GHSA-xqp3-jq6g-x3qm File Browser: Authentication Bypass via Proxy Auth Header Forgery
SummaryWhen 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 EnabledThis 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_ldapIn 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 checksCloud deployments may have misconfigured security groups or load balancersInternal networks often lack strict micro-segmentationThe 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 CauseThe `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 ValidationThe 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:**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 VerificationUnlike 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:**Problem 3: Automatic User CreationIf 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`](https://github.com/filebrowser/filebrowser/blob/main/auth/proxy.go), lines 30-63:**This auto-creation has no opt-in flag — it is always active when proxy auth is enabled.Complete Attack FlowProof of ConceptHere 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"`PrerequisitesFileBrowser with proxy auth enabled:Server is reachable directly (not exclusively behind the reverse proxy)Step 1: Confirm attacker (non-admin) is blocked**Result:** `403 Forbidden` — non-admin users cannot access `/api/settings`Step 2: Forge admin identity — no credentials needed**Result:** Valid JWT token returned for admin user (ID: 1, `perm.admin: true`)Step 3: Access admin-only endpoints with forged token**Result:** `200 OK` - complete server settings returned:Step 4: Enumerate all user accounts**Result:** All user accounts with full details (usernames, permissions, scopes, commands)Step 5: Impersonate any other user**Result:** Full file listing of testuser's scopeStep 6: Auto-create a new user account**Result:** New user `backdoor_account` created in the database with default permissions, JWT returnedValidated ResultsTested against `filebrowser/filebrowser:latest` Docker image on 2026-03-09:| Test | Result |
|------|--------|
| Attacker token (non-admin) -> `GET /api/settings` | **`403 Forbidden`** (blocked) |
| Forged header `X-Remote-User: admin` -> `POST /api/login` | **`200 OK`** — valid admin JWT (608 bytes) |
| Forged admin token -> `GET /api/settings` | **`200 OK`** — full server config returned |
| Forged admin token -> `GET /api/users` | **`200 OK`** — all user accounts listed |
| Forged header `X-Remote-User: testuser` | **`200 OK`** — testuser JWT, files accessible |
| Forged header `X-Remote-User: nonexistent_user` | **`200 OK`** — new user auto-created, JWT returned |ImpactAn unauthenticated attacker who can reach the FileBrowser instance directly can:**Full admin takeover** — impersonate the admin user and gain complete control**Read all server settings** — shell configuration, permissions, branding, rules**Enumerate and impersonate all users** — access every user's files without credentials**Create unlimited backdoor accounts** — auto-creation generates persistent accounts**Modify server configuration** — enable command execution, change shell, alter rules**Chain with other vulnerabilities** — gain admin access -> enable shell mode -> achieve RCE**Attack cost:** Zero credentials. One HTTP header.Suggested RemediationFix 1: Add trusted proxy IP validation (recommended)Fix 2: Make auto-user-creation opt-inAdd a configuration flag `auth.proxy.createUsers` (default: `false`) so administrators must explicitly enable automatic account creation.Fix 3: Documentation warningClearly document that when using proxy auth:FileBrowser **MUST NOT** be directly accessible from untrusted networksBind to `127.0.0.1` or use firewall rules to ensure only the reverse proxy can reach itThe reverse proxy **MUST** strip/overwrite the configured header from client requestsReferences**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
CVSS v3.1 score: 9.1