diff --git a/backend/frontend/frontend_excluded.go b/backend/frontend/frontend_excluded.go
index 2e0e35ef..a3e5a691 100644
--- a/backend/frontend/frontend_excluded.go
+++ b/backend/frontend/frontend_excluded.go
@@ -2,8 +2,11 @@
 
 package frontend
 
-import "github.com/gin-gonic/gin"
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/pocket-id/pocket-id/backend/internal/service"
+)
 
-func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) error {
+func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc, oidcService *service.OidcService) error {
 	return ErrFrontendNotIncluded
 }
diff --git a/backend/frontend/frontend_included.go b/backend/frontend/frontend_included.go
index 568449ef..42813f44 100644
--- a/backend/frontend/frontend_included.go
+++ b/backend/frontend/frontend_included.go
@@ -4,6 +4,7 @@ package frontend
 
 import (
 	"bytes"
+	"context"
 	"embed"
 	"fmt"
 	"io"
@@ -17,6 +18,8 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/pocket-id/pocket-id/backend/internal/middleware"
+	"github.com/pocket-id/pocket-id/backend/internal/service"
+	"github.com/pocket-id/pocket-id/backend/internal/utils"
 )
 
 //go:embed all:dist/*
@@ -54,7 +57,28 @@ func init() {
 	}
 }
 
-func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) error {
+// validateRedirectURI validates that the redirect_uri is in the client's allowed callback URLs
+func validateRedirectURI(ctx any, oidcService *service.OidcService, clientID, redirectURI string) (bool, error) {
+	client, err := oidcService.GetClient(ctx.(context.Context), clientID)
+	if err != nil {
+		return false, err
+	}
+
+	// If the client has no callback URLs configured, reject the redirect URI
+	if len(client.CallbackURLs) == 0 {
+		return false, fmt.Errorf("client has no callback URLs configured")
+	}
+
+	// Validate the redirect URI against the client's callback URLs
+	_, err = utils.GetCallbackURLFromList(client.CallbackURLs, redirectURI)
+	if err != nil {
+		return false, err
+	}
+
+	return true, nil
+}
+
+func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc, oidcService *service.OidcService) error {
 	distFS, err := fs.Sub(frontendFS, "dist")
 	if err != nil {
 		return fmt.Errorf("failed to create sub FS: %w", err)
@@ -92,12 +116,17 @@ func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) e
 
 		if path == "index.html" {
 			// Check if this is an OAuth2 authorization request with response_mode=form_post
-			// In that case, we need to allow form submissions to the redirect_uri
+			// In that case, we need to validate and allow form submissions to the redirect_uri
 			responseMode := c.Query("response_mode")
 			redirectURI := c.Query("redirect_uri")
-			if responseMode == "form_post" && redirectURI != "" {
-				// Set the allowed form-action in CSP to include the redirect URI
-				middleware.SetAllowedFormAction(c, redirectURI)
+			clientID := c.Query("client_id")
+			if responseMode == "form_post" && redirectURI != "" && clientID != "" {
+				// Validate the redirect_uri against the client's allowlist
+				isValid, err := validateRedirectURI(c.Request.Context(), oidcService, clientID, redirectURI)
+				if err == nil && isValid {
+					// Set the allowed form-action in CSP to include the redirect URI
+					middleware.SetAllowedFormAction(c, redirectURI)
+				}
 			}
 
 			nonce := middleware.GetCSPNonce(c)
diff --git a/backend/internal/bootstrap/router_bootstrap.go b/backend/internal/bootstrap/router_bootstrap.go
index 6803734d..fc957bd6 100644
--- a/backend/internal/bootstrap/router_bootstrap.go
+++ b/backend/internal/bootstrap/router_bootstrap.go
@@ -61,7 +61,7 @@ func initRouter(db *gorm.DB, svc *services) (utils.Service, error) {
 	r.Use(middleware.NewErrorHandlerMiddleware().Add())
 
 	frontendRateLimitMiddleware := middleware.NewRateLimitMiddleware().Add(rate.Every(100*time.Millisecond), 300)
-	err := frontend.RegisterFrontend(r, frontendRateLimitMiddleware)
+	err := frontend.RegisterFrontend(r, frontendRateLimitMiddleware, svc.oidcService)
 	if errors.Is(err, frontend.ErrFrontendNotIncluded) {
 		slog.Warn("Frontend is not included in the build. Skipping frontend registration.")
 	} else if err != nil {