Svelte Sveltekit Security

Svelte / SvelteKit Security Audit

Audit Svelte and SvelteKit apps. Covers Svelte 3/4/5 and SvelteKit 1/2.

When this skill applies

• Reviewing Svelte components for XSS sinks
• Auditing SvelteKit server load functions, form actions, +server.ts routes
• Reviewing hooks.server.ts middleware
• Checking $env module usage for accidental leaks
• Auditing form actions for auth and CSRF

Workflow

Follow ../_shared/audit-workflow.md.

Phase 1: Stack detection

grep -E '"(svelte|@sveltejs/kit)":' package.json
find . -name 'svelte.config.*' -name '*.svelte' -not -path '*/node_modules/*' | head

Phase 2: Inventory

# XSS sinks
grep -rn '@html' src/ 2>/dev/null

# Server load functions (run on server)
find src -name '+page.server.ts' -o -name '+layout.server.ts' 2>/dev/null

# Universal load (runs both server and client)
find src -name '+page.ts' -o -name '+layout.ts' 2>/dev/null

# Endpoint handlers
find src -name '+server.ts' 2>/dev/null

# Form actions
grep -rn 'export const actions' src/ 2>/dev/null

# Hooks (middleware)
find src -name 'hooks.server.ts' -o -name 'hooks.client.ts' 2>/dev/null

# Env modules
grep -rn '\$env/static/private\|\$env/static/public\|\$env/dynamic/private\|\$env/dynamic/public' src/ 2>/dev/null

Phase 3: Detection — the checks

{@html} XSS

• SVK-XSS-1 Every {@html foo} reviewed. Same sink as dangerouslySetInnerHTML. Sanitize with DOMPurify or use a safe Markdown renderer.

{@html post.content}

  import DOMPurify from 'isomorphic-dompurify';
  $: clean = DOMPurify.sanitize(post.content);

{@html clean}

Server vs universal load functions

+page.server.ts runs ONLY on server. +page.ts runs on server during SSR AND on client during navigation.

• SVK-LOAD-1 Code in +page.ts (universal load) must not import server-only modules (database, secret env). Use $env/static/private → will fail at build if a universal load imports it.
• SVK-LOAD-2 Data returned from server load serializes to the client during hydration. Don't return secrets in the load function's return object.
• SVK-LOAD-3 Universal load returning data fetched via authenticated API: the fetch on the client uses the user's cookies (good), but the server-side fetch during SSR uses no cookies by default — need to forward.

// +page.ts (universal)
export const load = async ({ fetch, params }) => {
  // SvelteKit's fetch forwards cookies during SSR — verify
  const res = await fetch(`/api/posts/${params.id}`);
  return { post: await res.json() };
};

+server.ts route handlers

API endpoints. Same scrutiny as any REST endpoint.

• SVK-API-1 Every handler checks auth. SvelteKit provides no implicit auth.
• SVK-API-2 Input validation via Zod / valibot on request body and URL params.
• SVK-API-3 CORS configured if endpoint is cross-origin reachable.
• SVK-API-4 No accidental data exposure — return only what caller is authorized to see.

// +server.ts
import { json, error } from '@sveltejs/kit';

export const GET = async ({ locals, params }) => {
  if (!locals.user) throw error(401);
  
  const item = await db.items.findFirst({
    where: { id: params.id, tenantId: locals.user.tenantId },
  });
  if (!item) throw error(404);
  
  return json(item);
};

Form actions

Form actions are POST endpoints generated from actions exports in +page.server.ts.

• SVK-FA-1 Every action checks auth.
• SVK-FA-2 Input validated; don't trust formData.get(...) directly.
• SVK-FA-3 Authz on the resource being mutated.
• SVK-FA-4 Actions return data that ships to the client — don't return secrets or other-user data.
• SVK-FA-5 SvelteKit has built-in CSRF protection (Origin header check); verify it's not disabled via csrf: false in svelte.config.js.

// +page.server.ts
import { fail, redirect } from '@sveltejs/kit';
import { z } from 'zod';

const DeleteSchema = z.object({ postId: z.string().uuid() });

export const actions = {
  delete: async ({ request, locals }) => {
    if (!locals.user) throw redirect(303, '/login');
    
    const formData = await request.formData();
    const parsed = DeleteSchema.safeParse({ postId: formData.get('postId') });
    if (!parsed.success) return fail(400, { error: 'Invalid input' });
    
    const post = await db.posts.findFirst({ where: { id: parsed.data.postId } });
    if (!post || post.authorId !== locals.user.id) throw error(403);
    
    await db.posts.delete({ where: { id: post.id } });
    return { success: true };
  },
};

hooks.server.ts

Runs before every request. Place for auth resolution + global headers.

• SVK-HK-1 Auth resolution in handle sets event.locals.user — every downstream load/action/route should use locals.user, not re-read cookies.
• SVK-HK-2 Global headers (CSP, HSTS, etc.) set here via event.setHeaders or response transformation.
• SVK-HK-3 Errors caught in handleError don't leak details in client response; log server-side.

$env module usage

SvelteKit splits env vars into 4 modules:

| Module | Where | Risk | |--------|-------|------| | $env/static/private | Server only, bundled at build | Safe | | $env/static/public | Both (must start with PUBLIC_) | Public, treat as ships to client | | $env/dynamic/private | Server only, at runtime | Safe | | $env/dynamic/public | Both (must start with PUBLIC_) | Public |

• SVK-ENV-1 Server-only secrets imported from $env/static/private or $env/dynamic/private. Never PUBLIC_*.
• SVK-ENV-2 Variables prefixed with PUBLIC_ (required for $env/*/public) are NOT secrets. Same trap as NEXT_PUBLIC_.
• SVK-ENV-3 SvelteKit refuses to bundle $env/static/private imports into client code — build will fail. So the prefix model is enforced. But check that no one tried PUBLIC_DATABASE_URL to "make it work".

Svelte 5 runes

Svelte 5 introduced runes ($state, $derived, $effect). Security implications:

• SVK-R5-1 $state synced to localStorage / sessionStorage doesn't store secrets.
• SVK-R5-2 $effect running on the client doesn't accidentally execute server-side intent (legacy code mixing Svelte 4 onMount with $effect).

Dependencies

• SVK-DEP-1 SvelteKit version current. SvelteKit 1.x had a CSRF disable issue (CVE-2023-44389 class); ensure 2.x or patched 1.x.
• SVK-DEP-2 Adapters (@sveltejs/adapter-node, @sveltejs/adapter-vercel, etc.) current.

Phase 4: Triage

Critical: {@html} with raw user input; form action without auth; data with secrets returned from load.

Phase 5: Report

Use ../_shared/findings-schema.md. Prefix IDs with SVK-.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: hlsitechio
• Source: hlsitechio/claude-skills-security
• License: MIT

Install and usage instructions live in the source repository linked above.