diff --git a/src/vs/platform/utilityProcess/common/envKey.ts b/src/vs/platform/utilityProcess/common/envKey.ts
new file mode 100644
index 00000000000000..8d88705c7dcaec
--- /dev/null
+++ b/src/vs/platform/utilityProcess/common/envKey.ts
@@ -0,0 +1,16 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+/**
+ * Whether `key` is a name Node.js accepts on process environments.
+ *
+ * Names beginning with a digit or containing characters other than `[A-Za-z0-9_]`
+ * are rejected: forwarding such keys to a forked/spawned child environment makes
+ * Electron throw `TypeError: Invalid value for env`, breaking the child process
+ * (e.g. the extension host). They must be filtered at the boundary.
+ */
+export function isValidEnvVariableKey(key: string): boolean {
+	return /^[A-Za-z_][A-Za-z0-9_]*$/.test(key);
+}
\ No newline at end of file
diff --git a/src/vs/platform/utilityProcess/electron-main/utilityProcess.ts b/src/vs/platform/utilityProcess/electron-main/utilityProcess.ts
index c1dd46b4c1771f..612ac9bdaa98b2 100644
--- a/src/vs/platform/utilityProcess/electron-main/utilityProcess.ts
+++ b/src/vs/platform/utilityProcess/electron-main/utilityProcess.ts
@@ -16,6 +16,7 @@ import { ITelemetryService } from '../../telemetry/common/telemetry.js';
 import { ILifecycleMainService } from '../../lifecycle/electron-main/lifecycleMainService.js';
 import { removeDangerousEnvVariables } from '../../../base/common/processes.js';
 import { deepClone } from '../../../base/common/objects.js';
+import { isValidEnvVariableKey } from '../common/envKey.js';
 import { isWindows } from '../../../base/common/platform.js';
 import { isUNCAccessRestrictionsDisabled, getUNCHostAllowlist } from '../../../base/node/unc.js';
 
@@ -293,8 +294,17 @@ export class UtilityProcess extends Disposable {
 		// Remove any environment variables that are not allowed
 		removeDangerousEnvVariables(env);
 
-		// Ensure all values are strings, otherwise the process will not start
+		// Ensure all values are strings, otherwise the process will not start (and valid keys are forwarded)
 		for (const key of Object.keys(env)) {
+			if (!isValidEnvVariableKey(key)) {
+				// An environment variable whose name Node.js considers invalid (e.g. leading digit,
+				// or characters outside [A-Za-z0-9_]) would break the utility process boundary:
+				// Electron's spawn() throws `TypeError: Invalid value for env`. Such keys leak in
+				// from the parent `process.env` via implicit (global) inheritance, so we strip them here.
+				console.warn(`[utilityProcess] Ignoring environment variable with invalid name: '${key}'`);
+				delete env[key];
+				continue;
+			}
 			env[key] = String(env[key]);
 		}
 
diff --git a/src/vs/platform/utilityProcess/test/common/envKey.test.ts b/src/vs/platform/utilityProcess/test/common/envKey.test.ts
new file mode 100644
index 00000000000000..980dde10fe4431
--- /dev/null
+++ b/src/vs/platform/utilityProcess/test/common/envKey.test.ts
@@ -0,0 +1,50 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as assert from 'assert';
+import { isValidEnvVariableKey } from '../common/envKey.js';
+
+suite('envKey', () => {
+
+	test('accepts common valid variable names', () => {
+		assert.strictEqual(isValidEnvVariableKey('PATH'), true);
+		assert.strictEqual(isValidEnvVariableKey('HOME'), true);
+		assert.strictEqual(isValidEnvVariableKey('USERNAME'), true);
+		assert.strictEqual(isValidEnvVariableKey('VSCODE_ESM_ENTRYPOINT'), true);
+		assert.strictEqual(isValidEnvVariableKey('NODE_UNC_HOST_ALLOWLIST'), true);
+		assert.strictEqual(isValidEnvVariableKey('_private'), true);
+	});
+
+	test('accepts digits after a leading letter/underscore', () => {
+		assert.strictEqual(isValidEnvVariableKey('a1'), true);
+		assert.strictEqual(isValidEnvVariableKey('a1b2'), true);
+		assert.strictEqual(isValidEnvVariableKey('ABC_123'), true);
+	});
+
+	test('rejects names starting with a digit', () => {
+		assert.strictEqual(isValidEnvVariableKey('1'), false);
+		assert.strictEqual(isValidEnvVariableKey('1HOME'), false);
+		assert.strictEqual(isValidEnvVariableKey('123'), false);
+		assert.strictEqual(isValidEnvVariableKey('0a'), false);
+	});
+
+	test('rejects names containing illegal characters, regardless of case', () => {
+		assert.strictEqual(isValidEnvVariableKey('a-b'), false);        // hyphen
+		assert.strictEqual(isValidEnvVariableKey('a b'), false);        // space
+		assert.strictEqual(isValidEnvVariableKey('a+b'), false);        // plus
+		assert.strictEqual(isValidEnvVariableKey('a(b)'), false);       // parens
+		assert.strictEqual(isValidEnvVariableKey('clion_g++'), false);  // real-world case
+		assert.strictEqual(isValidEnvVariableKey('CLION_G++'), false);  // case variant
+		assert.strictEqual(isValidEnvVariableKey('IntelliJ IDEA'), false);
+		assert.strictEqual(isValidEnvVariableKey('CommonProgramFiles(x86)'), false);
+		assert.strictEqual(isValidEnvVariableKey('ProgramFiles(x86)'), false);
+		assert.strictEqual(isValidEnvVariableKey('PROGRAMFILES(X86)'), false);
+		assert.strictEqual(isValidEnvVariableKey('CommonProgramFiles(X86)'), false);
+	});
+
+	test('rejects empty and non-string inputs', () => {
+		assert.strictEqual(isValidEnvVariableKey(''), false);
+	});
+});
\ No newline at end of file