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fix: Fix issue where configured non-loopback addresses become invalid due to DNS rebinding protection. #44

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fix: Fix issue where configured non-loopback addresses become invalid due to DNS rebinding protection. #44

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26 changes: 17 additions & 9 deletions src/main/kotlin/net/portswigger/mcp/KtorServerManager.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,9 @@ class KtorServerManager(private val api: MontoyaApi) : ServerManager {
install(CORS) {
allowHost("localhost:${config.port}")
allowHost("127.0.0.1:${config.port}")
if (config.host.lowercase() !in setOf("localhost", "127.0.0.1")) {
allowHost("${config.host}:${config.port}")
}

allowMethod(HttpMethod.Get)
allowMethod(HttpMethod.Post)
Expand All @@ -64,7 +67,7 @@ class KtorServerManager(private val api: MontoyaApi) : ServerManager {
val referer = call.request.header("Referer")
val userAgent = call.request.header("User-Agent")

if (origin != null && !isValidOrigin(origin)) {
if (origin != null && !isValidOrigin(origin, config.host)) {
api.logging().logToOutput("Blocked DNS rebinding attack from origin: $origin")
call.respond(HttpStatusCode.Forbidden)
return@intercept
Expand All @@ -74,13 +77,13 @@ class KtorServerManager(private val api: MontoyaApi) : ServerManager {
return@intercept
}

if (host != null && !isValidHost(host, config.port)) {
if (host != null && !isValidHost(host, config.port, config.host)) {
api.logging().logToOutput("Blocked DNS rebinding attack from host: $host")
call.respond(HttpStatusCode.Forbidden)
return@intercept
}

if (referer != null && !isValidReferer(referer)) {
if (referer != null && !isValidReferer(referer, config.host)) {
api.logging().logToOutput("Blocked suspicious request from referer: $referer")
call.respond(HttpStatusCode.Forbidden)
return@intercept
Expand Down Expand Up @@ -135,12 +138,13 @@ class KtorServerManager(private val api: MontoyaApi) : ServerManager {
executor.awaitTermination(10, TimeUnit.SECONDS)
}

private fun isValidOrigin(origin: String): Boolean {
private fun isValidOrigin(origin: String, configuredHost: String): Boolean {
try {
val url = URI(origin).toURL()
val hostname = url.host.lowercase()

val allowedHosts = setOf("localhost", "127.0.0.1")
val allowedHosts = mutableSetOf("localhost", "127.0.0.1")
allowedHosts.add(configuredHost.lowercase())

return hostname in allowedHosts
} catch (_: Exception) {
Expand All @@ -159,13 +163,15 @@ class KtorServerManager(private val api: MontoyaApi) : ServerManager {
return browserIndicators.any { userAgentLower.contains(it) }
}

private fun isValidHost(host: String, expectedPort: Int): Boolean {
private fun isValidHost(host: String, expectedPort: Int, configuredHost: String): Boolean {
try {
val parts = host.split(":")
val hostname = parts[0].lowercase()
val port = if (parts.size > 1) parts[1].toIntOrNull() else null

val allowedHosts = setOf("localhost", "127.0.0.1")
val allowedHosts = mutableSetOf("localhost", "127.0.0.1")
allowedHosts.add(configuredHost.lowercase())

if (hostname !in allowedHosts) {
return false
}
Expand All @@ -180,12 +186,14 @@ class KtorServerManager(private val api: MontoyaApi) : ServerManager {
}
}

private fun isValidReferer(referer: String): Boolean {
private fun isValidReferer(referer: String, configuredHost: String): Boolean {
try {
val url = URI(referer).toURL()
val hostname = url.host.lowercase()

val allowedHosts = setOf("localhost", "127.0.0.1")
val allowedHosts = mutableSetOf("localhost", "127.0.0.1")
allowedHosts.add(configuredHost.lowercase())

return hostname in allowedHosts

} catch (_: Exception) {
Expand Down