src/WebhookServer.Core/Execution/Native/InteractiveProcessLauncher.cs

@@ -0,0 +1,287 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using System.Text;
+using Microsoft.Win32.SafeHandles;
+using static WebhookServer.Core.Execution.Native.NativeMethods;
+
+namespace WebhookServer.Core.Execution.Native;
+
+/// <summary>
+/// Launches a child process inside the active console session under whoever is
+/// logged in at the keyboard. Required when running as SYSTEM (the service account)
+/// because <c>Process.Start</c> would land the child in Session 0 where it can't
+/// show UI on the user's desktop.
+///
+/// Caller must already be SYSTEM (the service runs as SYSTEM by default) — only
+/// SYSTEM can call <c>WTSQueryUserToken</c>.
+/// </summary>
+[SupportedOSPlatform("windows")]
+internal static class InteractiveProcessLauncher
+{
+    public sealed class LaunchOptions
+    {
+        public required string FileName { get; init; }
+        public required IReadOnlyList<string> Arguments { get; init; }
+        public string? WorkingDirectory { get; init; }
+        public IReadOnlyDictionary<string, string>? ExtraEnvVars { get; init; }
+        public byte[]? StdinBytes { get; init; }
+    }
+
+    public sealed class LaunchResult : IDisposable
+    {
+        public required IntPtr ProcessHandle { get; init; }
+        public required uint ProcessId { get; init; }
+        public required StreamReader Stdout { get; init; }
+        public required StreamReader Stderr { get; init; }
+
+        public void Dispose()
+        {
+            try { Stdout.Dispose(); } catch { }
+            try { Stderr.Dispose(); } catch { }
+            if (ProcessHandle != IntPtr.Zero)
+                try { CloseHandle(ProcessHandle); } catch { }
+        }
+    }
+
+    public static LaunchResult Launch(LaunchOptions opts)
+    {
+        var sessionId = WTSGetActiveConsoleSessionId();
+        if (sessionId == INVALID_SESSION_ID)
+            throw new InvalidOperationException("No active console session - is anyone logged in at the keyboard?");
+
+        if (!WTSQueryUserToken(sessionId, out var userToken))
+            throw LastError("WTSQueryUserToken (must run as SYSTEM)");
+
+        IntPtr primaryToken = IntPtr.Zero;
+        IntPtr envBlock = IntPtr.Zero;
+        IntPtr stdoutRead = IntPtr.Zero, stdoutWrite = IntPtr.Zero;
+        IntPtr stderrRead = IntPtr.Zero, stderrWrite = IntPtr.Zero;
+        IntPtr stdinRead = IntPtr.Zero, stdinWrite = IntPtr.Zero;
+        var pi = new PROCESS_INFORMATION();
+        bool succeeded = false;
+
+        try
+        {
+            if (!DuplicateTokenEx(userToken, (uint)MAXIMUM_ALLOWED, IntPtr.Zero,
+                    SECURITY_IMPERSONATION_LEVEL.SecurityImpersonation,
+                    TOKEN_TYPE.TokenPrimary, out primaryToken))
+                throw LastError("DuplicateTokenEx");
+
+            if (!CreateEnvironmentBlock(out envBlock, primaryToken, false))
+                throw LastError("CreateEnvironmentBlock");
+
+            if (opts.ExtraEnvVars is { Count: > 0 })
+                envBlock = AppendEnvVars(envBlock, opts.ExtraEnvVars);
+
+            CreateInheritablePipe(out stdoutRead, out stdoutWrite, parentReads: true);
+            CreateInheritablePipe(out stderrRead, out stderrWrite, parentReads: true);
+            CreateInheritablePipe(out stdinRead, out stdinWrite, parentReads: false);
+
+            var si = new STARTUPINFO
+            {
+                cb = Marshal.SizeOf<STARTUPINFO>(),
+                dwFlags = STARTF_USESTDHANDLES,
+                hStdInput = stdinRead,
+                hStdOutput = stdoutWrite,
+                hStdError = stderrWrite,
+                lpDesktop = @"winsta0\default",
+            };
+
+            var commandLine = BuildCommandLine(opts.FileName, opts.Arguments);
+
+            if (!CreateProcessAsUser(
