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Add support for DNS rebinding protections #284

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9758c23
Add support for DNS rebinding protections
ddworken May 30, 2025
7d44740
Extract DNS rebinding validation logic to separate methods in Spring …
ddworken Jun 27, 2025
145b592
Add javadocs
ddworken Jun 27, 2025
47e9791
Merge branch 'main' into main
ddworken Jul 14, 2025
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93 changes: 91 additions & 2 deletions ...main/java/io/modelcontextprotocol/server/transport/WebFluxSseServerTransportProvider.java
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,11 @@ public class WebFluxSseServerTransportProvider implements McpServerTransportProv
*/
private volatile boolean isClosing = false;

/**
* DNS rebinding protection configuration.
*/
private final DnsRebindingProtection dnsRebindingProtection;

/**
* Constructs a new WebFlux SSE server transport provider instance with the default
* SSE endpoint.
Expand All @@ -117,8 +122,10 @@ public class WebFluxSseServerTransportProvider implements McpServerTransportProv
* @param messageEndpoint The endpoint URI where clients should send their JSON-RPC
* messages. This endpoint will be communicated to clients during SSE connection
* setup. Must not be null.
* @deprecated Use {@link #builder()} instead.
* @throws IllegalArgumentException if either parameter is null
*/
@Deprecated
public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String messageEndpoint) {
this(objectMapper, messageEndpoint, DEFAULT_SSE_ENDPOINT);
}
Expand All @@ -130,10 +137,12 @@ public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String messa
* @param messageEndpoint The endpoint URI where clients should send their JSON-RPC
* messages. This endpoint will be communicated to clients during SSE connection
* setup. Must not be null.
* @deprecated Use {@link #builder()} instead.
* @throws IllegalArgumentException if either parameter is null
*/
@Deprecated
public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String messageEndpoint, String sseEndpoint) {
this(objectMapper, DEFAULT_BASE_URL, messageEndpoint, sseEndpoint);
this(objectMapper, DEFAULT_BASE_URL, messageEndpoint, sseEndpoint, null);
}

/**
Expand All @@ -144,10 +153,31 @@ public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String messa
* @param messageEndpoint The endpoint URI where clients should send their JSON-RPC
* messages. This endpoint will be communicated to clients during SSE connection
* setup. Must not be null.
* @deprecated Use {@link #builder()} instead.
* @throws IllegalArgumentException if either parameter is null
*/
@Deprecated
public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String baseUrl, String messageEndpoint,
String sseEndpoint) {
this(objectMapper, baseUrl, messageEndpoint, sseEndpoint, null);
}

/**
* Constructs a new WebFlux SSE server transport provider instance with optional DNS
* rebinding protection.
* @param objectMapper The ObjectMapper to use for JSON serialization/deserialization
* of MCP messages. Must not be null.
* @param baseUrl webflux message base path
* @param messageEndpoint The endpoint URI where clients should send their JSON-RPC
* messages. This endpoint will be communicated to clients during SSE connection
* setup. Must not be null.
* @param sseEndpoint The endpoint URI where clients establish their SSE connections.
* @param dnsRebindingProtection The DNS rebinding protection configuration (may be
* null).
* @throws IllegalArgumentException if required parameters are null
*/
private WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String baseUrl, String messageEndpoint,
String sseEndpoint, DnsRebindingProtection dnsRebindingProtection) {
Assert.notNull(objectMapper, "ObjectMapper must not be null");
Assert.notNull(baseUrl, "Message base path must not be null");
Assert.notNull(messageEndpoint, "Message endpoint must not be null");
Expand All @@ -157,6 +187,7 @@ public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String baseU
this.baseUrl = baseUrl;
this.messageEndpoint = messageEndpoint;
this.sseEndpoint = sseEndpoint;
this.dnsRebindingProtection = dnsRebindingProtection;
this.routerFunction = RouterFunctions.route()
.GET(this.sseEndpoint, this::handleSseConnection)
.POST(this.messageEndpoint, this::handleMessage)
Expand Down Expand Up @@ -255,6 +286,12 @@ private Mono<ServerResponse> handleSseConnection(ServerRequest request) {
return ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE).bodyValue("Server is shutting down");
}

// Validate headers
Mono<ServerResponse> validationError = validateDnsRebindingProtection(request);
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@chemicL chemicL Aug 13, 2025 •
edited
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It's not a valid style to return a null Mono reference. If there's a synchronous action that does not block, simply return a boolean to drive the behaviour.

