diff --git a/Sources/TerminalNotificationPolicy.swift b/Sources/TerminalNotificationPolicy.swift
index 3a5e8a606e14..d811ea4c5852 100644
--- a/Sources/TerminalNotificationPolicy.swift
+++ b/Sources/TerminalNotificationPolicy.swift
@@ -619,7 +619,9 @@ private final class NotificationHookProcessRun: @unchecked Sendable {
             posix_spawnattr_setsigmask(&attributes, &emptyMask),
             operation: "clear inherited signal mask"
         )
-        let flags = Int16(POSIX_SPAWN_SETPGROUP | POSIX_SPAWN_SETSIGMASK)
+        // Keep unrelated app descriptors out of hooks. The dup2 actions above
+        // preserve the hook's standard streams.
+        let flags = Int16(POSIX_SPAWN_SETPGROUP | POSIX_SPAWN_SETSIGMASK | POSIX_SPAWN_CLOEXEC_DEFAULT)
         try throwIfPOSIXError(posix_spawnattr_setflags(&attributes, flags), operation: "set spawn flags")
         try throwIfPOSIXError(posix_spawnattr_setpgroup(&attributes, 0), operation: "set process group")
         let arguments = ["/bin/sh", "-c", hook.command]
diff --git a/cmuxTests/NotificationAndMenuBarTests.swift b/cmuxTests/NotificationAndMenuBarTests.swift
index 8408191df7c3..d429875c9703 100644
--- a/cmuxTests/NotificationAndMenuBarTests.swift
+++ b/cmuxTests/NotificationAndMenuBarTests.swift
@@ -1,6 +1,8 @@
 import XCTest
 import AppKit
+import Darwin
 import SwiftUI
+import Testing
 import UniformTypeIdentifiers
 import WebKit
 import ObjectiveC.runtime
@@ -35,6 +37,47 @@ private final class NotificationHookEvaluationResultBox: @unchecked Sendable {
     }
 }
 
+@Suite("Notification hook process isolation")
+struct NotificationHookProcessIsolationTests {
+    @Test
+    func hookDoesNotInheritUnrelatedParentFileDescriptor() async throws {
+        var unrelatedPipe = [Int32](repeating: -1, count: 2)
+        try #require(Darwin.pipe(&unrelatedPipe) == 0)
+        defer {
+            for descriptor in unrelatedPipe where descriptor >= 0 {
+                Darwin.close(descriptor)
+            }
+        }
+        let unrelatedDescriptor = try #require(unrelatedPipe.first)
+        try #require(unrelatedDescriptor > STDERR_FILENO)
+
+        let request = TerminalNotificationPolicyRequest(
+            tabId: UUID(),
+            surfaceId: nil,
+            title: "Title",
+            subtitle: "",
+            body: "Body",
+            cwd: FileManager.default.temporaryDirectory.path,
+            isAppFocused: false,
+            isFocusedPanel: false
+        )
+        let hook = CmuxResolvedNotificationHook(
+            id: "descriptor-isolation",
+            command: "if [ -e /dev/fd/\(unrelatedDescriptor) ]; then printf '{\"notification\":{\"body\":\"inherited\"}}'; else cat; fi",
+            timeoutSeconds: 5,
+            sourcePath: "/tmp/cmux.json",
+            cwd: FileManager.default.temporaryDirectory.path
+        )
+
+        let result = await TerminalNotificationPolicyEngine.evaluate(
+            request: request,
+            hooks: [hook]
+        )
+        let envelope = try result.get()
+        #expect(envelope.notification.body == "Body")
+    }
+}
+
 final class TerminalNotificationPolicyEngineTests: XCTestCase {
     private func evaluate(
         request: TerminalNotificationPolicyRequest,