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Validate message endpoint in SSE client transport
Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>
1 parent 9520323 commit 2256c26

6 files changed

Lines changed: 269 additions & 10 deletions

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‎mcp-core/src/main/java/io/modelcontextprotocol/client/transport/DefaultSseMessageEndpointValidator.java‎

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/*
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* Copyright 2026-2026 the original author or authors.
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*/
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package io.modelcontextprotocol.client.transport;
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import java.net.URI;
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import java.net.URISyntaxException;
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import io.modelcontextprotocol.util.Assert;
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/**
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* Default {@link SseMessageEndpointValidator} that validates the {@code message} endpoint
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* advertised by an SSE server. Message endpoints must be a relative URI, without path
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* traversal or authority.
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*
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* @author Daniel Garnier-Moiroux
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*/
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public final class DefaultSseMessageEndpointValidator implements SseMessageEndpointValidator {
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@Override
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public void validate(URI sseUri, String messageEndpoint) throws InvalidSseMessageEndpointException {
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Assert.hasText(messageEndpoint, "messageEndpoint must not be empty");
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URI endpointUri;
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try {
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endpointUri = new URI(messageEndpoint);
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}
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catch (URISyntaxException ex) {
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throw new InvalidSseMessageEndpointException("messageEndpoint is not a valid URI: " + ex.getMessage(),
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messageEndpoint);
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}
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if (endpointUri.isAbsolute()) {
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// Exclude absolute URIs e.g. https://example.com/mcp
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throw new InvalidSseMessageEndpointException("messageEndpoint must be a relative path, not an absolute URI",
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messageEndpoint);
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}
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if (endpointUri.getRawAuthority() != null) {
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// Exclude network paths e.g. //example.com/mcp
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throw new InvalidSseMessageEndpointException(
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"messageEndpoint must be a relative path and must not contain an authority", messageEndpoint);
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}
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// Exclude path-traversal
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String decodedPath = endpointUri.getPath();
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if (decodedPath != null) {
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for (String segment : decodedPath.split("/", -1)) {
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if (".".equals(segment) || "..".equals(segment)) {
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throw new InvalidSseMessageEndpointException(
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"messageEndpoint must not contain path-traversal segments", messageEndpoint);
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}
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}
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}
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}
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}

‎mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java‎

Lines changed: 36 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -16,8 +16,6 @@
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import java.util.function.Consumer;
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import java.util.function.Function;
1818

19-
import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import io.modelcontextprotocol.client.transport.ResponseSubscribers.ResponseEvent;
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import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
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import io.modelcontextprotocol.client.transport.customizer.McpSyncHttpClientRequestCustomizer;
@@ -33,6 +31,8 @@
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import io.modelcontextprotocol.spec.ProtocolVersions;
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import io.modelcontextprotocol.util.Assert;
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import io.modelcontextprotocol.util.Utils;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import reactor.core.Disposable;
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import reactor.core.publisher.Flux;
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import reactor.core.publisher.Mono;
@@ -117,6 +117,11 @@ public class HttpClientSseClientTransport implements McpClientTransport {
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*/
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private final McpAsyncHttpClientRequestCustomizer httpRequestCustomizer;
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/**
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* Validator for the message endpoint;
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*/
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private final SseMessageEndpointValidator messageEndpointValidator;
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/**
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* Creates a new transport instance with custom HTTP client builder, object mapper,
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* and headers.
@@ -127,22 +132,26 @@ public class HttpClientSseClientTransport implements McpClientTransport {
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* @param jsonMapper the object mapper for JSON serialization/deserialization
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* @param httpRequestCustomizer customizer for the requestBuilder before executing
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* requests
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* @param messageEndpointValidator validator for the message endpoint
130136
* @throws IllegalArgumentException if objectMapper, clientBuilder, or headers is null
131137
*/
132138
HttpClientSseClientTransport(HttpClient httpClient, HttpRequest.Builder requestBuilder, String baseUri,
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String sseEndpoint, McpJsonMapper jsonMapper, McpAsyncHttpClientRequestCustomizer httpRequestCustomizer) {
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String sseEndpoint, McpJsonMapper jsonMapper, McpAsyncHttpClientRequestCustomizer httpRequestCustomizer,
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SseMessageEndpointValidator messageEndpointValidator) {
134141
Assert.notNull(jsonMapper, "jsonMapper must not be null");
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Assert.hasText(baseUri, "baseUri must not be empty");
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Assert.hasText(sseEndpoint, "sseEndpoint must not be empty");
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Assert.notNull(httpClient, "httpClient must not be null");
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Assert.notNull(requestBuilder, "requestBuilder must not be null");
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Assert.notNull(httpRequestCustomizer, "httpRequestCustomizer must not be null");
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Assert.notNull(messageEndpointValidator, "messageEndpointValidator must not be null");
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this.baseUri = URI.create(baseUri);
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this.sseEndpoint = sseEndpoint;
142150
this.jsonMapper = jsonMapper;
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this.httpClient = httpClient;
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this.requestBuilder = requestBuilder;
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this.httpRequestCustomizer = httpRequestCustomizer;
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this.messageEndpointValidator = messageEndpointValidator;
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}
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148157
@Override
@@ -178,6 +187,8 @@ public static class Builder {
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179188
private Duration connectTimeout = Duration.ofSeconds(10);
180189

