diff --git a/sdk/src/sandbox.ts b/sdk/src/sandbox.ts
index e941e2783..d25854aff 100644
--- a/sdk/src/sandbox.ts
+++ b/sdk/src/sandbox.ts
@@ -437,6 +437,28 @@ export interface SandboxSpawnOptions {
   signal?: AbortSignal;
 }
 
+/**
+ * Inject environment variables into the config's `process.env` field as
+ * `KEY=VALUE` strings.  This is the explicit channel for passing env vars
+ * to the sandboxed child -- the parent process environment is NOT inherited
+ * by the sandbox (security: prevents secret leakage).
+ */
+function injectEnvIntoConfig(
+  config: ContainerConfig,
+  env: { [key: string]: string | undefined },
+): void {
+  if (!config.process) {
+    config.process = { commandLine: '' };
+  }
+  const entries: string[] = config.process.env ? [...config.process.env] : [];
+  for (const [key, value] of Object.entries(env)) {
+    if (value !== undefined) {
+      entries.push(`${key}=${value}`);
+    }
+  }
+  config.process.env = entries;
+}
+
 /**
  * Internal helper: resolves the executor binary path and spawns a PTY process.
  */
@@ -446,6 +468,12 @@ function spawnWithConfig(
   workingDirectory?: string,
   env?: { [key: string]: string | undefined },
 ): pty.IPty {
+  // Inject env vars into config.process.env so they are passed explicitly to
+  // the sandboxed child via the JSON config (not via process inheritance).
+  if (env) {
+    injectEnvIntoConfig(config, env);
+  }
+
   const { executablePath, args, logger, startTime } = prepareSpawn(config, options);
 
   try {
@@ -455,7 +483,6 @@ function spawnWithConfig(
       rows: 80,
       ...options.ptyOptions,
       cwd: workingDirectory || process.cwd(),
-      env: env ?? options.ptyOptions?.env,
     };
 
     diagLog(`spawnWithConfig: spawning PTY process, cwd=${ptyOpts.cwd}`);
@@ -557,12 +584,17 @@ export function spawnSandboxFromConfig(
   env?: { [key: string]: string | undefined }
 ): pty.IPty | ChildProcess {
   if (options.usePty === false) {
+    // Inject env vars into config.process.env so they are passed explicitly to
+    // the sandboxed child via the JSON config (not via process inheritance).
+    if (env) {
+      injectEnvIntoConfig(config, env);
+    }
+
     const { executablePath, args, logger, startTime } = prepareSpawn(config, options);
     try {
       const child = spawn(executablePath, args, {
         cwd: workingDirectory || process.cwd(),
         stdio: ['pipe', 'pipe', 'pipe'],
-        ...(env ? { env: { ...process.env, ...env } as NodeJS.ProcessEnv } : {}),
       });
       child.on('close', (code) => {
         logger?.log('info', 'mxc.spawn.exit', {
diff --git a/src/Cargo.toml b/src/Cargo.toml
index 8a769ca36..2c3ef5196 100644
--- a/src/Cargo.toml
+++ b/src/Cargo.toml
@@ -40,6 +40,7 @@ windows = { version = "0.62", features = [
     "Win32_Security",
     "Win32_System_Com",
     "Win32_System_Console",
+    "Win32_System_Environment",
     "Win32_System_Ole",
     "Win32_System_Threading",
     "Win32_System_Pipes",
diff --git a/src/wxc_common/src/base_container_runner.rs b/src/wxc_common/src/base_container_runner.rs
index 85510443d..82af9763f 100644
--- a/src/wxc_common/src/base_container_runner.rs
+++ b/src/wxc_common/src/base_container_runner.rs
@@ -38,6 +38,29 @@ use sandbox_spec::base_container_layout::{
     NetworkPolicy as FbsNetworkPolicy, NetworkPolicyArgs, SandboxSpec, SandboxSpecArgs,
 };
 
+/// `CREATE_UNICODE_ENVIRONMENT` -- tells the API that the environment block is UTF-16 encoded.
+const CREATE_UNICODE_ENVIRONMENT: u32 = 0x0000_0400;
+
+/// Serialize `KEY=VALUE` pairs into a double-null-terminated UTF-16 environment block.
+///
+/// Entries are sorted case-insensitively by key as required by `CreateProcessW`.
+fn encode_env_block(env_vars: &[String]) -> Vec<u16> {
+    let mut entries: Vec<(&str, &str)> =
+        env_vars.iter().filter_map(|e| e.split_once('=')).collect();
+
+    entries.sort_by(|(a, _), (b, _)| a.to_ascii_uppercase().cmp(&b.to_ascii_uppercase()));
+
+    let mut block = Vec::new();
+    for (key, value) in &entries {
+        for ch in format!("{}={}", key, value).encode_utf16() {
+            block.push(ch);
+        }
+        block.push(0);
+    }
+    block.push(0);
+    block
+}
+
 /// Function pointer type matching `Experimental_CreateProcessInSandbox` from processmodel.dll.
 type PfnCreateProcessInSandbox = unsafe extern "system" fn(
     application_name: *const u16,
@@ -543,6 +566,28 @@ impl ScriptRunner for BaseContainerRunner {
         };
         let mut pi: PROCESS_INFORMATION = unsafe { std::mem::zeroed() };
 
+        // Environment block for the sandboxed child.
+        // If the caller specified explicit env vars, use only those.
+        // Otherwise, pass NULL to let the OS provide the default environment
+        // for the sandbox (CreateProcessInSandbox handles this internally).
+        let env_block: Option<Vec<u16>> = if request.env.is_empty() {
+            // TODO: consider calling CreateEnvironmentBlock(NULL, FALSE) here
+            // for a cleansed default env if the OS API doesn't do it for us.
+            None
+        } else {
+            Some(encode_env_block(&request.env))
+        };
+
+        let env_ptr = env_block
+            .as_ref()
+            .map(|b| b.as_ptr() as *const c_void)
+            .unwrap_or(ptr::null());
+        let creation_flags = if env_block.is_some() {
+            CREATE_UNICODE_ENVIRONMENT
+        } else {
+            0
+        };
+
         let _ = writeln!(logger, "launching: {}", request.script_code);
         let _ = writeln!(logger, "identity: {identity}");
 
@@ -558,8 +603,8 @@ impl ScriptRunner for BaseContainerRunner {
                 ptr::null(),             // processAttributes (must be NULL)
                 ptr::null(),             // threadAttributes  (must be NULL)
                 0,                       // inheritHandles    (must be FALSE)
-                0,                       // creationFlags
-                ptr::null(),             // environment       (must be NULL)
+                creation_flags,          // creationFlags
+                env_ptr,                 // environment
                 cwd_ptr,                 // currentDirectory
                 &si,                     // startupInfo
                 identity_wide.as_ptr(),  // identity