  • boolean passesDnsRebindingProtectionCheck(ServerRequest request)
  • Mono<ServerResponse failDnsRebindingProtection()
if (!passesDnsRebindingProtectionCheck(request)) {
    return failDnsRebindingProtection();
}

I think the style you're using in this PR is inspired by errors from Go. The style present in the codebase of this project should be consistent.

if (validationError != null) {
return validationError;
}

return ServerResponse.ok()
.contentType(MediaType.TEXT_EVENT_STREAM)
.body(Flux.<ServerSentEvent<?>>create(sink -> {
Expand Down Expand Up @@ -299,6 +336,19 @@ private Mono<ServerResponse> handleMessage(ServerRequest request) {
return ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE).bodyValue("Server is shutting down");
}

// Always validate Content-Type for POST requests
String contentType = request.headers().contentType().map(MediaType::toString).orElse(null);
if (contentType == null || !contentType.toLowerCase().startsWith("application/json")) {
logger.warn("Invalid Content-Type header: '{}'", contentType);
return ServerResponse.badRequest().bodyValue(new McpError("Content-Type must be application/json"));
}

// Validate headers for POST requests if DNS rebinding protection is configured
Mono<ServerResponse> validationError = validateDnsRebindingProtection(request);
if (validationError != null) {
return validationError;
}

if (request.queryParam("sessionId").isEmpty()) {
return ServerResponse.badRequest().bodyValue(new McpError("Session ID missing in message endpoint"));
}
Expand Down Expand Up @@ -396,6 +446,8 @@ public static class Builder {

private String sseEndpoint = DEFAULT_SSE_ENDPOINT;

private DnsRebindingProtection dnsRebindingProtection;

/**
* Sets the ObjectMapper to use for JSON serialization/deserialization of MCP
* messages.
Expand Down Expand Up @@ -446,6 +498,22 @@ public Builder sseEndpoint(String sseEndpoint) {
return this;
}

/**
* Sets the DNS rebinding protection configuration.
* <p>
* When set, this configuration will be used to create a header validator that
* enforces DNS rebinding protection rules. This will override any previously set
* header validator.
* @param config The DNS rebinding protection configuration
* @return this builder instance
* @throws IllegalArgumentException if config is null
*/
public Builder dnsRebindingProtection(DnsRebindingProtection config) {
Assert.notNull(config, "DNS rebinding protection config must not be null");
this.dnsRebindingProtection = config;
return this;
}

/**
* Builds a new instance of {@link WebFluxSseServerTransportProvider} with the
* configured settings.
Expand All @@ -456,9 +524,30 @@ public WebFluxSseServerTransportProvider build() {
Assert.notNull(objectMapper, "ObjectMapper must be set");
Assert.notNull(messageEndpoint, "Message endpoint must be set");

return new WebFluxSseServerTransportProvider(objectMapper, baseUrl, messageEndpoint, sseEndpoint);
return new WebFluxSseServerTransportProvider(objectMapper, baseUrl, messageEndpoint, sseEndpoint,
dnsRebindingProtection);
}

}

/**
* Validates DNS rebinding protection for the given request.
* @param request The incoming server request
* @return A ServerResponse with forbidden status if validation fails, or null if
* validation passes
*/
private Mono<ServerResponse> validateDnsRebindingProtection(ServerRequest request) {
if (dnsRebindingProtection != null) {
String hostHeader = request.headers().asHttpHeaders().getFirst("Host");
String originHeader = request.headers().asHttpHeaders().getFirst("Origin");
if (!dnsRebindingProtection.isValid(hostHeader, originHeader)) {
logger.warn("DNS rebinding protection validation failed - Host: '{}', Origin: '{}'", hostHeader,
originHeader);
return ServerResponse.status(HttpStatus.FORBIDDEN)
.bodyValue("DNS rebinding protection validation failed");
}
}
return null;
}

}
72 changes: 71 additions & 1 deletion .../main/java/io/modelcontextprotocol/server/transport/WebMvcSseServerTransportProvider.java
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -106,6 +106,11 @@ public class WebMvcSseServerTransportProvider implements McpServerTransportProvi
*/
private volatile boolean isClosing = false;

/**
* DNS rebinding protection configuration.
*/
private final DnsRebindingProtection dnsRebindingProtection;

/**
* Constructs a new WebMvcSseServerTransportProvider instance with the default SSE
* endpoint.
Expand All @@ -114,8 +119,10 @@ public class WebMvcSseServerTransportProvider implements McpServerTransportProvi
* @param messageEndpoint The endpoint URI where clients should send their JSON-RPC
* messages via HTTP POST. This endpoint will be communicated to clients through the
* SSE connection's initial endpoint event.
* @deprecated Use {@link #builder()} instead.
* @throws IllegalArgumentException if either objectMapper or messageEndpoint is null
*/
@Deprecated
public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String messageEndpoint) {
this(objectMapper, messageEndpoint, DEFAULT_SSE_ENDPOINT);
}
Expand All @@ -128,10 +135,12 @@ public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String messag
* messages via HTTP POST. This endpoint will be communicated to clients through the
* SSE connection's initial endpoint event.
* @param sseEndpoint The endpoint URI where clients establish their SSE connections.
* @deprecated Use {@link #builder()} instead.
* @throws IllegalArgumentException if any parameter is null
*/
@Deprecated
public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String messageEndpoint, String sseEndpoint) {
this(objectMapper, "", messageEndpoint, sseEndpoint);
this(objectMapper, "", messageEndpoint, sseEndpoint, null);
}

