seccon ep 2023 ctf4b workshop web1 problem4

chall4.ts

import { ChallRes } from "./types.ts";
import { getFlag } from "./flags.ts";
import { Form, FormFile } from "https://deno.land/x/[email protected]/mod.ts";
import { join } from "https://deno.land/[email protected]/path/mod.ts";

const getFilename = (image: FormFile | FormFile[]) => {
  if (Array.isArray(image)) {
    // Case for FormFile[]
    return image.map((file) => file.filename);
  } else {
    // Case for FormFile
    return [image.filename];
  }
};

const doNotPathTraversal = (filename: string[]) => {
  const myReplace = (str: string) => {
    while (str.includes("../")) {
      str = str.replaceAll("../", "");
    }
    return str;
  };
  return filename.map((file) => myReplace(file));
};

export function chall4(req: Form): ChallRes {
  const filename = getFilename(req.files.image);
  const safeFilename = doNotPathTraversal(filename);
  const savePath = join("public", "images", ...safeFilename);

  // TODO: implement file saving
  // Deno.writeFileSync(savePath, req.files.image.content);

  if (savePath === "../etc/passwd") {
    return { flag: getFlag("chall4"), message: "okay...nice hacking!" };
  } else {
    return { error: `OK! savePath -> ${savePath}` };
  }
}