diff --git a/bootstrap/app.php b/bootstrap/app.php
index 4bb5a5dd..6c2d2140 100644
--- a/bootstrap/app.php
+++ b/bootstrap/app.php
@@ -104,6 +104,33 @@ return Application::configure(basePath: dirname(__DIR__))
         ]);
     })
     ->withExceptions(function (Exceptions $exceptions) {
+        // Every credential this application stores encrypted, named again
+        // here so a failed validation does not write it back out in clear.
+        //
+        // A ValidationException flashes the request's input into the
+        // session so the form can be repopulated, minus this list. The
+        // framework's own three cover the login and password forms; none
+        // of the settings screens' credentials were on it, and
+        // config/session.php stores sessions in the database by default
+        // with `encrypt => false`. So a mistyped storage form put the
+        // secret access key in clear into the same database whose dump the
+        // `encrypted` cast exists to survive — and a service account key
+        // file, which is most likely to fail validation exactly when it
+        // was pasted incompletely, put a private key there.
+        //
+        // Merged with the framework's defaults rather than replacing them.
+        // The cost is that these fields come back blank after a failed
+        // save, which is what every one of these screens already does on a
+        // successful one: they are write-only, and a blank means "keep
+        // what is stored".
+        $exceptions->dontFlash([
+            'secret',           // ExternalStorageSettingsController (S3)
+            'key_file',         // ExternalStorageSettingsController (GCS)
+            'bind_password',    // LdapSettingsController
+            'client_secret',    // SocialLoginSettingsController, EmailSettingsController
+            'secret_key',       // CaptchaSettingsController
+        ]);
+
         // RFC 7807 for /api/* only. Everything else — web pages, Inertia
         // requests, the public share links — keeps Laravel's own handling
         // untouched, which is why this is scoped by path rather than by
diff --git a/tests/Feature/Platform/SecretsNotFlashedTest.php b/tests/Feature/Platform/SecretsNotFlashedTest.php
new file mode 100644
index 00000000..2c7184f6
--- /dev/null
+++ b/tests/Feature/Platform/SecretsNotFlashedTest.php
@@ -0,0 +1,118 @@
+<?php
+
+declare(strict_types=1);
+
+use App\Models\User;
+
+/**
+ * A failed save must not write the credential back out in clear.
+ *
+ * Laravel flashes the request's input into the session when validation
+ * fails, so the form can be repopulated. Its own exclusion list is
+ * current_password / password / password_confirmation -- written for the
+ * login and password forms, and covering none of the credentials the
+ * settings screens take. config/session.php stores sessions in the
+ * database by default and does not encrypt them, so anything flashed
+ * lands in clear in the same database the `encrypted` casts exist to
+ * protect.
+ *
+ * Each of these submits a form that fails validation *and* carries a
+ * secret, then reads the old input back the way the form would.
+ */
+beforeEach(function () {
+    $this->admin = User::factory()->create();
+});
+
+test('a rejected storage form does not flash the secret access key', function () {
+    // Fails on the missing bucket, which is required.
+    $this->actingAs($this->admin)->patch('/system/settings/storage', [
+        'active' => true,
+        'provider' => 's3',
+        'access_key' => 'AKIAEXAMPLE',
+        'secret' => 'super-secret-access-key',
+        'region' => 'us-east-1',
+        'use_path_style' => false,
+    ])->assertSessionHasErrors('bucket');
+
+    expect(session()->getOldInput('secret'))->toBeNull()
+        // The rest of the form still comes back, or the screen would clear
+        // itself every time somebody mistypes one field.
+        ->and(session()->getOldInput('region'))->toBe('us-east-1');
+});
+
+test('a rejected storage form does not flash the service account key file', function () {
+    // The sharpest case: serviceAccountKeyRule() is what catches a paste
+    // that lost its last line, so the request most likely to fail here is
+    // the one carrying a private key.
+    $key = json_encode([
+        'type' => 'service_account',
+        'project_id' => 'example',
+        'private_key_id' => 'abc',
+        'private_key' => "-----BEGIN PRIVATE KEY-----\nMIIBVAIBADAN\n-----END PRIVATE KEY-----\n",
+        'client_email' => 'svc@example.iam.gserviceaccount.com',
+    ]);
+
+    $this->actingAs($this->admin)->patch('/system/settings/storage', [
+        'active' => true,
+        'provider' => 'gcs',
+        'key_file' => $key,
+        'use_path_style' => false,
+    ])->assertSessionHasErrors('bucket');
+
+    expect(session()->getOldInput('key_file'))->toBeNull()
+        ->and(json_encode(session()->all()))->not->toContain('BEGIN PRIVATE KEY');
+});
+
+test('a rejected ldap form does not flash the bind password', function () {
+    // Fails on the missing email_attribute, which is required.
+    $this->actingAs($this->admin)->patch('/system/settings/ldap', [
+        'active' => true,
+        'port' => 636,
+        'encryption' => 'ssl',
+        'bind_dn' => 'cn=admin,dc=example,dc=test',
+        'bind_password' => 'super-secret-bind-password',
+        'name_attribute' => 'cn',
+        'auto_provision' => false,
+        'auto_approve' => false,
+    ])->assertSessionHasErrors('email_attribute');
+
+    expect(session()->getOldInput('bind_password'))->toBeNull()
+        ->and(session()->getOldInput('bind_dn'))->toBe('cn=admin,dc=example,dc=test');
+});
+
+test('a rejected social login form does not flash the client secret', function () {
+    $this->actingAs($this->admin)->patch('/system/settings/social-login/google', [
+        'enabled' => true,
+        'client_id' => '',
+        'client_secret' => 'super-secret-client-secret',
+    ])->assertSessionHasErrors();
+
+    expect(session()->getOldInput('client_secret'))->toBeNull();
+});
+
+test('a rejected captcha form does not flash the secret key', function () {
+    // Fails on the missing site_key, which turnstile requires.
+    $this->actingAs($this->admin)->patch('/system/settings/captcha', [
+        'provider' => 'turnstile',
+        'secret_key' => 'super-secret-captcha-key',
+        'on_login' => false,
+        'on_registration' => false,
+        'on_password_reset' => false,
+        'on_public_comments' => false,
+    ])->assertSessionHasErrors('site_key');
+
+    expect(session()->getOldInput('secret_key'))->toBeNull()
+        ->and(session()->getOldInput('provider'))->toBe('turnstile');
+});
+
+test('the framework defaults are kept, not replaced', function () {
+    // dontFlash() merges, so adding to it must not drop password.
+    $this->actingAs($this->admin)->put('/settings/password', [
+        'current_password' => 'wrong-password',
+        'password' => 'a-brand-new-password',
+        'password_confirmation' => 'a-brand-new-password',
+    ])->assertSessionHasErrors('current_password');
+
+    expect(session()->getOldInput('password'))->toBeNull()
+        ->and(session()->getOldInput('current_password'))->toBeNull();
+});