+                    primaryToken,
+                    null,
+                    commandLine,
+                    IntPtr.Zero, IntPtr.Zero,
+                    bInheritHandles: true,
+                    CREATE_UNICODE_ENVIRONMENT | CREATE_NO_WINDOW | NORMAL_PRIORITY_CLASS,
+                    envBlock,
+                    string.IsNullOrEmpty(opts.WorkingDirectory) ? null : opts.WorkingDirectory,
+                    ref si,
+                    out pi))
+            {
+                throw LastError("CreateProcessAsUser");
+            }
+
+            // Close child-side handles in our process so EOF propagates when the child exits.
+            CloseHandle(stdoutWrite); stdoutWrite = IntPtr.Zero;
+            CloseHandle(stderrWrite); stderrWrite = IntPtr.Zero;
+            CloseHandle(stdinRead); stdinRead = IntPtr.Zero;
+
+            // Pipe stdin if provided, then close the write end.
+            if (opts.StdinBytes is { Length: > 0 })
+            {
+                using var stdinStream = new FileStream(new SafeFileHandle(stdinWrite, ownsHandle: true), FileAccess.Write);
+                stdinStream.Write(opts.StdinBytes, 0, opts.StdinBytes.Length);
+                stdinStream.Flush();
+                stdinWrite = IntPtr.Zero; // ownership transferred to the FileStream
+            }
+            else
+            {
+                CloseHandle(stdinWrite); stdinWrite = IntPtr.Zero;
+            }
+
+            CloseHandle(pi.hThread);
+
+            var stdout = new StreamReader(new FileStream(new SafeFileHandle(stdoutRead, true), FileAccess.Read), Encoding.UTF8);
+            var stderr = new StreamReader(new FileStream(new SafeFileHandle(stderrRead, true), FileAccess.Read), Encoding.UTF8);
+            stdoutRead = IntPtr.Zero;
+            stderrRead = IntPtr.Zero;
+
+            succeeded = true;
+            return new LaunchResult
+            {
+                ProcessHandle = pi.hProcess,
+                ProcessId = pi.dwProcessId,
+                Stdout = stdout,
+                Stderr = stderr,
+            };
+        }
+        finally
+        {
+            if (userToken != IntPtr.Zero) CloseHandle(userToken);
+            if (primaryToken != IntPtr.Zero) CloseHandle(primaryToken);
+            if (envBlock != IntPtr.Zero) DestroyEnvironmentBlock(envBlock);
+            if (!succeeded)
+            {
+                if (stdoutRead != IntPtr.Zero) CloseHandle(stdoutRead);
+                if (stdoutWrite != IntPtr.Zero) CloseHandle(stdoutWrite);
+                if (stderrRead != IntPtr.Zero) CloseHandle(stderrRead);
+                if (stderrWrite != IntPtr.Zero) CloseHandle(stderrWrite);
+                if (stdinRead != IntPtr.Zero) CloseHandle(stdinRead);
+                if (stdinWrite != IntPtr.Zero) CloseHandle(stdinWrite);
+                if (pi.hProcess != IntPtr.Zero) CloseHandle(pi.hProcess);
+                if (pi.hThread != IntPtr.Zero) CloseHandle(pi.hThread);
+            }
+        }
+    }
+
+    public static async Task<int> WaitAsync(IntPtr processHandle, TimeSpan timeout, CancellationToken ct)
+    {
+        // Poll WaitForSingleObject in 100ms slices to honor cancellation/timeout cheaply.
+        var deadline = DateTime.UtcNow + timeout;
+        while (true)
+        {
+            ct.ThrowIfCancellationRequested();
+            var remaining = deadline - DateTime.UtcNow;
+            if (remaining <= TimeSpan.Zero) throw new TimeoutException();
+
+            var slice = (uint)Math.Min(100, (int)remaining.TotalMilliseconds);
+            var result = WaitForSingleObject(processHandle, slice);
+            if (result == 0) // WAIT_OBJECT_0
+            {
+                if (!GetExitCodeProcess(processHandle, out var code))
+                    throw LastError("GetExitCodeProcess");
+                return (int)code;
+            }