190+
private SseMessageEndpointValidator messageEndpointValidator = new DefaultSseMessageEndpointValidator();
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181192
/**
182193
* Creates a new builder instance.
183194
*/
@@ -297,14 +308,27 @@ public Builder connectTimeout(Duration connectTimeout) {
297308
return this;
298309
}
299310

311+
/**
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* Sets the validator that ensure the message endpoint returned over the SSE
313+
* connection is valid.
314+
* @param messageEndpointValidator the validator
315+
* @return this builder
316+
*/
317+
public Builder messageEndpointValidator(SseMessageEndpointValidator messageEndpointValidator) {
318+
Assert.notNull(messageEndpointValidator, "messageEndpointValidator must not be null");
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this.messageEndpointValidator = messageEndpointValidator;
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return this;
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}
322+
300323
/**
301324
* Builds a new {@link HttpClientSseClientTransport} instance.
302325
* @return a new transport instance
303326
*/
304327
public HttpClientSseClientTransport build() {
305328
HttpClient httpClient = this.clientBuilder.connectTimeout(this.connectTimeout).build();
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return new HttpClientSseClientTransport(httpClient, requestBuilder, baseUri, sseEndpoint,
307-
jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, httpRequestCustomizer);
330+
jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, httpRequestCustomizer,
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messageEndpointValidator);
308332
}
309333

310334
}
@@ -342,6 +366,14 @@ public Mono<Void> connect(Function<Mono<JSONRPCMessage>, Mono<JSONRPCMessage>> h
342366
try {
343367
if (ENDPOINT_EVENT_TYPE.equals(responseEvent.sseEvent().event())) {
344368
String messageEndpointUri = responseEvent.sseEvent().data();
369+
try {
370+
messageEndpointValidator.validate(uri, messageEndpointUri);
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}
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catch (InvalidSseMessageEndpointException e) {
373+
sink.error(e);
374+
this.messageEndpointSink.tryEmitError(e);
375+
return Flux.error(e);
376+
}
345377
if (this.messageEndpointSink.tryEmitValue(messageEndpointUri).isSuccess()) {
346378
sink.success();
347379
return Flux.empty(); // No further processing needed

‎mcp-core/src/main/java/io/modelcontextprotocol/client/transport/InvalidSseMessageEndpointException.java‎

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/*
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* Copyright 2026-2026 the original author or authors.
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*/
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package io.modelcontextprotocol.client.transport;
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/**
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* Exception thrown when the {@code message} endpoint returned from the SSE connection is
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* not valid.
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*
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* @author Daniel Garnier-Moiroux
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*/
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public class InvalidSseMessageEndpointException extends Exception {
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private final String messageEndpoint;
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public InvalidSseMessageEndpointException(String message, String messageEndpoint) {
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super(message);
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this.messageEndpoint = messageEndpoint;
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}
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public String getMessageEndpoint() {
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return messageEndpoint;
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}
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}

‎mcp-core/src/main/java/io/modelcontextprotocol/client/transport/SseMessageEndpointValidator.java‎

