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DefaultSseMessageEndpointValidator allows same-origin message endpoints #946

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Merged
Kehrlann merged 1 commit into from
May 4, 2026
+25 −21
Merged

DefaultSseMessageEndpointValidator allows same-origin message endpoints #946

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25 changes: 14 additions & 11 deletions ...ain/java/io/modelcontextprotocol/client/transport/DefaultSseMessageEndpointValidator.java
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Original file line number Diff line number Diff line change
Expand Up @@ -10,8 +10,8 @@

/**
* Default {@link SseMessageEndpointValidator} that validates the {@code message} endpoint
* advertised by an SSE server. Message endpoints must be a relative URI, without path
* traversal or authority.
* advertised by an SSE server. Message endpoints must either have the same origin as the
* SSE uri, or be a relative uri.
*
* @author Daniel Garnier-Moiroux
*/
Expand All @@ -30,16 +30,19 @@ public void validate(URI sseUri, String messageEndpoint) throws InvalidSseMessag
messageEndpoint);
}

if (endpointUri.isAbsolute()) {
// Exclude absolute URIs e.g. https://example.com/mcp
throw new InvalidSseMessageEndpointException("messageEndpoint must be a relative path, not an absolute URI",
messageEndpoint);
}
if (endpointUri.isAbsolute() || endpointUri.getRawAuthority() != null) {
String scheme = endpointUri.getScheme();
String host = endpointUri.getHost();
int port = endpointUri.getPort();

if (endpointUri.getRawAuthority() != null) {
// Exclude network paths e.g. //example.com/mcp
throw new InvalidSseMessageEndpointException(
"messageEndpoint must be a relative path and must not contain an authority", messageEndpoint);
boolean sameScheme = scheme != null && scheme.equalsIgnoreCase(sseUri.getScheme());
boolean sameHost = host != null && host.equalsIgnoreCase(sseUri.getHost());
boolean samePort = port == sseUri.getPort();

if (!sameScheme || !sameHost || !samePort) {
throw new InvalidSseMessageEndpointException(
"messageEndpoint must be a relative path or a same-origin URI", messageEndpoint);
}
}

// Exclude path-traversal
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21 changes: 11 additions & 10 deletions ...ava/io/modelcontextprotocol/client/transport/DefaultSseMessageEndpointValidatorTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,7 @@ class DefaultSseMessageEndpointValidatorTests {
private final DefaultSseMessageEndpointValidator validator = new DefaultSseMessageEndpointValidator();

@ParameterizedTest
@ValueSource(strings = { "/messages", "messages?session=abc", "/" })
@ValueSource(strings = { "/messages", "messages?session=abc", "/", "https://mcp.example.com/messages" })
void valid(String endpoint) {
assertThatCode(() -> validator.validate(SSE_URI, endpoint)).doesNotThrowAnyException();
}
Expand All @@ -41,32 +41,33 @@ void invalidEmpty(String endpoint) {
@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")
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
.hasMessageContaining("must not contain path-traversal segments")
.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" })
@ValueSource(strings = { "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")
// Absolute URIs must be same-origin.
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
.hasMessageContaining("must be a relative path or a same-origin URI")
.asInstanceOf(type(InvalidSseMessageEndpointException.class))
.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
.isEqualTo(endpoint);

}

@ParameterizedTest
@ValueSource(strings = { "//example/messages", "//user:secret@example/messages" })
@ValueSource(strings = { "//example/messages", "//user:secret@example/messages", "//mcp.example.com/messages" })
void invalidNetworkReference(String endpoint) {
// `//host/...` introduces an authority and is therefore not a pure path.
// It is missing a scheme, so it fails same-origin check.
assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
.hasMessageContaining("must not contain an authority")
.hasMessageContaining("must be a relative path or a same-origin URI")
.asInstanceOf(type(InvalidSseMessageEndpointException.class))
.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
.isEqualTo(endpoint);
Expand Down
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