+            if (result == 0xFFFFFFFF)
+                throw LastError("WaitForSingleObject");
+            await Task.Yield();
+        }
+    }
+
+    public static void Kill(IntPtr processHandle)
+    {
+        try { TerminateProcess(processHandle, 1); } catch { }
+    }
+
+    private static void CreateInheritablePipe(out IntPtr read, out IntPtr write, bool parentReads)
+    {
+        var sa = new SECURITY_ATTRIBUTES
+        {
+            nLength = Marshal.SizeOf<SECURITY_ATTRIBUTES>(),
+            bInheritHandle = 1,
+            lpSecurityDescriptor = IntPtr.Zero,
+        };
+        if (!CreatePipe(out read, out write, ref sa, 0))
+            throw LastError("CreatePipe");
+
+        // Mark the parent's end non-inheritable so it's not duplicated into the child.
+        var parentEnd = parentReads ? read : write;
+        if (!SetHandleInformation(parentEnd, HANDLE_FLAG_INHERIT, 0))
+            throw LastError("SetHandleInformation");
+    }
+
+    private static IntPtr AppendEnvVars(IntPtr existingBlock, IReadOnlyDictionary<string, string> extras)
+    {
+        // The existing block is a sequence of null-terminated WCHAR strings ending with
+        // an extra null. Walk it byte-pair by byte-pair to find the end.
+        var parsed = new List<string>();
+        var offset = 0;
+        while (true)
+        {
+            var ch0 = Marshal.ReadInt16(existingBlock, offset);
+            if (ch0 == 0) break;
+            var entry = Marshal.PtrToStringUni(existingBlock + offset)!;
+            parsed.Add(entry);
+            offset += (entry.Length + 1) * sizeof(char);
+        }
+        DestroyEnvironmentBlock(existingBlock);
+
+        var combined = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        foreach (var entry in parsed)
+        {
+            var eq = entry.IndexOf('=');
+            if (eq <= 0) continue;
+            combined[entry.Substring(0, eq)] = entry.Substring(eq + 1);
+        }
+        foreach (var (k, v) in extras) combined[k] = v;
+
+        var sb = new StringBuilder();
+        foreach (var (k, v) in combined)
+        {
+            sb.Append(k).Append('=').Append(v).Append('\0');
+        }
+        sb.Append('\0');
+
+        var bytes = Encoding.Unicode.GetBytes(sb.ToString());
+        var ptr = Marshal.AllocHGlobal(bytes.Length);
+        Marshal.Copy(bytes, 0, ptr, bytes.Length);
+        return ptr;
+    }
+
+    /// <summary>
+    /// Build a Windows command line string from a filename + arg list using the
+    /// quoting rules consumed by CommandLineToArgvW.
+    /// </summary>
+    private static string BuildCommandLine(string fileName, IReadOnlyList<string> args)
+    {
+        var sb = new StringBuilder();
+        AppendArg(sb, fileName);
+        foreach (var arg in args)
+        {
+            sb.Append(' ');
+            AppendArg(sb, arg);
+        }
+        return sb.ToString();
+    }
+
+    private static void AppendArg(StringBuilder sb, string arg)
+    {
+        // Empty arg → "".
+        if (arg.Length == 0) { sb.Append("\"\""); return; }
+
+        var needsQuoting = arg.IndexOfAny(new[] { ' ', '\t', '\n', '\v', '"' }) >= 0;
+        if (!needsQuoting) { sb.Append(arg); return; }
+
+        sb.Append('"');
+        var backslashes = 0;
+        foreach (var ch in arg)
+        {
+            if (ch == '\\') { backslashes++; continue; }
+            if (ch == '"')
+            {
+                sb.Append('\\', backslashes * 2 + 1);
+                sb.Append('"');
+                backslashes = 0;
+                continue;
+            }
+            if (backslashes > 0) { sb.Append('\\', backslashes); backslashes = 0; }
+            sb.Append(ch);
+        }
+        // Trailing backslashes before closing quote must be doubled.
+        if (backslashes > 0) sb.Append('\\', backslashes * 2);
+        sb.Append('"');
+    }
+}

