From f44db693f8bc3e4566c69204d74bb89ce5895c66 Mon Sep 17 00:00:00 2001
From: Matt Kane <m@mk.gg>
Date: Wed, 5 Oct 2022 14:07:09 +0100
Subject: [PATCH] feat: add prefix filtering and loop detection for local
 images (#83)

* feat: add prefix filtering for local images

* fix: prevent infinite loop

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
---
 example/netlify/functions/ipx.ts |  1 +
 package.json                     |  2 +-
 src/index.ts                     | 60 +++++++++++++++++++++++++++++---
 3 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/example/netlify/functions/ipx.ts b/example/netlify/functions/ipx.ts
index ff10e2c..4d691e4 100644
--- a/example/netlify/functions/ipx.ts
+++ b/example/netlify/functions/ipx.ts
@@ -10,6 +10,7 @@ export const handler = createIPXHandler({
   domains: [
     'www.netlify.com'
   ],
+  localPrefix: '/img/',
   basePath: '/.netlify/builders/ipx/',
   responseHeaders: {
     'Strict-Transport-Security': 'max-age=31536000',
diff --git a/package.json b/package.json
index 18cba1e..aab5228 100644
--- a/package.json
+++ b/package.json
@@ -14,7 +14,7 @@
     "prepack": "yarn build",
     "lint": "yarn eslint --ext .ts,.js,.mjs src",
     "test": "ava",
-    "dev": "cd example && netlify dev"
+    "dev": "netlify dev"
   },
   "dependencies": {
     "@netlify/functions": "^1.3.0",
diff --git a/src/index.ts b/src/index.ts
index cfcbd32..57fb956 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -7,14 +7,41 @@ import etag from 'etag'
 import { loadSourceImage } from './http'
 import { decodeBase64Params, doPatternsMatchUrl, RemotePattern } from './utils'
 export interface IPXHandlerOptions extends Partial<IPXOptions> {
+  /**
+   * Path to cache directory
+   * @default os.tmpdir() /ipx-cache
+   */
   cacheDir?: string
+  /**
+   * Base path for IPX requests
+   * @default /_ipx/
+   */
   basePath?: string
   propsEncoding?: 'base64' | undefined
+  /**
+   * Bypass domain check for remote images
+   */
   bypassDomainCheck?: boolean
+  /**
+   * Restrict local image access to a specific prefix
+   */
+  localPrefix?: string
+  /**
+   * Patterns used to verify remote image URLs
+   */
   remotePatterns?: RemotePattern[]
+  /**
+   * Add custom headers to response
+   */
   responseHeaders?: Record<string, string>
 }
 
+const SUBREQUEST_HEADER = 'x-ipx-subrequest'
+
+const plainText = {
+  'Content-Type': 'text/plain'
+}
+
 export function createIPXHandler ({
   cacheDir = join(tmpdir(), 'ipx-cache'),
   basePath = '/_ipx/',
@@ -22,13 +49,26 @@ export function createIPXHandler ({
   bypassDomainCheck,
   remotePatterns,
   responseHeaders,
+  localPrefix,
   ...opts
 }: IPXHandlerOptions = {}) {
   const ipx = createIPX({ ...opts, dir: join(cacheDir, 'cache') })
   if (!basePath.endsWith('/')) {
     basePath = `${basePath}/`
   }
+  if (localPrefix && !localPrefix.startsWith('/')) {
+    localPrefix = `/${localPrefix}`
+  }
   const handler: Handler = async (event, _context) => {
+    if (event.headers[SUBREQUEST_HEADER]) {
+      // eslint-disable-next-line no-console
+      console.error('Source image loop detected')
+      return {
+        statusCode: 400,
+        body: 'Source image loop detected',
+        headers: plainText
+      }
+    }
     let domains = (opts as IPXOptions).domains || []
     const remoteURLPatterns = remotePatterns || []
     const requestEtag = event.headers['if-none-match']
@@ -43,18 +83,28 @@ export function createIPXHandler ({
       if (params.error) {
         return {
           statusCode: 400,
-          body: params.error
+          body: params.error,
+          headers: plainText
         }
       }
       id = params.id
       modifiers = params.modifiers
     }
 
-    const requestHeaders: Record<string, string> = {}
+    const requestHeaders: Record<string, string> = {
+      [SUBREQUEST_HEADER]: '1'
+    }
     const isLocal = !id.startsWith('http://') && !id.startsWith('https://')
     if (isLocal) {
       const url = new URL(event.rawUrl)
       url.pathname = id
+      if (localPrefix && !url.pathname.startsWith(localPrefix)) {
+        return {
+          statusCode: 400,
+          body: 'Invalid source image path',
+          headers: plainText
+        }
+      }
       id = url.toString()
       if (event.headers.cookie) {
         requestHeaders.cookie = event.headers.cookie
@@ -70,7 +120,8 @@ export function createIPXHandler ({
       if (!parsedUrl.host) {
         return {
           statusCode: 403,
-          body: 'Hostname is missing: ' + id
+          body: 'Hostname is missing: ' + id,
+          headers: plainText
         }
       }
 
@@ -107,7 +158,8 @@ export function createIPXHandler ({
           `)
           return {
             statusCode: 403,
-            body: 'URL not on allowlist: ' + id
+            body: 'URL not on allowlist: ' + id,
+            headers: plainText
           }
         }
       }