✍️ Description

Hi, with PsiTransfer we can upload files and protect them with a password. However, there is an IDOR that let an attacker retrieve arbitrary files and get the AES encrypted data of these files. All is left is to perform an offline bruteforce to crack the password of this file and get the associated metadata of this file and to get the direct link like /files/sid++UUIDv4

If you protect a file with a password and you want to access it using : http://localhost:3000/2507a65aac91 you have to enter the correct password associated with the file in order to access the metada containing the direct link http://localhost:3000/files/2507a65aac91++dff0ccf6-4bef-4ab5-aef1-3e5da03f8926

Under the hood, the client is sending a request to retrieve http://localhost:3000/2507a65aac91.json which is publically accessible as long as we guessed the 6 bytes sid. Then an attacker can download the file and easily bruteforce the password.

In https://github.com/psi-4ward/psitransfer/blob/c53c6ad0edb0cf36de87b9d13ef6d8d4ba260bd7/lib/endpoints.js#L155

app.get(`${ config.baseUrl }:sid`, (req, res, next) => {
  if (req.url.endsWith('.json')) {
    const sid = req.params.sid.substr(0, req.params.sid.length - 5);
    /**/
    res.json({
      items: db.get(sid).map(data => {
        const item = Object.assign(data, { url: `${ config.baseUrl }files/${ sid }++${ data.key }` });
        if (item.metadata.password) {
          return AES.encrypt(JSON.stringify(data), item.metadata.password).toString();// IDOR
        } else {
          return item;
        }
      }),
     /**/
    });
  } /**/
});

🕵️‍♂️ Proof of Concept

http://localhost:3000/2507a65aac91 require a password

http://localhost:3000/2507a65aac91.json doesn't require a password, offline bruteforce is possible

💥 Impact

Attackers could have the ability to crack the passwords of the protected files and retrieve the direct URL of the protected files and download them.

Mitigation

You should perform the decryption in the server side, that way, users can't access the encrypted data of other users.