src/WebhookServer.Core/Execution/Native/NativeMethods.cs

@@ -0,0 +1,144 @@
+using System.ComponentModel;
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace WebhookServer.Core.Execution.Native;
+
+/// <summary>
+/// Win32 P/Invoke layer for launching processes in another user's session.
+/// Used by <see cref="InteractiveProcessLauncher"/>; not intended for general use.
+/// </summary>
+[SupportedOSPlatform("windows")]
+internal static class NativeMethods
+{
+    public const uint INVALID_SESSION_ID = 0xFFFFFFFF;
+    public const int MAXIMUM_ALLOWED = 0x02000000;
+
+    public const uint CREATE_UNICODE_ENVIRONMENT = 0x00000400;
+    public const uint CREATE_NO_WINDOW = 0x08000000;
+    public const uint CREATE_NEW_CONSOLE = 0x00000010;
+    public const uint NORMAL_PRIORITY_CLASS = 0x00000020;
+
+    public const int STARTF_USESTDHANDLES = 0x00000100;
+
+    public const int HANDLE_FLAG_INHERIT = 1;
+
+    public const uint INFINITE = 0xFFFFFFFF;
+
+    public enum SECURITY_IMPERSONATION_LEVEL
+    {
+        SecurityAnonymous = 0,
+        SecurityIdentification = 1,
+        SecurityImpersonation = 2,
+        SecurityDelegation = 3,
+    }
+
+    public enum TOKEN_TYPE
+    {
+        TokenPrimary = 1,
+        TokenImpersonation = 2,
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    public struct SECURITY_ATTRIBUTES
+    {
+        public int nLength;
+        public IntPtr lpSecurityDescriptor;
+        public int bInheritHandle;
+    }
+
+    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
+    public struct STARTUPINFO
+    {
+        public int cb;
+        public string? lpReserved;
+        public string? lpDesktop;
+        public string? lpTitle;
+        public uint dwX;
+        public uint dwY;
+        public uint dwXSize;
+        public uint dwYSize;
+        public uint dwXCountChars;
+        public uint dwYCountChars;
+        public uint dwFillAttribute;
+        public uint dwFlags;
+        public ushort wShowWindow;
+        public ushort cbReserved2;
+        public IntPtr lpReserved2;
+        public IntPtr hStdInput;
+        public IntPtr hStdOutput;
+        public IntPtr hStdError;
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    public struct PROCESS_INFORMATION
+    {
+        public IntPtr hProcess;
+        public IntPtr hThread;
+        public uint dwProcessId;
+        public uint dwThreadId;
+    }
+
+    [DllImport("kernel32.dll")]
+    public static extern uint WTSGetActiveConsoleSessionId();
+
+    [DllImport("wtsapi32.dll", SetLastError = true)]
+    public static extern bool WTSQueryUserToken(uint sessionId, out IntPtr phToken);
+
+    [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
+    public static extern bool DuplicateTokenEx(
+        IntPtr hExistingToken,
+        uint dwDesiredAccess,
+        IntPtr lpTokenAttributes,
+        SECURITY_IMPERSONATION_LEVEL impersonationLevel,
+        TOKEN_TYPE tokenType,
+        out IntPtr phNewToken);
+
+    [DllImport("userenv.dll", SetLastError = true)]
+    public static extern bool CreateEnvironmentBlock(
+        out IntPtr lpEnvironment,
+        IntPtr hToken,
+        bool bInherit);
+
+    [DllImport("userenv.dll", SetLastError = true)]
+    public static extern bool DestroyEnvironmentBlock(IntPtr lpEnvironment);
+
+    [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
+    public static extern bool CreateProcessAsUser(
+        IntPtr hToken,
+        string? lpApplicationName,
+        string? lpCommandLine,
+        IntPtr lpProcessAttributes,
+        IntPtr lpThreadAttributes,
+        bool bInheritHandles,
+        uint dwCreationFlags,
+        IntPtr lpEnvironment,
+        string? lpCurrentDirectory,
+        ref STARTUPINFO lpStartupInfo,
+        out PROCESS_INFORMATION lpProcessInformation);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    public static extern bool CreatePipe(
+        out IntPtr hReadPipe,
+        out IntPtr hWritePipe,
+        ref SECURITY_ATTRIBUTES lpPipeAttributes,
+        uint nSize);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    public static extern bool SetHandleInformation(IntPtr hObject, int dwMask, int dwFlags);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    public static extern bool CloseHandle(IntPtr hObject);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    public static extern bool GetExitCodeProcess(IntPtr hProcess, out uint lpExitCode);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    public static extern uint WaitForSingleObject(IntPtr hHandle, uint dwMilliseconds);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    public static extern bool TerminateProcess(IntPtr hProcess, uint uExitCode);
+
+    public static Win32Exception LastError(string what) =>
+        new(Marshal.GetLastWin32Error(), $"{what} failed");
+}

src/WebhookServer.Core/Execution/ProcessExecutor.cs

@@ -1,9 +1,13 @@
 using System.Diagnostics;
+using System.Runtime.Versioning;
+using System.Security;
 using System.Text;
+using WebhookServer.Core.Execution.Native;
 using WebhookServer.Core.Models;
 
 namespace WebhookServer.Core.Execution;
 
+[SupportedOSPlatform("windows")]
 public sealed class ProcessExecutor : IExecutor
 {
     /// <summary>Per-stream cap on captured output (excess is dropped and StdoutTruncated set).</summary>
@@ -12,23 +16,40 @@ public sealed class ProcessExecutor : IExecutor
     public async Task<ExecutionResult> RunAsync(EndpointConfig endpoint, ExecutionContext ctx, CancellationToken ct)
     {
         var startedAt = DateTimeOffset.UtcNow;
-        var psi = BuildStartInfo(endpoint, ctx);
+        var mode = endpoint.RunAs?.Mode ?? RunAsMode.Service;
+        return mode switch
+        {
+            RunAsMode.InteractiveUser => await RunInteractiveAsync(endpoint, ctx, startedAt, ct).ConfigureAwait(false),
+            _ => await RunWithProcessAsync(endpoint, ctx, startedAt, ct).ConfigureAwait(false),
+        };
+    }
+
+    // ---------------- Process path: handles Service (default) and SpecificUser. ----------------
+
+    private async Task<ExecutionResult> RunWithProcessAsync(EndpointConfig endpoint, ExecutionContext ctx, DateTimeOffset startedAt, CancellationToken ct)
+    {
+        var (psi, envVars) = BuildStartInfo(endpoint, ctx);
+        foreach (var (k, v) in envVars)
+            psi.Environment[k] = v;
+
+        if (endpoint.RunAs?.Mode == RunAsMode.SpecificUser)
+        {
+            try { ApplySpecificUser(psi, endpoint.RunAs); }
+            catch (Exception ex) { return Failed(ctx.RunId, startedAt, ex.Message); }
+        }
 