/**
Expand All @@ -144,10 +153,32 @@ public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String messag
* messages via HTTP POST. This endpoint will be communicated to clients through the
* SSE connection's initial endpoint event.
* @param sseEndpoint The endpoint URI where clients establish their SSE connections.
* @deprecated Use {@link #builder()} instead.
* @throws IllegalArgumentException if any parameter is null
*/
@Deprecated
public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String baseUrl, String messageEndpoint,
String sseEndpoint) {
this(objectMapper, baseUrl, messageEndpoint, sseEndpoint, null);
}

/**
* Constructs a new WebMvcSseServerTransportProvider instance with DNS rebinding
* protection.
* @param objectMapper The ObjectMapper to use for JSON serialization/deserialization
* of messages.
* @param baseUrl The base URL for the message endpoint, used to construct the full
* endpoint URL for clients.
* @param messageEndpoint The endpoint URI where clients should send their JSON-RPC
* messages via HTTP POST. This endpoint will be communicated to clients through the
* SSE connection's initial endpoint event.
* @param sseEndpoint The endpoint URI where clients establish their SSE connections.
* @param dnsRebindingProtection The DNS rebinding protection configuration (may be
* null).
* @throws IllegalArgumentException if any required parameter is null
*/
private WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String baseUrl, String messageEndpoint,
String sseEndpoint, DnsRebindingProtection dnsRebindingProtection) {
Assert.notNull(objectMapper, "ObjectMapper must not be null");
Assert.notNull(baseUrl, "Message base URL must not be null");
Assert.notNull(messageEndpoint, "Message endpoint must not be null");
Expand All @@ -157,6 +188,7 @@ public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String baseUr
this.baseUrl = baseUrl;
this.messageEndpoint = messageEndpoint;
this.sseEndpoint = sseEndpoint;
this.dnsRebindingProtection = dnsRebindingProtection;
this.routerFunction = RouterFunctions.route()
.GET(this.sseEndpoint, this::handleSseConnection)
.POST(this.messageEndpoint, this::handleMessage)
Expand Down Expand Up @@ -246,6 +278,12 @@ private ServerResponse handleSseConnection(ServerRequest request) {
return ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE).body("Server is shutting down");
}

// Validate headers
ServerResponse validationError = validateDnsRebindingProtection(request);
if (validationError != null) {
return validationError;
}

String sessionId = UUID.randomUUID().toString();
logger.debug("Creating new SSE connection for session: {}", sessionId);

Expand Down Expand Up @@ -299,6 +337,19 @@ private ServerResponse handleMessage(ServerRequest request) {
return ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE).body("Server is shutting down");
}

// Always validate Content-Type for POST requests
String contentType = request.headers().asHttpHeaders().getFirst("Content-Type");
if (contentType == null || !contentType.toLowerCase().startsWith("application/json")) {
logger.warn("Invalid Content-Type header: '{}'", contentType);
return ServerResponse.badRequest().body(new McpError("Content-Type must be application/json"));
}

// Validate headers for POST requests if DNS rebinding protection is configured
ServerResponse validationError = validateDnsRebindingProtection(request);
if (validationError != null) {
return validationError;
}

if (request.param("sessionId").isEmpty()) {
return ServerResponse.badRequest().body(new McpError("Session ID missing in message endpoint"));
}
Expand Down Expand Up @@ -416,4 +467,23 @@ public void close() {

}

/**
* Validates DNS rebinding protection for the given request.
* @param request The incoming server request
* @return A ServerResponse with forbidden status if validation fails, or null if
* validation passes
*/
private ServerResponse validateDnsRebindingProtection(ServerRequest request) {
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I believe a similar strategy as in the HttpServletSseServerTransportProvider can be applied here (isValid + fail).

if (dnsRebindingProtection != null) {
String hostHeader = request.headers().asHttpHeaders().getFirst("Host");
String originHeader = request.headers().asHttpHeaders().getFirst("Origin");
if (!dnsRebindingProtection.isValid(hostHeader, originHeader)) {
logger.warn("DNS rebinding protection validation failed - Host: '{}', Origin: '{}'", hostHeader,
originHeader);
return ServerResponse.status(HttpStatus.FORBIDDEN).body("DNS rebinding protection validation failed");
}
}
return null;
}

}
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