::ptr;
+
+use windows_sys::Win32::Foundation::INVALID_HANDLE_VALUE;
+use windows_sys::Win32::Foundation::LocalFree;
+use windows_sys::Win32::Security::ACCESS_ALLOWED_ACE;
+use windows_sys::Win32::Security::ACE_HEADER;
+use windows_sys::Win32::Security::Authorization::GetSecurityInfo;
+use windows_sys::Win32::Security::Authorization::SE_FILE_OBJECT;
+use windows_sys::Win32::Security::CONTAINER_INHERIT_ACE;
+use windows_sys::Win32::Security::DACL_SECURITY_INFORMATION;
+use windows_sys::Win32::Security::EqualSid;
+use windows_sys::Win32::Security::GetAce;
+use windows_sys::Win32::Security::GetSecurityDescriptorControl;
+use windows_sys::Win32::Security::OBJECT_INHERIT_ACE;
+use windows_sys::Win32::Security::OWNER_SECURITY_INFORMATION;
+use windows_sys::Win32::Security::SE_DACL_PROTECTED;
+use windows_sys::Win32::Security::TOKEN_USER;
+use windows_sys::Win32::Storage::FileSystem::BY_HANDLE_FILE_INFORMATION;
+use windows_sys::Win32::Storage::FileSystem::CreateFileW;
+use windows_sys::Win32::Storage::FileSystem::FILE_ALL_ACCESS;
+use windows_sys::Win32::Storage::FileSystem::FILE_ATTRIBUTE_DIRECTORY;
+use windows_sys::Win32::Storage::FileSystem::FILE_ATTRIBUTE_REPARSE_POINT;
+use windows_sys::Win32::Storage::FileSystem::FILE_FLAG_BACKUP_SEMANTICS;
+use windows_sys::Win32::Storage::FileSystem::FILE_FLAG_OPEN_REPARSE_POINT;
+use windows_sys::Win32::Storage::FileSystem::FILE_LIST_DIRECTORY;
+use windows_sys::Win32::Storage::FileSystem::FILE_READ_ATTRIBUTES;
+use windows_sys::Win32::Storage::FileSystem::FILE_SHARE_READ;
+use windows_sys::Win32::Storage::FileSystem::FILE_SHARE_WRITE;
+use windows_sys::Win32::Storage::FileSystem::GetFileInformationByHandle;
+use windows_sys::Win32::Storage::FileSystem::OPEN_EXISTING;
+use windows_sys::Win32::Storage::FileSystem::READ_CONTROL;
+use windows_sys::Win32::System::SystemServices::ACCESS_ALLOWED_ACE_TYPE;
+
+/// Validates an existing daemon socket directory without creating or changing it.
+/// Connect using the returned path and retain the handle through connection setup
+/// to prevent replacement of the validated directory. Ancestor junctions are
+/// resolved before validation; the socket directory itself must not be a reparse point.
+pub fn validate_private_socket_path(socket_path: &Path) -> io::Result<(PathBuf, OwnedHandle)> {
+    let absolute = std::path::absolute(socket_path)?;
+    let directory = absolute.parent().ok_or(io::ErrorKind::InvalidInput)?;
+    let socket_name = absolute.file_name().ok_or(io::ErrorKind::InvalidInput)?;
+    let (directory, guard) = validate_private_directory(directory)?;
+    Ok((directory.join(socket_name), guard))
+}
+
+pub(super) fn validate_private_directory(directory: &Path) -> io::Result<(PathBuf, OwnedHandle)> {
+    let parent = directory.parent().ok_or(io::ErrorKind::InvalidInput)?;
+    let directory_name = directory.file_name().ok_or(io::ErrorKind::InvalidInput)?;
+    let directory = std::fs::canonicalize(parent)?.join(directory_name);
+    let path_wide: Vec<u16> = directory.as_os_str().encode_wide().chain(Some(0)).collect();
+    // Metadata-only access does not enforce sharing restrictions on Windows.
+    // Directory listing access makes omission of FILE_SHARE_DELETE pin the path.
+    let handle = unsafe {
+        CreateFileW(
+            path_wide.as_ptr(),
+            READ_CONTROL | FILE_READ_ATTRIBUTES | FILE_LIST_DIRECTORY,
+            FILE_SHARE_READ | FILE_SHARE_WRITE,
+            ptr::null(),
+            OPEN_EXISTING,
+            FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OPEN_REPARSE_POINT,
+            /*htemplatefile*/ 0,
+        )
+    };
+    if handle == INVALID_HANDLE_VALUE {
+        return Err(io::Error::last_os_error());
+    }
+    let guard = unsafe { OwnedHandle::from_raw_handle(handle as _) };
+    let mut info: BY_HANDLE_FILE_INFORMATION = unsafe { std::mem::zeroed() };
+    if unsafe { GetFileInformationByHandle(handle, &mut info) } == 0 {
+        return Err(io::Error::last_os_error());
+    }
+    if info.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY == 0
+        || info.dwFileAttributes & FILE_ATTRIBUTE_REPARSE_POINT != 0
+    {
+        return Err(io::Error::new(
+            io::ErrorKind::PermissionDenied,
+            "untrusted socket directory",
+        ));
+    }
+    let user = crate::windows_security::current_user()?;
+    let user_sid = unsafe { (*(user.as_ptr().cast::<TOKEN_USER>())).User.Sid };
+    let mut owner = ptr::null_mut();
+    let mut dacl = ptr::null_mut();
+    let mut descriptor = ptr::null_mut();
+    let status = unsafe {
+        GetSecurityInfo(
+            handle,
+            SE_FILE_OBJECT,
+            OWNER_SECURITY_INFORMATION | DACL_SECURITY_INFORMATION,
+            &mut owner,
+            ptr::null_mut(),
+            &mut dacl,
+            ptr::null_mut(),
+            &mut descriptor,
+        )
+    };
+    if status != 0 {
+        return Err(io::Error::from_raw_os_error(status as i32));
+    }
+    // Accept exactly the protected, inheritable user-only ACL set at daemon bind.
+    let trusted = unsafe {
+        let mut control = 0;
+        let mut revision = 0;
+        let mut ace = ptr::null_mut();
+        !owner.is_null()
+            && EqualSid(owner, user_sid) != 0
+            && GetSecurityDescriptorControl(descriptor, &mut control, &mut revision) != 0
+            && control & SE_DACL_PROTECTED != 0
+            && !dacl.is_null()
+            && (*dacl).AceCount == 1
+            && GetAce(dacl, /*dwaceindex*/ 0, &mut ace) != 0
+            && (*ace.cast::<ACE_HEADER>()).AceType == ACCESS_ALLOWED_ACE_TYPE as u8
+            && (*ace.cast::<ACE_HEADER>()).AceFlags
+                == (CONTAINER_INHERIT_ACE | OBJECT_INHERIT_ACE) as u8
+            && (*ace.cast::<ACCESS_ALLOWED_ACE>()).Mask == FILE_ALL_ACCESS
+            && EqualSid(
+                ptr::addr_of_mut!((*ace.cast::<ACCESS_ALLOWED_ACE>()).SidStart).cast(),
+                user_sid,
+            ) != 0
+    };
+    unsafe { LocalFree(descriptor as _) };
+    if !trusted {
+        return Err(io::Error::new(
+            io::ErrorKind::PermissionDenied,
+            "socket directory is not private to the current user",
+        ));
+    }
+    Ok((directory, guard))
+}