         using var process = new Process { StartInfo = psi, EnableRaisingEvents = true };
 
         try
         {
             if (!process.Start())
-            {
                 return Failed(ctx.RunId, startedAt, "process failed to start");
-            }
         }
         catch (Exception ex)
         {
             return Failed(ctx.RunId, startedAt, $"launch error: {ex.Message}");
         }
 
-        // stdin
         if (endpoint.DataPassing.StdinJson)
         {
             try
@@ -42,15 +63,14 @@ public sealed class ProcessExecutor : IExecutor
             }
             finally
             {
-                try { process.StandardInput.Close(); } catch { /* swallow */ }
+                try { process.StandardInput.Close(); } catch { }
             }
         }
         else
         {
-            try { process.StandardInput.Close(); } catch { /* swallow */ }
+            try { process.StandardInput.Close(); } catch { }
         }
 
-        // Capture stdout/stderr in parallel, with per-stream cap.
         var stdoutTask = ReadCappedAsync(process.StandardOutput, ct);
         var stderrTask = ReadCappedAsync(process.StandardError, ct);
 
@@ -66,8 +86,8 @@ public sealed class ProcessExecutor : IExecutor
         catch (OperationCanceledException) when (!ct.IsCancellationRequested)
         {
             timedOut = true;
-            try { process.Kill(entireProcessTree: true); } catch { /* swallow */ }
-            try { await process.WaitForExitAsync(CancellationToken.None).ConfigureAwait(false); } catch { /* swallow */ }
+            try { process.Kill(entireProcessTree: true); } catch { }
+            try { await process.WaitForExitAsync(CancellationToken.None).ConfigureAwait(false); } catch { }
         }
 
         var (stdout, stdoutTrunc) = await stdoutTask.ConfigureAwait(false);
@@ -87,7 +107,72 @@ public sealed class ProcessExecutor : IExecutor
         };
     }
 
-    private static ProcessStartInfo BuildStartInfo(EndpointConfig endpoint, ExecutionContext ctx)
+    // ---------------- Interactive path: launches into the active console session. ----------------
+
+    private static async Task<ExecutionResult> RunInteractiveAsync(EndpointConfig endpoint, ExecutionContext ctx, DateTimeOffset startedAt, CancellationToken ct)
+    {
+        var (psi, envVars) = BuildStartInfo(endpoint, ctx);
+
+        InteractiveProcessLauncher.LaunchResult launch;
+        try
+        {
+            launch = InteractiveProcessLauncher.Launch(new InteractiveProcessLauncher.LaunchOptions
+            {
+                FileName = psi.FileName,
+                Arguments = psi.ArgumentList.ToList(),
+                WorkingDirectory = string.IsNullOrEmpty(psi.WorkingDirectory) ? null : psi.WorkingDirectory,
+                ExtraEnvVars = envVars,
+                StdinBytes = endpoint.DataPassing.StdinJson ? ctx.BodyBytes : null,
+            });
+        }
+        catch (Exception ex)
+        {
+            return Failed(ctx.RunId, startedAt, $"interactive launch error: {ex.Message}");
+        }
+
+        try
+        {
+            var stdoutTask = ReadCappedAsync(launch.Stdout, ct);
+            var stderrTask = ReadCappedAsync(launch.Stderr, ct);
+
+            var timeout = TimeSpan.FromSeconds(Math.Max(1, endpoint.TimeoutSeconds));
+            bool timedOut = false;
+            int exitCode = -1;
+            try
+            {
+                exitCode = await InteractiveProcessLauncher.WaitAsync(launch.ProcessHandle, timeout, ct).ConfigureAwait(false);
+            }
+            catch (TimeoutException)
+            {
+                timedOut = true;
+                InteractiveProcessLauncher.Kill(launch.ProcessHandle);
+            }
+
+            var (stdout, stdoutTrunc) = await stdoutTask.ConfigureAwait(false);
+            var (stderr, stderrTrunc) = await stderrTask.ConfigureAwait(false);
+
+            return new ExecutionResult
+            {
+                RunId = ctx.RunId,
+                ExitCode = timedOut ? -1 : exitCode,
+                Stdout = stdout,
+                Stderr = stderr,
+                StdoutTruncated = stdoutTrunc,
+                StderrTruncated = stderrTrunc,
+                StartedAt = startedAt,
+                CompletedAt = DateTimeOffset.UtcNow,
+                TimedOut = timedOut,
+            };
+        }
+        finally
+        {
+            launch.Dispose();
+        }
+    }
+
+    // ---------------- Shared psi construction. ----------------
+
+    private static (ProcessStartInfo psi, Dictionary<string, string> envVars) BuildStartInfo(EndpointConfig endpoint, ExecutionContext ctx)
     {
         var psi = new ProcessStartInfo
         {
@@ -131,18 +216,19 @@ public sealed class ProcessExecutor : IExecutor
                 psi.ArgumentList.Add(arg);
         }
 