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,27 @@
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/*
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* Copyright 2026-2026 the original author or authors.
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*/
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package io.modelcontextprotocol.client.transport;
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import java.net.URI;
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/**
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* Validate the that message endpoint in the SSE transport is valid. Throws
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* {@link InvalidSseMessageEndpointException} when then endpoint is not valid.
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*
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* @author Daniel Garnier-Moiroux
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*/
15+
@FunctionalInterface
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public interface SseMessageEndpointValidator {
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/**
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* Validate the message endpoint coming from an SSE connection. Throws if not valid.
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* @param sseUri the URI used to establish the SSE connection
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* @param messageEndpoint the message endpoint from the SSE connection
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* @throws InvalidSseMessageEndpointException error thrown if the message endpoint is
23+
* not valid.
24+
*/
25+
void validate(URI sseUri, String messageEndpoint) throws InvalidSseMessageEndpointException;
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27+
}

‎mcp-core/src/test/java/io/modelcontextprotocol/client/transport/DefaultSseMessageEndpointValidatorTests.java‎

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1+
/*
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* Copyright 2026-2026 the original author or authors.
3+
*/
4+
package io.modelcontextprotocol.client.transport;
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import java.net.URI;
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8+
import org.junit.jupiter.params.ParameterizedTest;
9+
import org.junit.jupiter.params.provider.NullSource;
10+
import org.junit.jupiter.params.provider.ValueSource;
11+
12+
import static org.assertj.core.api.Assertions.assertThatCode;
13+
import static org.assertj.core.api.Assertions.assertThatThrownBy;
14+
import static org.assertj.core.api.InstanceOfAssertFactories.type;
15+
16+
/**
17+
* Tests for {@link DefaultSseMessageEndpointValidator}.
18+
*
19+
* @author Daniel Garnier-Moiroux
20+
*/
21+
class DefaultSseMessageEndpointValidatorTests {
22+
23+
private static final URI SSE_URI = URI.create("https://mcp.example.com/sse");
24+
25+
private final DefaultSseMessageEndpointValidator validator = new DefaultSseMessageEndpointValidator();
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27+
@ParameterizedTest
28+
@ValueSource(strings = { "/messages", "messages?session=abc", "/" })
29+
void valid(String endpoint) {
30+
assertThatCode(() -> validator.validate(SSE_URI, endpoint)).doesNotThrowAnyException();
31+
}
32+
33+
@ParameterizedTest
34+
@ValueSource(strings = { "", " ", "\t" })
35+
@NullSource
36+
void invalidEmpty(String endpoint) {
37+
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).isInstanceOf(IllegalArgumentException.class)
38+
.hasMessageContaining("messageEndpoint must not be empty");
39+
}
40+
41+
@ParameterizedTest
42+
@ValueSource(strings = { "/foo/../bar", "/foo/./bar", "../bar", "./bar", "/foo/%2E%2E/bar", "/foo/%2e/bar" })
43+
void invalidPathTraversal(String endpoint) {
44+
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).hasMessageContaining("path-traversal")
45+
.asInstanceOf(type(InvalidSseMessageEndpointException.class))
46+
.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
47+
.isEqualTo(endpoint);
48+
}
49+
50+
@ParameterizedTest
51+
@ValueSource(strings = { "https://mcp.example.com/messages", "https://127.0.0.1/messages",
52+
"https://mcp.example.com:8443/messages", "http://localhost:1234/messages", "file:///etc/passwd",
53+
"gopher://mcp.example.com/_test" })
54+
void invalidAbsoluteUris(String endpoint) {
55+
// Even an absolute URI on the same origin must be rejected: the contract
56+
// is that the messageEndpoint is a path-only relative reference.
57+
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).hasMessageContaining("must be a relative path")
58+
.asInstanceOf(type(InvalidSseMessageEndpointException.class))
59+
.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
60+
.isEqualTo(endpoint);
61+
62+
}
63+
64+
@ParameterizedTest
65+
@ValueSource(strings = { "//example/messages", "//user:secret@example/messages" })
66+
void invalidNetworkReference(String endpoint) {
67+
// `//host/...` introduces an authority and is therefore not a pure path.
68+
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
69+
.hasMessageContaining("must not contain an authority")
70+
.asInstanceOf(type(InvalidSseMessageEndpointException.class))
71+
.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
72+
.isEqualTo(endpoint);
73+
}
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}

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