Guard against out of order messages in the exchange.

src/FreeDSx/Sasl/Challenge/CramMD5Challenge.php

@@ -108,6 +108,12 @@ private function validateClientResponse(
         Message $received,
         CramMD5Options $options,
     ): void {
+        # The client sent a response without us sending a challenge...
+        if (!$this->context->has('challenge')) {
+            $this->context->setIsComplete(true);
+
+            return;
+        }
         if (!$received->has('username')) {
             throw new SaslException('The client response must have a username.');
         }

tests/unit/Challenge/CramMD5ChallengeTest.php

@@ -67,6 +67,21 @@ public function testChallengeWithFromServerWithClientCorrectResponse(): void
         self::assertTrue($context->isComplete());
     }
 
+    public function testChallengeFromServerWithNoIssuedChallengeDoesNotAuthenticate(): void
+    {
+        $serverChallenge = new CramMD5Challenge(true);
+        $validate = static fn (string $username, string $challenge): string => hash_hmac('md5', $challenge, 'bar');
+
+        // A digest computed offline over an empty challenge, sent before the server issued one.
+        $context = $serverChallenge->challenge(
+            'foo ' . hash_hmac('md5', '', 'bar'),
+            (new CramMD5Options())->setPasswordCallback(Closure::fromCallable($validate)),
+        );
+
+        self::assertFalse($context->isAuthenticated());
+        self::assertTrue($context->isComplete());
+    }
+
     public function testPasswordCallableReceivesEncodedChallengeMatchingWhatClientUses(): void
     {
         $serverChallenge = new CramMD5Challenge(true);

tests/unit/Challenge/DigestMD5ChallengeTest.php

@@ -16,6 +16,7 @@
 use FreeDSx\Sasl\Challenge\DigestMD5Challenge;
 use FreeDSx\Sasl\Encoder\DigestMD5Encoder;
 use FreeDSx\Sasl\Exception\SaslException;
+use FreeDSx\Sasl\Message;
 use FreeDSx\Sasl\Options\DigestMD5Options;
 use FreeDSx\Sasl\SaslContext;
 use PHPUnit\Framework\TestCase;
@@ -119,6 +120,33 @@ public function testGenerateServerChallengeForClientInServerMode(): void
         self::assertNotEmpty($challenge->get('nonce'), 'The nonce must be generated.');
     }
 
+    public function testServerModeWithNoIssuedChallengeDoesNotAuthenticate(): void
+    {
+        $serverChallenge = new DigestMD5Challenge(true);
+
+        // A client response sent before the server issued its nonce.
+        $clientResponse = $this->encoder->encode(
+            new Message([
+                'username' => 'foo',
+                'realm' => '',
+                'nonce' => 'x',
+                'cnonce' => str_repeat('a', 14),
+                'nc' => '00000001',
+                'qop' => 'auth',
+                'digest-uri' => 'ldap/host',
+                'response' => str_repeat('a', 32),
+            ]),
+            new SaslContext(),
+        );
+
+        $context = $serverChallenge->challenge(
+            $clientResponse,
+            (new DigestMD5Options())->setPassword('bar'),
+        );
+
+        self::assertFalse($context->isAuthenticated());
+    }
+
     public function testGenerateServerResponseToClientResponse(): void
     {
         self::markTestSkipped('Test does not work on newer PHP due to deprecated ciphers it seems. Needs investigation.');