fix(#259): validate host in pairing string to prevent SSRF (#344)

Add robust validation for hosts provided through URI or Base64 pairing
strings to prevent Server-Side Request Forgery (SSRF). The application
now rejects loopback addresses (like localhost, 127.0.0.1, 0.0.0.0),
link-local addresses, and any-local addresses, while continuing to allow
remote connections to site-local addresses, custom remote domains,
and VPN networks (like Tailscale). The DNS resolution operates securely
on the IO dispatcher to ensure main-thread safety. Included tests verify
both rejection of invalid hosts and seamless parsing of valid ones.

app/src/main/java/com/m57/hermescontrol/ui/connect/ConnectViewModel.kt

@@ -39,6 +39,22 @@ class ConnectViewModel(private val app: Application) : ViewModel() {
         loadSavedValues()
     }
 
+    private suspend fun isHostAllowed(host: String): Boolean =
+        withContext(Dispatchers.IO) {
+            if (host == "localhost" || host == "127.0.0.1" || host == "0.0.0.0") return@withContext false
+            if (host.startsWith("169.254.")) return@withContext false
+            if (host == "metadata.google.internal") return@withContext false
+            try {
+                val address = java.net.InetAddress.getByName(host)
+                if (address.isLoopbackAddress || address.isAnyLocalAddress || address.isLinkLocalAddress) {
+                    return@withContext false
+                }
+                true
+            } catch (e: Exception) {
+                false
+            }
+        }
+
     fun loadSavedValues() {
         val savedToken = AuthManager.getToken() ?: ""
         val savedHost = AuthManager.getHost()
@@ -267,15 +283,25 @@ class ConnectViewModel(private val app: Application) : ViewModel() {
                 val port = uri.getQueryParameter("port")
                 val token = uri.getQueryParameter("token")
                 if (host != null && port != null && token != null) {
-                    _uiState.update {
-                        it.copy(
-                            host = host,
-                            port = port,
-                            token = token,
-                            errorMessage = null,
-                        )
+                    viewModelScope.launch {
+                        if (!isHostAllowed(host)) {
+                            _uiState.update {
+                                it.copy(
+                                    errorMessage = app.getString(R.string.connect_error_invalid_host),
+                                )
+                            }
+                            return@launch
+                        }
+                        _uiState.update {
+                            it.copy(
+                                host = host,
+                                port = port,
+                                token = token,
+                                errorMessage = null,
+                            )
+                        }
+                        connect()
                     }
-                    connect()
                     return
                 }
             }
@@ -293,15 +319,25 @@ class ConnectViewModel(private val app: Application) : ViewModel() {
                     val port = json.get("port")?.asInt?.toString() ?: json.get("port")?.asString
                     val token = json.get("token")?.asString
                     if (host != null && port != null && token != null) {
-                        _uiState.update {
-                            it.copy(
-                                host = host,
-                                port = port,
-                                token = token,
-                                errorMessage = null,
-                            )
+                        viewModelScope.launch {
+                            if (!isHostAllowed(host)) {
+                                _uiState.update {
+                                    it.copy(
+                                        errorMessage = app.getString(R.string.connect_error_invalid_host),
+                                    )
+                                }
+                                return@launch
+                            }
+                            _uiState.update {
+                                it.copy(
+                                    host = host,
+                                    port = port,
+                                    token = token,
+                                    errorMessage = null,
+                                )
+                            }
+                            connect()
                         }
-                        connect()
                         return
                     }
                     // Decoded valid JSON but missing required fields