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[Server] Harden JSON-RPC input parsing against malformed input and resource exhaustion (#383)

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* fix: harden JSON-RPC input parsing against malformed input and DoS

MessageFactory::create() detected batches via $input[0] === '{', which
mishandled valid-but-non-object JSON: a scalar ("5") or leading-whitespace
object silently returned [] (with a PHP warning) or could reach a foreach
over a non-array. It also had no bound on batch size, letting one small
request expand into arbitrarily many operations (amplification).

- Detect single-vs-batch from the decoded JSON type (object vs list array)
  instead of the raw first byte; reject scalars and empty payloads as
  InvalidInputMessageException rather than crashing or no-oping.
- Cap batch size (maxBatchSize, default 100) and reject oversized batches.
- Reject non-object batch elements as InvalidInputMessageException instead
  of letting them hit a TypeError.
- Bound the StreamableHttpTransport POST body (maxBodyBytes, default 4 MiB),
  returning 413, with an incremental read so unknown-size/chunked bodies
  cannot exhaust memory.

* docs: document input-hardening limits in CHANGELOG and transports guide

Add CHANGELOG entries and docs/transports.md sections for the JSON-RPC
batch size cap (maxBatchSize) and the StreamableHttpTransport request
body size cap (maxBodyBytes), plus the type-based single-vs-batch
message detection.
This commit is contained in:
Antoine Bluchet 2026-07-06 00:10:13 +02:00 • committed by GitHub
commit df5379c047
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CHANGELOG.md
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@ -31,6 +31,9 @@ All notable changes to `mcp/sdk` will be documented in this file.
* [BC Break] `ResourceDefinition::__construct()` signature changed — `$title` parameter added between `$name` and `$description`. Callers using positional arguments must switch to named arguments.
* [BC Break] `ResourceTemplate::__construct()` signature changed — `$title` parameter added between `$name` and `$description`. Callers using positional arguments must switch to named arguments.
* [BC Break] `McpResource` and `McpResourceTemplate` attribute signatures changed — `$title` parameter added between `$name` and `$description`. Callers using positional arguments must switch to named arguments.
* Harden JSON-RPC input parsing: single-message vs batch is now decided from the decoded JSON type (object → single, list array → batch) instead of the raw first byte. Scalars, empty payloads, and non-object batch elements are surfaced as `InvalidInputMessageException` entries instead of triggering warnings or a `TypeError`.
* Add `maxBatchSize` (default `100`) to `MessageFactory` — oversized JSON-RPC batches are rejected before any message is constructed, guarding against amplification.
* Add `maxBodyBytes` (default 4 MiB) to `StreamableHttpTransport` — POST bodies exceeding the cap are rejected with `413`. Unknown-size/chunked bodies are read incrementally and stopped at the cap so they cannot exhaust memory.
0.5.0
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