+        var envVars = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+        {
+            ["WEBHOOK_RUN_ID"] = ctx.RunId,
+            ["WEBHOOK_SLUG"] = ctx.Slug,
+        };
+
         if (endpoint.DataPassing.EnvVars)
         {
-            foreach (var (k, v) in ctx.Headers)
-                psi.Environment[$"WEBHOOK_HEADER_{Sanitize(k)}"] = v;
-            foreach (var (k, v) in ctx.Query)
-                psi.Environment[$"WEBHOOK_QUERY_{Sanitize(k)}"] = v;
+            foreach (var (k, v) in ctx.Headers) envVars[$"WEBHOOK_HEADER_{Sanitize(k)}"] = v;
+            foreach (var (k, v) in ctx.Query) envVars[$"WEBHOOK_QUERY_{Sanitize(k)}"] = v;
         }
 
-        psi.Environment["WEBHOOK_RUN_ID"] = ctx.RunId;
-        psi.Environment["WEBHOOK_SLUG"] = ctx.Slug;
-
-        return psi;
+        return (psi, envVars);
     }
 
     private static void AddPwshArgs(ProcessStartInfo psi, EndpointConfig endpoint)
@@ -175,6 +261,32 @@ public sealed class ProcessExecutor : IExecutor
         return endpoint.InlineCommand ?? "";
     }
 
+    private static void ApplySpecificUser(ProcessStartInfo psi, RunAsConfig runAs)
+    {
+        if (string.IsNullOrEmpty(runAs.Username))
+            throw new InvalidOperationException("RunAs.Username is required when Mode = SpecificUser");
+        if (runAs.Password?.Plaintext is not { Length: > 0 } password)
+            throw new InvalidOperationException("RunAs.Password is required when Mode = SpecificUser");
+
+        var (domain, user) = ParseUserSpec(runAs.Username);
+        psi.UserName = user;
+        if (!string.IsNullOrEmpty(domain)) psi.Domain = domain;
+
+        var ss = new SecureString();
+        foreach (var ch in password) ss.AppendChar(ch);
+        ss.MakeReadOnly();
+        psi.Password = ss;
+
+        psi.LoadUserProfile = runAs.LoadProfile;
+    }
+
+    private static (string Domain, string User) ParseUserSpec(string spec)
+    {
+        var bs = spec.IndexOf('\\');
+        if (bs > 0) return (spec.Substring(0, bs), spec.Substring(bs + 1));
+        return ("", spec);
+    }
+
     private static string Sanitize(string key)
     {
         var sb = new StringBuilder(key.Length);
@@ -203,7 +315,6 @@ public sealed class ProcessExecutor : IExecutor
             catch (IOException) { break; }
             if (n == 0) break;
 
-            // Cheap byte estimate (ASCII-ish); good enough as a guard rail.
             if (!truncated)
             {
                 if (byteEstimate + n > MaxOutputBytes)
@@ -218,7 +329,6 @@ public sealed class ProcessExecutor : IExecutor
                     byteEstimate += n;
                 }
             }
-            // Else keep draining without storing to keep the pipe from blocking.
         }
 
         return (sb.ToString(), truncated);

src/WebhookServer.Core/Models/EndpointConfig.cs

@@ -42,4 +42,6 @@ public sealed class EndpointConfig
     public bool Serialize { get; set; }
 
     public CallbackConfig? Callback { get; set; }
+
+    public RunAsConfig? RunAs { get; set; }
 }

