diff --git a/README.md b/README.md
index 78a1aac..537d461 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,15 @@ token validator. Never expose a client secret or token in browser code.
 
 The administrative API is intentionally excluded from this package.
 
+## Transport and correlation
+
+All discovery, token, and JWKS requests enforce certificate and hostname
+verification, reject HTTP redirects, and use bounded connect/read timeouts.
+Callers may pass a safe correlation identifier as the optional final argument
+to `discover`, `exchange`, and ID-token validation methods; the SDK propagates
+it as `X-Correlation-ID`. The SDK rejects malformed values and never logs
+authorization codes, client secrets, or tokens.
+
 ## Security
 
 Only explicit HTTPS endpoints are accepted. JWT signing is pinned to RS256, keys
diff --git a/src/Oidc/AuthorizationCodeClient.php b/src/Oidc/AuthorizationCodeClient.php
index 3d19386..1b56eb3 100644
--- a/src/Oidc/AuthorizationCodeClient.php
+++ b/src/Oidc/AuthorizationCodeClient.php
@@ -10,7 +10,7 @@ public function __construct(private readonly ClientInterface $http)
     {
     }
 
-    public function exchange(OidcClientConfiguration $configuration, string $code, string $codeVerifier): OidcTokenSet
+    public function exchange(OidcClientConfiguration $configuration, string $code, string $codeVerifier, ?string $correlationId = null): OidcTokenSet
     {
         if ($code === '' || $codeVerifier === '') {
             throw new OidcException('Authorization code and PKCE verifier are required.');
@@ -29,10 +29,11 @@ public function exchange(OidcClientConfiguration $configuration, string $code, s
 
         try {
             $response = $this->http->request('POST', $configuration->tokenEndpoint, [
-                'timeout' => $configuration->httpTimeoutSeconds,
-                'connect_timeout' => $configuration->httpTimeoutSeconds,
-                'http_errors' => false,
-                'headers' => ['Accept' => 'application/json'],
+                ...OidcHttpRequestOptions::strict(
+                    $configuration->httpTimeoutSeconds,
+                    ['Accept' => 'application/json'],
+                    $correlationId,
+                ),
                 'form_params' => $form,
             ]);
         } catch (\Throwable $exception) {
diff --git a/src/Oidc/IdTokenValidator.php b/src/Oidc/IdTokenValidator.php
index c414d76..2d2e5cc 100644
--- a/src/Oidc/IdTokenValidator.php
+++ b/src/Oidc/IdTokenValidator.php
@@ -18,9 +18,9 @@ public function __construct(private readonly ClientInterface $http)
 
     /** @return array<string, mixed> */
     /** @return array<string, mixed> */
-    public function validate(OidcClientConfiguration $configuration, string $idToken, ?string $expectedNonce = null): array
+    public function validate(OidcClientConfiguration $configuration, string $idToken, ?string $expectedNonce = null, ?string $correlationId = null): array
     {
-        $payload = $this->verifiedClaims($configuration, $idToken);
+        $payload = $this->verifiedClaims($configuration, $idToken, $correlationId);
         $this->assertIssuerAndAudience($configuration, $payload);
         if (trim((string) ($payload['sub'] ?? '')) === '') {
             throw new OidcException('ID token subject is missing.');
@@ -37,9 +37,9 @@ public function validate(OidcClientConfiguration $configuration, string $idToken
     }
 
     /** @return array<string, mixed> */
-    public function validateLogoutToken(OidcClientConfiguration $configuration, string $logoutToken): array
+    public function validateLogoutToken(OidcClientConfiguration $configuration, string $logoutToken, ?string $correlationId = null): array
     {
-        $payload = $this->verifiedClaims($configuration, $logoutToken);
+        $payload = $this->verifiedClaims($configuration, $logoutToken, $correlationId);
         $this->assertIssuerAndAudience($configuration, $payload);
         if (! is_numeric($payload['iat'] ?? null) || (int) $payload['iat'] > time() + 30 || trim((string) ($payload['jti'] ?? '')) === '') {
             throw new OidcException('Back-channel logout token time or identifier claims are invalid.');
@@ -49,18 +49,18 @@ public function validateLogoutToken(OidcClientConfiguration $configuration, stri
     }
 
     /** @return array<string, mixed> */
-    private function verifiedClaims(OidcClientConfiguration $configuration, string $token): array
+    private function verifiedClaims(OidcClientConfiguration $configuration, string $token, ?string $correlationId): array
     {
         $header = $this->decodeHeader($token);
         if (($header['alg'] ?? null) !== 'RS256' || ! is_string($header['kid'] ?? null) || $header['kid'] === '') {
             throw new OidcException('JWT header does not use an allowed signing algorithm.');
         }
 
