Validate message endpoint in SSE client transport

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

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

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+package io.modelcontextprotocol.client.transport;
+
+import java.net.URI;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.NullSource;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.assertj.core.api.InstanceOfAssertFactories.type;
+
+/**
+ * Tests for {@link DefaultSseMessageEndpointValidator}.
+ *
+ * @author Daniel Garnier-Moiroux
+ */
+class DefaultSseMessageEndpointValidatorTests {
+
+	private static final URI SSE_URI = URI.create("https://mcp.example.com/sse");
+
+	private final DefaultSseMessageEndpointValidator validator = new DefaultSseMessageEndpointValidator();
+
+	@ParameterizedTest
+	@ValueSource(strings = { "/messages", "messages?session=abc", "/" })
+	void valid(String endpoint) {
+		assertThatCode(() -> validator.validate(SSE_URI, endpoint)).doesNotThrowAnyException();
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "", " ", "\t" })
+	@NullSource
+	void invalidEmpty(String endpoint) {
+		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).isInstanceOf(IllegalArgumentException.class)
+			.hasMessageContaining("messageEndpoint must not be empty");
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "/foo/../bar", "/foo/./bar", "../bar", "./bar", "/foo/%2E%2E/bar", "/foo/%2e/bar" })
+	void invalidPathTraversal(String endpoint) {
+		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).hasMessageContaining("path-traversal")
+			.asInstanceOf(type(InvalidSseMessageEndpointException.class))
+			.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
+			.isEqualTo(endpoint);
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "https://mcp.example.com/messages", "https://127.0.0.1/messages",
+			"https://mcp.example.com:8443/messages", "http://localhost:1234/messages", "file:///etc/passwd",
+			"gopher://mcp.example.com/_test" })
+	void invalidAbsoluteUris(String endpoint) {
+		// Even an absolute URI on the same origin must be rejected: the contract
+		// is that the messageEndpoint is a path-only relative reference.
+		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).hasMessageContaining("must be a relative path")
+			.asInstanceOf(type(InvalidSseMessageEndpointException.class))
+			.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
+			.isEqualTo(endpoint);
+
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "//example/messages", "//user:secret@example/messages" })
+	void invalidNetworkReference(String endpoint) {
+		// `//host/...` introduces an authority and is therefore not a pure path.
+		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
+			.hasMessageContaining("must not contain an authority")
+			.asInstanceOf(type(InvalidSseMessageEndpointException.class))
+			.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
+			.isEqualTo(endpoint);
+	}
+
+}