x.sessions: verify HMAC signatures when extracting sessions IDs from cookies (#26199)

Author: fleximus

vlib/x/sessions/sessions.v

@@ -170,15 +170,19 @@ pub fn (mut s Sessions[T]) resave[X](mut ctx X, data T) ! {
 }
 
 // get_session_id retrieves the current session id, if it is set.
+// The HMAC signature is verified when extracting from cookies.
 pub fn (s &Sessions[T]) get_session_id[X](ctx X) ?string {
 	// first check session id from `ctx`
 	sid_from_ctx := ctx.CurrentSession.session_id
 	if sid_from_ctx != '' {
 		return sid_from_ctx
 	} else if cookie := ctx.get_cookie(s.cookie_options.cookie_name) {
-		// check request headers for the session_id cookie
-		a := cookie.split('.')
-		return a[0]
+		// check request headers for the session_id cookie and verify HMAC signature
+		sid, valid := verify_session_id(cookie, s.secret)
+		if valid {
+			return sid
+		}
+		return none
 	} else {
 		// check the Set-Cookie headers on the response for a session id
 		for cookie in ctx.res.cookies() {

vlib/x/sessions/tests/session_test.v

@@ -19,5 +19,44 @@ fn test_session_id() {
 	verified_sid, valid := sessions.verify_session_id(sid_with_hmac, secret)
 
 	assert unverified_sid == verified_sid
-	assert valid == true
+	assert valid
+}
+
+fn test_forged_signature_rejected() {
+	// Create a valid session ID
+	sid, _ := sessions.new_session_id(secret)
+
+	// Forge a cookie with valid session ID but invalid signature
+	forged_cookie := '${sid}.INVALID_SIGNATURE'
+	verified_sid, valid := sessions.verify_session_id(forged_cookie, secret)
+
+	// Forged signature must be rejected
+	assert !valid
+	assert verified_sid == sid
+}
+
+fn test_wrong_secret_rejected() {
+	// Create a session with one secret
+	_, signed_cookie := sessions.new_session_id(secret)
+
+	// Try to verify with a different secret
+	wrong_secret := 'wrong_secret'.bytes()
+	_, valid := sessions.verify_session_id(signed_cookie, wrong_secret)
+
+	// Must be rejected when using wrong secret
+	assert !valid
+}
+
+fn test_malformed_cookie_rejected() {
+	// Cookie without signature separator
+	_, valid1 := sessions.verify_session_id('just_a_session_id', secret)
+	assert !valid1
+
+	// Empty cookie
+	_, valid2 := sessions.verify_session_id('', secret)
+	assert !valid2
+
+	// Cookie with empty parts
+	_, valid3 := sessions.verify_session_id('.', secret)
+	assert !valid3
 }