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Merged
koic merged 1 commit into from
Nov 7, 2026
Merged

Scrub the header and body values echoed into error responses #671

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21 changes: 15 additions & 6 deletions lib/mcp/server/transports/streamable_http_transport.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -905,7 +905,7 @@ def handle_modern(request, header_version, body_string: nil)
message: "Unsupported protocol version",
data: {
supported: MCP::Configuration::SUPPORTED_MODERN_PROTOCOL_VERSIONS,
requested: header_version,
requested: printable_value(header_version),
},
id: body[:id],
)
Expand Down Expand Up @@ -983,7 +983,7 @@ def validate_modern_headers(request, body, header_version)
meta_version = params.is_a?(Hash) && params[:_meta].is_a?(Hash) ? params.dig(:_meta, :"io.modelcontextprotocol/protocolVersion") : nil
if meta_version && meta_version != header_version
return header_mismatch_response(
"MCP-Protocol-Version header value '#{header_version}' does not match body value '#{meta_version}'",
"MCP-Protocol-Version header value '#{printable_value(header_version)}' does not match body value '#{printable_value(meta_version)}'",
body[:id],
)
end
Expand All @@ -1002,7 +1002,7 @@ def validate_modern_headers(request, body, header_version)
end
if method_header != body[:method]
return header_mismatch_response(
"Mcp-Method header value '#{method_header}' does not match body value '#{body[:method]}'",
"Mcp-Method header value '#{printable_value(method_header)}' does not match body value '#{printable_value(body[:method])}'",
body[:id],
)
end
Expand All @@ -1013,15 +1013,15 @@ def validate_modern_headers(request, body, header_version)
if body_name
if name_header.to_s.empty?
return header_mismatch_response(
"Mcp-Name header is required for `#{body[:method]}`",
"Mcp-Name header is required for `#{printable_value(body[:method])}`",
body[:id],
)
end

decoded_name = decode_header_value(name_header)
if decoded_name != body_name
return header_mismatch_response(
"Mcp-Name header value '#{decoded_name}' does not match body value '#{body_name}'",
"Mcp-Name header value '#{printable_value(decoded_name)}' does not match body value '#{printable_value(body_name)}'",
body[:id],
)
end
Expand Down Expand Up @@ -1571,6 +1571,15 @@ def write_listen_closing_result(subscription)
end
end

# A client-supplied value for an error message or `data` member. A header value reaches the transport as
# ASCII-8BIT from a Rack server, or tagged UTF-8 from a test request or a framework, and a body value comes
# out of the JSON parser; a value holding bytes that are not valid UTF-8 would make the response's `to_json`
# raise and turn the intended 400 into a 500 with an exception report. Such bytes are replaced with U+FFFD;
# a valid value comes through unchanged.
def printable_value(value)
value.to_s.dup.force_encoding(Encoding::UTF_8).scrub
end

def header_mismatch_response(message, id)
json_rpc_error_response(
status: 400,
Expand Down Expand Up @@ -2055,7 +2064,7 @@ def validate_protocol_version_header(request)
json_rpc_error_response(
status: 400,
code: JsonRpcHandler::ErrorCode::INVALID_REQUEST,
message: "Bad Request: Unsupported protocol version: #{header_value}. Supported versions: #{supported}",
message: "Bad Request: Unsupported protocol version: #{printable_value(header_value)}. Supported versions: #{supported}",
)
end

Expand Down
84 changes: 84 additions & 0 deletions test/mcp/server/transports/streamable_http_transport_test.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -6327,6 +6327,90 @@ def string
assert_equal "2027-01-01", body.dig("error", "data", "requested")
end

test "modern POST answers 400 with -32022 when the requested version is not valid UTF-8" do
# The echoed `requested` member is scrubbed, where `to_json` on the raw value raised and the rescue answered 500 with an exception report.
reported = []
original_reporter = MCP.configuration.exception_reporter
MCP.configuration.exception_reporter = ->(error, _context) { reported << error }
version = "2026-07-28\xFF".dup.force_encoding(Encoding::UTF_8)

response = @transport.handle_request(modern_rack_request(modern_body("ping", {}), version: version))

assert_equal 400, response[0]
body = JSON.parse(response[2][0])
assert_equal(-32022, body.dig("error", "code"))
assert_equal "2026-07-28\uFFFD", body.dig("error", "data", "requested")
assert_empty reported
ensure
MCP.configuration.exception_reporter = original_reporter
end

test "modern POST answers 400 with -32020 when a mirror header that mismatches is not valid UTF-8" do
reported = []
original_reporter = MCP.configuration.exception_reporter
MCP.configuration.exception_reporter = ->(error, _context) { reported << error }
invalid_method = "tools/call\xFF".dup.force_encoding(Encoding::UTF_8)
invalid_name = "=?base64?#{["file:///\xFF".b].pack("m0")}?="

by_method = @transport.handle_request(modern_rack_request(
modern_body("ping", {}),
headers: { "HTTP_MCP_METHOD" => invalid_method },
))
by_name = @transport.handle_request(modern_rack_request(
modern_body("resources/read", { uri: "file:///a.txt" }),
headers: { "HTTP_MCP_NAME" => invalid_name },
))

[by_method, by_name].each do |response|
assert_equal 400, response[0]
body = JSON.parse(response[2][0])
assert_equal(-32020, body.dig("error", "code"))
assert_includes body.dig("error", "message"), "\uFFFD"
end
assert_empty reported
ensure
MCP.configuration.exception_reporter = original_reporter
end

test "modern POST answers 400 with -32020 when the body's envelope version is not valid UTF-8" do
# The body side of the mismatch message comes out of the JSON parser, which passes such bytes through.
reported = []
original_reporter = MCP.configuration.exception_reporter
MCP.configuration.exception_reporter = ->(error, _context) { reported << error }
body_string = modern_body("ping", {}, version: "2026-07-28X").b.sub("2026-07-28X", "2026-07-28\xFF".b)

response = @transport.handle_request(modern_rack_request(body_string))

assert_equal 400, response[0]
body = JSON.parse(response[2][0])
assert_equal(-32020, body.dig("error", "code"))
assert_includes body.dig("error", "message"), "\uFFFD"
assert_empty reported
ensure
MCP.configuration.exception_reporter = original_reporter
end

test "modern POST echoes a valid non-ASCII mirror header given as bytes in its -32020 message" do
# A Rack server hands the header over as ASCII-8BIT; json 3.1 refuses to serialize such a string
# when it holds bytes above 0x7F, valid UTF-8 or not, so the mismatch answered 500 there.
reported = []
original_reporter = MCP.configuration.exception_reporter
MCP.configuration.exception_reporter = ->(error, _context) { reported << error }

response = @transport.handle_request(modern_rack_request(
modern_body("ping", {}),
headers: { "HTTP_MCP_METHOD" => "p\xC3\xAFng".b },
))

assert_equal 400, response[0]
body = JSON.parse(response[2][0])
assert_equal(-32020, body.dig("error", "code"))
assert_includes body.dig("error", "message"), "p\u00EFng"
assert_empty reported
ensure
MCP.configuration.exception_reporter = original_reporter
end

test "modern POST with a header/body version mismatch returns 400 with -32020" do
# Header names the supported modern version; the body envelope claims another.
response = @transport.handle_request(modern_rack_request(
Expand Down
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