fix(auth): disable URL caching in FoundationClientEngine (#4238)

The FoundationClientEngine is the base HTTP client used by the AWS SDK
service clients (Cognito Identity / Identity Provider, etc.) for credential
and token exchange. It previously used URLSession.shared, whose default
configuration includes an on-disk URLCache. As a result, responses carrying
Cognito tokens and AWS credentials were persisted to the app container's
Cache.db, where they could be recovered by inspecting the device.

Use a dedicated URLSession with urlCache = nil and a
reloadIgnoringLocalCacheData policy so these responses are never written to
disk. This mirrors the cache-disabling behavior already applied to the
Hosted UI URLSession in AWSCognitoAuthPlugin+Configure.

Author: harsh62

AmplifyPlugins/Core/AmplifyCredentials/CustomHttpClientEngine/FoundationClientEngine.swift

@@ -11,10 +11,12 @@ import SmithyHTTPAPI
 
 @_spi(FoundationClientEngine)
 public struct FoundationClientEngine: HTTPClient {
+    let urlSession: URLSession
+
     public func send(request: SmithyHTTPAPI.HTTPRequest) async throws -> SmithyHTTPAPI.HTTPResponse {
         let urlRequest = try await URLRequest(from: request)
 
-        let (data, response) = try await URLSession.shared.data(for: urlRequest)
+        let (data, response) = try await urlSession.data(for: urlRequest)
         guard let httpURLResponse = response as? HTTPURLResponse else {
             // This shouldn't be necessary because we're only making HTTP requests.
             // `URLResponse` should always be a `HTTPURLResponse`.
@@ -30,7 +32,16 @@ public struct FoundationClientEngine: HTTPClient {
         return httpResponse
     }
 
-    public init() {}
+    public init() {
+        // These requests carry Cognito tokens and AWS credentials. Disable URL
+        // caching so that responses are never persisted to disk (e.g. Cache.db),
+        // where they could be recovered by inspecting the app container. This
+        // mirrors the cache-disabling behavior used for the Hosted UI session.
+        let configuration = URLSessionConfiguration.default
+        configuration.urlCache = nil
+        configuration.requestCachePolicy = .reloadIgnoringLocalCacheData
+        self.urlSession = URLSession(configuration: configuration)
+    }
 
     /// no-op
     func close() async {}

AmplifyPlugins/Core/AmplifyCredentialsTests/Utils/FoundationClientEngineTests.swift

@@ -0,0 +1,32 @@
+//
+// Copyright Amazon.com Inc. or its affiliates.
+// All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+@_spi(FoundationClientEngine)
+@testable import InternalAmplifyCredentials
+import XCTest
+
+class FoundationClientEngineTests: XCTestCase {
+
+    /// Given: A `FoundationClientEngine`.
+    /// When: The engine is initialized.
+    /// Then: Its `URLSession` has URL caching disabled so that responses
+    ///       carrying Cognito tokens / AWS credentials are never persisted
+    ///       to disk (e.g. Cache.db).
+    func test_urlSession_disablesCaching() {
+        let engine = FoundationClientEngine()
+        let configuration = engine.urlSession.configuration
+
+        XCTAssertNil(
+            configuration.urlCache,
+            "URLSession must not have a URLCache, otherwise credential responses are persisted to disk."
+        )
+        XCTAssertEqual(
+            configuration.requestCachePolicy,
+            .reloadIgnoringLocalCacheData
+        )
+    }
+}