-        $keys = $this->jwks($configuration->jwksUri, $configuration->httpTimeoutSeconds, false);
+        $keys = $this->jwks($configuration->jwksUri, $configuration->httpTimeoutSeconds, false, $correlationId);
         $keySet = JWK::parseKeySet($keys);
         $kid = $header['kid'];
         if (! isset($keySet[$kid])) {
-            $keySet = JWK::parseKeySet($this->jwks($configuration->jwksUri, $configuration->httpTimeoutSeconds, true));
+            $keySet = JWK::parseKeySet($this->jwks($configuration->jwksUri, $configuration->httpTimeoutSeconds, true, $correlationId));
         }
         $key = $keySet[$kid] ?? null;
         if (! $key instanceof Key) {
@@ -93,13 +93,13 @@ private function assertIssuerAndAudience(OidcClientConfiguration $configuration,
 
     /** @return array<string, mixed> */
     /** @return array<string, mixed> */
-    private function jwks(string $uri, int $timeoutSeconds, bool $refresh): array
+    private function jwks(string $uri, int $timeoutSeconds, bool $refresh, ?string $correlationId): array
     {
         if (! $refresh && isset($this->jwksByUri[$uri])) {
             return $this->jwksByUri[$uri];
         }
         try {
-            $response = $this->http->request('GET', $uri, ['timeout' => $timeoutSeconds, 'connect_timeout' => $timeoutSeconds, 'http_errors' => false]);
+            $response = $this->http->request('GET', $uri, OidcHttpRequestOptions::strict($timeoutSeconds, [], $correlationId));
         } catch (\Throwable $exception) {
             throw new OidcException('JWKS request failed.', 0, $exception);
         }
diff --git a/src/Oidc/OidcDiscoveryClient.php b/src/Oidc/OidcDiscoveryClient.php
index 5c8d71d..fddb01b 100644
--- a/src/Oidc/OidcDiscoveryClient.php
+++ b/src/Oidc/OidcDiscoveryClient.php
@@ -10,7 +10,7 @@ public function __construct(private readonly ClientInterface $http)
     {
     }
 
-    public function discover(string $issuer, int $timeoutSeconds = 5): OidcDiscoveryDocument
+    public function discover(string $issuer, int $timeoutSeconds = 5, ?string $correlationId = null): OidcDiscoveryDocument
     {
         $issuer = rtrim($issuer, '/');
         if (filter_var($issuer, FILTER_VALIDATE_URL) === false || parse_url($issuer, PHP_URL_SCHEME) !== 'https') {
@@ -18,12 +18,11 @@ public function discover(string $issuer, int $timeoutSeconds = 5): OidcDiscovery
         }
 
         try {
-            $response = $this->http->request('GET', $issuer.'/.well-known/openid-configuration', [
-                'timeout' => $timeoutSeconds,
-                'connect_timeout' => $timeoutSeconds,
-                'http_errors' => false,
-                'headers' => ['Accept' => 'application/json'],
-            ]);
+            $response = $this->http->request('GET', $issuer.'/.well-known/openid-configuration', OidcHttpRequestOptions::strict(
+                $timeoutSeconds,
+                ['Accept' => 'application/json'],
+                $correlationId,
+            ));
         } catch (\Throwable $exception) {
             throw new OidcException('OIDC discovery request failed.', 0, $exception);
         }
@@ -34,11 +33,17 @@ public function discover(string $issuer, int $timeoutSeconds = 5): OidcDiscovery
         }
 
         foreach (['authorization_endpoint', 'token_endpoint', 'jwks_uri'] as $field) {
-            if (! is_string($document[$field] ?? null) || $document[$field] === '') {
+            if (! is_string($document[$field] ?? null) || ! self::isHttpsUrl($document[$field])) {
                 throw new OidcException('OIDC discovery document is missing '.$field.'.');
             }
         }
 