src/WebhookServer.Core/Models/Enums.cs

@@ -53,3 +53,18 @@ public enum HttpsBindingKind
     PfxFile = 1,
     CertStoreThumbprint = 2,
 }
+
+public enum RunAsMode
+{
+    /// <summary>Run as whatever account the service itself runs under (default).</summary>
+    Service = 0,
+
+    /// <summary>Run as a specific username + password (batch logon, no UI).</summary>
+    SpecificUser = 1,
+
+    /// <summary>
+    /// Run in the active console session under whoever is logged in at the keyboard.
+    /// Lets hooks pop interactive UI on the user's desktop.
+    /// </summary>
+    InteractiveUser = 2,
+}

src/WebhookServer.Core/Models/RunAsConfig.cs

@@ -0,0 +1,21 @@
+namespace WebhookServer.Core.Models;
+
+public sealed class RunAsConfig
+{
+    public RunAsMode Mode { get; set; } = RunAsMode.Service;
+
+    /// <summary>
+    /// "DOMAIN\user" or "user@upn" or just "user" (local). Required when
+    /// <see cref="Mode"/> is <see cref="RunAsMode.SpecificUser"/>.
+    /// </summary>
+    public string? Username { get; set; }
+
+    /// <summary>DPAPI-protected password for SpecificUser mode.</summary>
+    public ProtectedString? Password { get; set; }
+
+    /// <summary>
+    /// When true, load the user's profile (HKCU + AppData) before running.
+    /// Slower; only needed for hooks that read user-scope settings.
+    /// </summary>
+    public bool LoadProfile { get; set; }
+}

src/WebhookServer.Core/Storage/ConfigStore.cs

@@ -55,6 +55,7 @@ public sealed class ConfigStore
         {
             ClearOne(ep.Bearer?.Secret);
             ClearOne(ep.Hmac?.Secret);
+            ClearOne(ep.RunAs?.Password);
             if (ep.Callback is { } cb)
             {
                 ClearOne(cb.Bearer?.Secret);
@@ -76,6 +77,7 @@ public sealed class ConfigStore
         {
             DecryptOne(ep.Bearer?.Secret);
             DecryptOne(ep.Hmac?.Secret);
+            DecryptOne(ep.RunAs?.Password);
             if (ep.Callback is { } cb)
             {
                 DecryptOne(cb.Bearer?.Secret);
@@ -91,6 +93,7 @@ public sealed class ConfigStore
         {
             EncryptOne(ep.Bearer?.Secret);
             EncryptOne(ep.Hmac?.Secret);
+            EncryptOne(ep.RunAs?.Password);
             if (ep.Callback is { } cb)
             {
                 EncryptOne(cb.Bearer?.Secret);

src/WebhookServer.Gui/ViewModels/EndpointEditorViewModel.cs

@@ -23,6 +23,8 @@ public sealed partial class EndpointEditorViewModel : ObservableObject
         Endpoint = JsonSerializer.Deserialize<EndpointConfig>(json, ConfigJson.Compact)!;
         Endpoint.Bearer ??= new BearerOptions();
         Endpoint.Hmac ??= new HmacOptions();
+        Endpoint.RunAs ??= new RunAsConfig();
+        Endpoint.RunAs.Password ??= new ProtectedString();
         IsNew = isNew;
     }
 
@@ -53,6 +55,58 @@ public sealed partial class EndpointEditorViewModel : ObservableObject
     public Visibility HmacVisible =>
         Endpoint.AuthMode == AuthMode.Hmac ? Visibility.Visible : Visibility.Collapsed;
 
+    public Array RunAsModes { get; } = Enum.GetValues(typeof(RunAsMode));
+
+    public RunAsMode SelectedRunAsMode
+    {
+        get => Endpoint.RunAs?.Mode ?? RunAsMode.Service;
+        set
+        {
+            Endpoint.RunAs ??= new RunAsConfig();
+            if (Endpoint.RunAs.Mode == value) return;
+            Endpoint.RunAs.Mode = value;
+            OnPropertyChanged();
+            OnPropertyChanged(nameof(SpecificUserVisible));
+        }
+    }
+
+    public Visibility SpecificUserVisible =>
+        SelectedRunAsMode == RunAsMode.SpecificUser ? Visibility.Visible : Visibility.Collapsed;
+
+    public string RunAsUsername
+    {
+        get => Endpoint.RunAs?.Username ?? "";
+        set
+        {
+            Endpoint.RunAs ??= new RunAsConfig();
+            Endpoint.RunAs.Username = string.IsNullOrEmpty(value) ? null : value;
+            OnPropertyChanged();
+        }
+    }
+
+    public string RunAsPassword
+    {
+        get => Endpoint.RunAs?.Password?.Plaintext ?? "";
+        set
+        {
+            Endpoint.RunAs ??= new RunAsConfig();
+            Endpoint.RunAs.Password ??= new ProtectedString();
+            Endpoint.RunAs.Password.Plaintext = string.IsNullOrEmpty(value) ? null : value;
+            OnPropertyChanged();
+        }
+    }
+
+    public bool RunAsLoadProfile
+    {
+        get => Endpoint.RunAs?.LoadProfile ?? false;
+        set
+        {
+            Endpoint.RunAs ??= new RunAsConfig();
+            Endpoint.RunAs.LoadProfile = value;
+            OnPropertyChanged();
+        }
+    }
+
     public string AllowedClientsText
     {
         get => string.Join(Environment.NewLine, Endpoint.AllowedClients);

