FileBrowser Quantum is a free, self-hosted, web-based file manager. Prior to version 1.3.2-beta, the `/api/auth/login` authentication endpoint does not execute in constant time. When a non-existent username is supplied, the server returns a `401`/`403` response almost immediately. When a valid username is provided, the server performs a bcrypt password comparison, causing a measurable delay in the response time. Version 1.3.2-beta patches the issue.

Affected: Go: github.com/gtsteffaniak/filebrowser/backend

### Summary

The `/api/auth/login` authentication endpoint does not execute in constant time. When a non-existent username is supplied, the server returns a `401`/`403` response almost immediately. When a valid username is provided, the server performs a bcrypt password comparison, causing a measurable delay in the response time.

### Details

The vulnerability exists in the `Auth` function of `JSONAuth` in `auth/json.go` (lines 45–52). The function performs a database lookup for the user prior to performing any password validation.

```go user, err := userStore.Get(username) if err != nil { return nil, fmt.Errorf("unable to get user from store: %v", err) } err = users.CheckPwd(password, user.Password) if err != nil { return nil, err } ```

If the username is not found, the function returns an error immediately. If the username is found, the function calls `CheckPwd`, which executes the bcrypt hash comparison. Because bcrypt is intentionally computationally expensive, this introduces a measurable delay in the response time.

As a result, an attacker can distinguish valid usernames from invalid ones by measuring the authentication response times.

In testing, responses for valid usernames consistently required approximately 40–50 ms due to the bcrypt comparison, while invalid usernames returned in approximately 1–4 ms.

### PoC

The script below automates this attack by calibrating the network latency using non-existent usernames to establish a baseline and then test…