feat(security): add THROTTLE_KEY env-var fallback for throttle.key (#4707)

checkThrottleEncryption() now consults the THROTTLE_KEY environment
variable when throttle.key is empty, mirroring the ENCRYPTION_KEY
fallback in Config/Encryption. This lets Docker/Compose deployments
supply the throttle HMAC secret without writing a shared value into a
read-only .env. An explicit throttle.key always takes precedence.

Adds regression tests to the existing security_helperTest suite and
documents THROTTLE_KEY in .env.example.

.env.example

@@ -62,6 +62,10 @@ database.tests.DBPrefix='ospos_'
 encryption.key=''
 # Persistent secret for HMAC-hashing login-throttle cache keys. Left blank and
 # provisioned on startup (php spark env:provision); independent of encryption.key.
+# For Docker/Compose, pass it via the THROTTLE_KEY env var instead:
+#   docker run -e THROTTLE_KEY="$(openssl rand -hex 32)" opensourcepos
+# THROTTLE_KEY is read as a fallback when throttle.key is empty, so no
+# shared secret needs to be committed or baked into the shipped image.
 throttle.key=''
 
 #--------------------------------------------------------------------

app/Helpers/security_helper.php

@@ -327,12 +327,16 @@ function checkEncryption(?CI3SecretConverter $converter = null): bool
  * Returns the persistent HMAC secret used to hash login-throttle cache keys.
  *
  * Behaviour:
- * - Key already present: returned immediately (no I/O).
+ * - Key already present (throttle.key or THROTTLE_KEY): returned immediately (no I/O).
  * - Key missing and .env IS writable: generates a fresh key and persists it.
  * - Key missing and .env NOT writable:
  *   throws — the key was presumably already provisioned externally
  *   (e.g. `php spark env:provision` at container startup).
  *
+ * THROTTLE_KEY is a real environment-variable fallback (mirroring ENCRYPTION_KEY):
+ * it is consulted only when throttle.key is empty, so it can be supplied via
+ * Docker/Compose without writing a shared secret into .env.
+ *
  * @return string the throttle key
  * @throws RuntimeException if the key cannot be provisioned
  */
@@ -340,6 +344,11 @@ function checkThrottleEncryption(): string
 {
     $key = (string) env('throttle.key', '');
 
+    if ($key === '') {
+        $envKey = getenv('THROTTLE_KEY');
+        $key    = $envKey === false ? '' : $envKey;
+    }
+
     if ($key !== '') {
         return $key;
     }

tests/helpers/security_helperTest.php

@@ -467,6 +467,59 @@ class security_helperTest extends CIUnitTestCase
         }
     }
 
+    public function testCheckThrottleEncryptionFallsBackToThrottleKeyEnvVar(): void
+    {
+        $throttleKey = bin2hex(random_bytes(32));
+
+        // Clear the dot-notation throttle.key so the THROTTLE_KEY fallback is
+        // exercised, and leave a .env with no throttle.key so any accidental
+        // provisioning would be visible in the file assertion below.
+        putenv('throttle.key');
+        unset($_ENV['throttle.key'], $_SERVER['throttle.key']);
+        file_put_contents($this->envPath, "# tmp\n");
+
+        $previous = getenv('THROTTLE_KEY');
+        putenv("THROTTLE_KEY=$throttleKey");
+
+        try {
+            $result = checkThrottleEncryption();
+
+            $this->assertSame($throttleKey, $result);
+            $this->assertStringNotContainsString('throttle.key=', file_get_contents($this->envPath), 'the THROTTLE_KEY fallback must not trigger provisioning/write');
+        } finally {
+            if ($previous === false) {
+                putenv('THROTTLE_KEY');
+            } else {
+                putenv("THROTTLE_KEY=$previous");
+            }
+        }
+    }
+
+    public function testCheckThrottleEncryptionPrefersThrottleKeyOverEnvVar(): void
+    {
+        $explicit = bin2hex(random_bytes(32));
+        $fallback = bin2hex(random_bytes(32));
+
+        putenv("throttle.key=$explicit");
+        $_ENV['throttle.key']    = $explicit;
+        $_SERVER['throttle.key'] = $explicit;
+
+        $previous = getenv('THROTTLE_KEY');
+        putenv("THROTTLE_KEY=$fallback");
+
+        try {
+            $result = checkThrottleEncryption();
+
+            $this->assertSame($explicit, $result, 'an explicit throttle.key must take precedence over the THROTTLE_KEY fallback');
+        } finally {
+            if ($previous === false) {
+                putenv('THROTTLE_KEY');
+            } else {
+                putenv("THROTTLE_KEY=$previous");
+            }
+        }
+    }
+
     // -- rotateEncryptionKey() — provisioning path, does write --
 
     public function testRotateEncryptionKeyPersistsNewKey(): void