lib/httpserver: adds flagsAuthKey command-line flag (#2758)

* lib/httpserver: adds flagsAuthKey command-line flag
It protects /flags endpoint with authKey.

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2753O

* Apply suggestions from code review

Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>

app/vmauth/README.md

@@ -140,7 +140,11 @@ Do not transfer Basic Auth headers in plaintext over untrusted networks. Enable
 
 Alternatively, [https termination proxy](https://en.wikipedia.org/wiki/TLS_termination_proxy) may be put in front of `vmauth`.
 
-It is recommended protecting `/-/reload` endpoint with `-reloadAuthKey` command-line flag, so external users couldn't trigger config reload.
+It is recommended protecting  following endpoints with authKeys:
+* `/-/reload` with `-reloadAuthKey` command-line flag, so external users couldn't trigger config reload.
+* `/flags` with `-flagsAuthkey` command-line flag, so unauthorized users couldn't get application command-line flags.
+* `/metrics` with `metricsAuthkey` command-line flag, so unauthorized users couldn't get access to [vmauth metrics](#monitoring).
+* `/debug/pprof` with `pprofAuthKey` command-line flag, so unauthorized users couldn't get access to [profiling information](#profiling).
 
 ## Monitoring
 

docs/CHANGELOG.md

@@ -17,6 +17,8 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 **Update notes:** this release introduces backwards-incompatible changes to communication protocol between `vmselect` and `vmstorage` nodes in cluster version of VictoriaMetrics because of added [query tracing](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#query-tracing), so `vmselect` and `vmstorage` nodes may log communication errors during the upgrade. These errors should stop after all the `vmselect` and `vmstorage` nodes are updated to new release. It is safe to downgrade to previous releases.
 
+* SECURITY: Adds `flagsAuthKey` command-line flag, which protects `/flags` endpoint and prevents application configuration disclosure. See [This issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2753).
+
 * FEATURE: support query tracing, which allows determining bottlenecks during query processing. See [these docs](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#query-tracing) and [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1403).
 * FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): add `cardinality` tab, which can help identifying the source of [high cardinality](https://docs.victoriametrics.com/FAQ.html#what-is-high-cardinality) and [high churn rate](https://docs.victoriametrics.com/FAQ.html#what-is-high-churn-rate) issues. See [this](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2233) and [this](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2730) feature requests and [these docs](https://docs.victoriametrics.com/#cardinality-explorer).
 * FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): small UX enhancements according to [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2638).

docs/vmauth.md

@@ -144,7 +144,11 @@ Do not transfer Basic Auth headers in plaintext over untrusted networks. Enable
 
 Alternatively, [https termination proxy](https://en.wikipedia.org/wiki/TLS_termination_proxy) may be put in front of `vmauth`.
 
-It is recommended protecting `/-/reload` endpoint with `-reloadAuthKey` command-line flag, so external users couldn't trigger config reload.
+It is recommended protecting  following endpoints with authKeys:
+* `/-/reload` with `-reloadAuthKey` command-line flag, so external users couldn't trigger config reload.
+* `/flags` with `flagsAuthkey` command-line flag, so external users couldn't get application configuriton.
+* `/metrics` with `metricsAuthkey` command-line flag, so external users couldn't get application metrics.
+* `/debug/pprof` with `pprofAuthKey` command-line flag, so external users couldn't get access to application profiling information.
 
 ## Monitoring
 

lib/httpserver/httpserver.go

@@ -48,6 +48,8 @@ var (
 	shutdownDelay               = flag.Duration("http.shutdownDelay", 0, `Optional delay before http server shutdown. During this delay, the server returns non-OK responses from /health page, so load balancers can route new requests to other servers`)
 	idleConnTimeout             = flag.Duration("http.idleConnTimeout", time.Minute, "Timeout for incoming idle http connections")
 	connTimeout                 = flag.Duration("http.connTimeout", 2*time.Minute, `Incoming http connections are closed after the configured timeout. This may help to spread the incoming load among a cluster of services behind a load balancer. Please note that the real timeout may be bigger by up to 10% as a protection against the thundering herd problem`)
+
+	flagsAuthKey = flag.String("flagsAuthKey", "", "Auth key for /flags endpoint. It must be passed via authKey query arg. It overrides httpAuth.* settings")
 )
 
 var (
@@ -284,6 +286,10 @@ func handlerWrapper(s *server, w http.ResponseWriter, r *http.Request, rh Reques
 		metricsHandlerDuration.UpdateDuration(startTime)
 		return
 	case "/flags":
+		if len(*flagsAuthKey) > 0 && r.FormValue("authKey") != *flagsAuthKey {
+			http.Error(w, "The provided authKey doesn't match -flagsAuthKey", http.StatusUnauthorized)
+			return
+		}
 		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 		flagutil.WriteFlags(w)
 		return