## Summary When FileBrowser is configured with proxy authentication (`auth.method=proxy`), any unauthenticated attacker who can reach the server directly can impersonate **any user - including admin** - by sending a single forged HTTP header. No credentials are required. Additionally, specifying a non-existent username causes the server to **automatically create a new user account**, providing an account creation primitive with no authorization. **This is an already known issue that has been documented in the documentation for several years, but has not been documented as a vulnerability before.** ## Severity **HIGH** - CVSS 3.1: **8.1** (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N) ## Affected Component - **File:** [`auth/proxy.go`](https://github.com/filebrowser/filebrowser/blob/main/auth/proxy.go), lines 21-28 - **CWE:** [CWE-287](https://cwe.mitre.org/data/definitions/287.html) (Improper Authentication), [CWE-290](https://cwe.mitre.org/data/definitions/290.html) (Authentication Bypass by Spoofing) - **Affected versions:** All versions supporting `auth.method=proxy` ## Prerequisite: Proxy Auth Must Be Enabled This vulnerability is **NOT exploitable on default configuration** (`auth.method=json`). It requires the administrator to have configured proxy authentication mode. However, this is a **common production deployment pattern** - many organizations run FileBrowser behind a reverse proxy that handles SSO/LDAP/OAuth authentication: - **nginx** + Authelia / Authentik - **Traefik** + OAuth2 Proxy - **Caddy** + forward_auth - **Apache** + mod_auth_ldap In these setups, the proxy authenticates the user and passes the username via HTTP header (e.g., `X-Remote-User`). FileBrowser trusts this header to identify the user. | Deployment Scenario | Exploitable? | |---|---| | Default install (`auth.method=json`) | **No** — JSON auth uses password verification | | `auth.method=proxy` + FileBrowser only reachable via proxy (bound to `127.0.0.1` or firewalled) | **No** - attacker cannot reach the server directly | | `auth.method=proxy` + FileBrowser port exposed to network | **Yes - full admin takeover** | 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`](https://github.com/filebrowser/filebrowser/blob/main/auth/proxy.go), lines 21-28:** ```go 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`](https://github.com/filebrowser/filebrowser/blob/main/http/auth.go), lines 121-137:** ```go 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

AI Analysis