+        foreach (['userinfo_endpoint', 'end_session_endpoint'] as $field) {
+            if (isset($document[$field]) && (! is_string($document[$field]) || ! self::isHttpsUrl($document[$field]))) {
+                throw new OidcException('OIDC discovery document contains an invalid '.$field.'.');
+            }
+        }
+
         return new OidcDiscoveryDocument(
             issuer: $issuer,
             authorizationEndpoint: (string) $document['authorization_endpoint'],
@@ -48,4 +53,9 @@ public function discover(string $issuer, int $timeoutSeconds = 5): OidcDiscovery
             endSessionEndpoint: is_string($document['end_session_endpoint'] ?? null) ? $document['end_session_endpoint'] : null,
         );
     }
+
+    private static function isHttpsUrl(string $url): bool
+    {
+        return filter_var($url, FILTER_VALIDATE_URL) !== false && parse_url($url, PHP_URL_SCHEME) === 'https';
+    }
 }
diff --git a/src/Oidc/OidcHttpRequestOptions.php b/src/Oidc/OidcHttpRequestOptions.php
new file mode 100644
index 0000000..e4d9f01
--- /dev/null
+++ b/src/Oidc/OidcHttpRequestOptions.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Novvor\IdentitySdk\Oidc;
+
+/**
+ * Builds the non-negotiable transport options for OIDC protocol calls.
+ *
+ * Consumers may choose a custom CA bundle on their HTTP client, but protocol
+ * calls can never opt out of certificate or hostname verification.
+ */
+final class OidcHttpRequestOptions
+{
+    /**
+     * @param array<string, string> $headers
+     * @return array<string, mixed>
+     */
+    public static function strict(int $timeoutSeconds, array $headers = [], ?string $correlationId = null): array
+    {
+        if ($timeoutSeconds < 1 || $timeoutSeconds > 30) {
+            throw new OidcException('OIDC HTTP timeout must be between one and thirty seconds.');
+        }
+
+        if ($correlationId !== null) {
+            $correlationId = trim($correlationId);
+            if ($correlationId === '' || strlen($correlationId) > 128 || preg_match('/^[A-Za-z0-9._:-]+$/', $correlationId) !== 1) {
+                throw new OidcException('OIDC correlation ID is invalid.');
+            }
+
+            $headers['X-Correlation-ID'] = $correlationId;
+        }
+
+        return [
+            'timeout' => $timeoutSeconds,
+            'connect_timeout' => $timeoutSeconds,
+            'http_errors' => false,
+            'allow_redirects' => false,
+            'verify' => true,
+            'headers' => $headers,
+        ];
+    }
+}
diff --git a/tests/OidcTransportSecurityTest.php b/tests/OidcTransportSecurityTest.php
new file mode 100644
index 0000000..4fc4db6
--- /dev/null
+++ b/tests/OidcTransportSecurityTest.php
@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Novvor\IdentitySdk\Tests;
+
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\Psr7\Response;
+use Novvor\IdentitySdk\Oidc\AuthorizationCodeClient;
+use Novvor\IdentitySdk\Oidc\OidcClientConfiguration;
+use Novvor\IdentitySdk\Oidc\OidcDiscoveryClient;
+use Novvor\IdentitySdk\Oidc\OidcException;
+use Novvor\IdentitySdk\Oidc\OidcHttpRequestOptions;
+use PHPUnit\Framework\TestCase;
+
+final class OidcTransportSecurityTest extends TestCase
+{
+    public function test_discovery_requires_https_endpoints(): void
+    {
+        $client = $this->client([
+            new Response(200, ['Content-Type' => 'application/json'], json_encode([
+                'issuer' => 'https://identity.example.com',
+                'authorization_endpoint' => 'https://identity.example.com/oauth/authorize',
+                'token_endpoint' => 'https://identity.example.com/oauth/token',
+                'jwks_uri' => 'https://identity.example.com/.well-known/jwks.json',
+            ], JSON_THROW_ON_ERROR)),
+        ]);
+
+        $document = (new OidcDiscoveryClient($client))->discover('https://identity.example.com');
+
+        self::assertSame('https://identity.example.com/oauth/token', $document->tokenEndpoint);
+    }
+
+    public function test_token_exchange_and_strict_transport_options_reject_invalid_correlation_id(): void
+    {
+        $client = $this->client([
+            new Response(200, ['Content-Type' => 'application/json'], json_encode([
+                'access_token' => 'access-token',
+                'id_token' => 'id-token',
+                'token_type' => 'Bearer',
+                'expires_in' => 300,
+            ], JSON_THROW_ON_ERROR)),
+        ]);
+
+        $tokens = (new AuthorizationCodeClient($client))->exchange($this->configuration(), 'code', str_repeat('a', 43));
+
+        self::assertSame('access-token', $tokens->accessToken);
+        self::assertSame([
+            'timeout' => 5,
+            'connect_timeout' => 5,
+            'http_errors' => false,
+            'allow_redirects' => false,
+            'verify' => true,
+            'headers' => ['Accept' => 'application/json', 'X-Correlation-ID' => 'corr-456'],
+        ], OidcHttpRequestOptions::strict(5, ['Accept' => 'application/json'], 'corr-456'));
+
+        $this->expectException(OidcException::class);
+        OidcHttpRequestOptions::strict(5, [], "invalid\nheader");
+    }
+
+    /** @param list<Response> $responses */
+    private function client(array $responses): Client
+    {
+        return new Client(['handler' => \GuzzleHttp\HandlerStack::create(new MockHandler($responses))]);
+    }
+
+    private function configuration(): OidcClientConfiguration
+    {
+        return new OidcClientConfiguration(
+            'https://identity.example.com',
+            'client',
+            'https://app.example.com/auth/oidc/callback',
+            'https://identity.example.com/oauth/authorize',
+            'https://identity.example.com/oauth/token',
+            'https://identity.example.com/.well-known/jwks.json',
+            'client-secret',
+        );
+    }
+}