Description

File Browser Vulnerable to Username Enumeration via Timing Attack in /api/login

Summary

The JSONAuth.Auth function contains a logic flaw that allows unauthenticated attackers to enumerate valid usernames by measuring the response time of the /api/login endpoint.

Details

The vulnerability exists due to a "short-circuit" evaluation in the authentication logic. When a username is not found in the database, the function returns immediately. However, if the username does exist, the code proceeds to verify the password using bcrypt (users.CheckPwd), which is a computationally expensive operation designed to be slow.

This difference in execution path creates a measurable timing discrepancy:

Invalid User: ~1ms execution (Database lookup only). Valid User: ~50ms+ execution (Database lookup + Bcrypt hashing).

In auth/json.go:

// auth/json.go line 54
u, err := usr.Get(srv.Root, cred.Username)
// VULNERABILITY:
// If 'err != nil' (User not found), the OR condition short-circuits.
// The second part (!users.CheckPwd) is NEVER executed.
//
// If 'err == nil' (User found), the code MUST execute users.CheckPwd (Bcrypt).
if err != nil || !users.CheckPwd(cred.Password, u.Password) {...
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PoC

The following Python script automates the attack. It first calibrates the network latency using random (non-existent) users to establish a baseline/threshold, and then tests a list of target usernames. Valid users are detected when the response time exceeds the calculated threshold.

import requests
import time
import random
import string
import statistics
import argparse

CALIBRATION_SAMPLES = 20...
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For example, in this case, I have guchihacker as the only valid user in the application.

I am going to use the exploit to list valid users. As we can see, the user guchihacker has been confirmed as a valid user by comparing the server response time.

Impact

An unauthenticated remote attacker can enumerate valid usernames. This significantly weakens the security posture by facilitating targeted brute-force attacks or credential stuffing against specific, known-valid accounts (e.g., 'admin', 'root', employee names).

I remain at your disposal for any questions you may have on this matter. Thank you very much.

Sincerely, Felix Sanchez (GUCHI)

Mitigation

Aliases

References