src/WebhookServer.Gui/Views/EndpointEditor.xaml

@@ -130,6 +130,50 @@
                     </StackPanel>
                 </GroupBox>
 
+                <GroupBox Header="Run as" Padding="6" Margin="0,8,0,0">
+                    <StackPanel>
+                        <Grid>
+                            <Grid.ColumnDefinitions>
+                                <ColumnDefinition Width="120"/>
+                                <ColumnDefinition Width="*"/>
+                            </Grid.ColumnDefinitions>
+                            <TextBlock Text="Identity" VerticalAlignment="Center"/>
+                            <ComboBox Grid.Column="1" ItemsSource="{Binding RunAsModes}" SelectedItem="{Binding SelectedRunAsMode, Mode=TwoWay}"/>
+                        </Grid>
+                        <TextBlock Foreground="Gray" FontStyle="Italic" FontSize="11" Margin="120,2,0,0"
+                                   Text="Service = run as the service account (default). InteractiveUser = the logged-in user's desktop session, lets hooks pop UI. SpecificUser = a named account with password."/>
+                        <StackPanel Visibility="{Binding SpecificUserVisible}">
+                            <Grid Margin="0,6,0,0">
+                                <Grid.ColumnDefinitions>
+                                    <ColumnDefinition Width="120"/>
+                                    <ColumnDefinition Width="*"/>
+                                </Grid.ColumnDefinitions>
+                                <TextBlock Text="Username" VerticalAlignment="Center"/>
+                                <TextBox Grid.Column="1" Text="{Binding RunAsUsername, UpdateSourceTrigger=PropertyChanged}" FontFamily="Consolas"/>
+                            </Grid>
+                            <TextBlock Foreground="Gray" FontStyle="Italic" FontSize="11" Margin="120,2,0,0"
+                                       Text="Examples: DOMAIN\justin   .\local-user   user@contoso.com"/>
+                            <Grid Margin="0,4,0,0">
+                                <Grid.ColumnDefinitions>
+                                    <ColumnDefinition Width="120"/>
+                                    <ColumnDefinition Width="*"/>
+                                </Grid.ColumnDefinitions>
+                                <TextBlock Text="Password" VerticalAlignment="Center"/>
+                                <TextBox Grid.Column="1" Text="{Binding RunAsPassword, UpdateSourceTrigger=PropertyChanged}" FontFamily="Consolas"/>
+                            </Grid>
+                            <Grid Margin="0,4,0,0">
+                                <Grid.ColumnDefinitions>
+                                    <ColumnDefinition Width="120"/>
+                                    <ColumnDefinition Width="*"/>
+                                </Grid.ColumnDefinitions>
+                                <TextBlock Text="Load profile" VerticalAlignment="Center"/>
+                                <CheckBox Grid.Column="1" IsChecked="{Binding RunAsLoadProfile}" VerticalAlignment="Center"
+                                          Content="Load the user's HKCU + AppData (slower; only needed if the script reads user-scoped state)"/>
+                            </Grid>
+                        </StackPanel>
+                    </StackPanel>
+                </GroupBox>
+
                 <GroupBox Header="Response" Padding="6" Margin="0,8,0,0">
                     <Grid>
                         <Grid.ColumnDefinitions>

src/WebhookServer.Service/AdminPipeServer.cs

@@ -255,6 +255,7 @@ internal sealed class AdminPipeServer : BackgroundService
     {
         if (incoming.Bearer is { } a) MergeProtected(a.Secret, prior.Bearer?.Secret);
         if (incoming.Hmac is { } h) MergeProtected(h.Secret, prior.Hmac?.Secret);
+        if (incoming.RunAs is { Password: { } runAsPwd }) MergeProtected(runAsPwd, prior.RunAs?.Password);
         if (incoming.Callback is { } cb)
         {
             if (cb.Bearer is { } cba) MergeProtected(cba.Secret, prior.Callback?.Bearer?.Secret);