diff --git a/apps/web/client/src/server/api/routers/project/helper.ts b/apps/web/client/src/server/api/routers/project/helper.ts
index e2f6037378..b522900dd5 100644
--- a/apps/web/client/src/server/api/routers/project/helper.ts
+++ b/apps/web/client/src/server/api/routers/project/helper.ts
@@ -1,4 +1,8 @@
-import { type Frame } from "@onlook/db";
+import { eq } from "drizzle-orm";
+import { type Frame, projects, userProjects, type DrizzleDb } from "@onlook/db";
+
+/** Type representing a db instance or transaction that has query capabilities */
+type DbOrTx = Pick<DrizzleDb, 'query'>;
 
 export function extractCsbPort(frames: Frame[]): number | null {
     if (!frames || frames.length === 0) return null;
@@ -17,3 +21,32 @@ export function extractCsbPort(frames: Frame[]): number | null {
     }
     return null;
 }
+
+/**
+ * Verifies that a user has access to a project by checking the userProjects table.
+ * @throws Error if the user does not have access to the project or if it doesn't exist
+ *
+ * Note: This function intentionally returns the same error message whether the project
+ * doesn't exist or the user lacks access to prevent information disclosure about
+ * project existence.
+ *
+ * Accepts either a db instance or a transaction to support atomic authorization checks.
+ */
+export async function verifyProjectAccess(
+    db: DbOrTx,
+    userId: string,
+    projectId: string,
+): Promise<void> {
+    const project = await db.query.projects.findFirst({
+        where: eq(projects.id, projectId),
+        with: {
+            userProjects: {
+                where: eq(userProjects.userId, userId),
+            },
+        },
+    });
+
+    if (!project || project.userProjects.length === 0) {
+        throw new Error('Unauthorized or not found');
+    }
+}
diff --git a/apps/web/client/src/server/api/routers/project/project.ts b/apps/web/client/src/server/api/routers/project/project.ts
index a110db5bbf..bfece724ee 100644
--- a/apps/web/client/src/server/api/routers/project/project.ts
+++ b/apps/web/client/src/server/api/routers/project/project.ts
@@ -37,7 +37,7 @@ import { and, eq, ne } from 'drizzle-orm';
 import { z } from 'zod';
 import { projectCreateRequestRouter } from './createRequest';
 import { fork } from './fork';
-import { extractCsbPort } from './helper';
+import { extractCsbPort, verifyProjectAccess } from './helper';
 
 export const projectRouter = createTRPCRouter({
     hasAccess: protectedProcedure
@@ -59,6 +59,7 @@ export const projectRouter = createTRPCRouter({
         .input(z.object({ projectId: z.string() }))
         .mutation(async ({ ctx, input }) => {
             try {
+                await verifyProjectAccess(ctx.db, ctx.user.id, input.projectId);
                 if (!env.FIRECRAWL_API_KEY) {
                     throw new Error('FIRECRAWL_API_KEY is not configured');
                 }
@@ -349,8 +350,9 @@ export const projectRouter = createTRPCRouter({
         .input(z.object({ id: z.string() }))
         .mutation(async ({ ctx, input }) => {
             await ctx.db.transaction(async (tx) => {
-                await tx.delete(projects).where(eq(projects.id, input.id));
+                await verifyProjectAccess(tx, ctx.user.id, input.id);
                 await tx.delete(userProjects).where(eq(userProjects.projectId, input.id));
+                await tx.delete(projects).where(eq(projects.id, input.id));
             });
         }),
     getPreviewProjects: protectedProcedure
@@ -365,6 +367,7 @@ export const projectRouter = createTRPCRouter({
             return projects.map((project) => fromDbProject(project.project));
         }),
     update: protectedProcedure.input(projectUpdateSchema).mutation(async ({ ctx, input }) => {
+        await verifyProjectAccess(ctx.db, ctx.user.id, input.id);
         const [updatedProject] = await ctx.db.update(projects).set({
             ...input,
             updatedAt: new Date(),
@@ -380,6 +383,7 @@ export const projectRouter = createTRPCRouter({
         projectId: z.string(),
         tag: z.string(),
     })).mutation(async ({ ctx, input }) => {
+        await verifyProjectAccess(ctx.db, ctx.user.id, input.projectId);
         const project = await ctx.db.query.projects.findFirst({
             where: eq(projects.id, input.projectId),
         });
@@ -404,6 +408,7 @@ export const projectRouter = createTRPCRouter({
         projectId: z.string(),
         tag: z.string(),
     })).mutation(async ({ ctx, input }) => {
+        await verifyProjectAccess(ctx.db, ctx.user.id, input.projectId);
         const project = await ctx.db.query.projects.findFirst({
             where: eq(projects.id, input.projectId),
         });