The MCP PHP SDK (Composer package mcp/sdk) is the official Model Context Protocol SDK for PHP. In versions 0.5.0 through 0.7.0, the HTTP client transport reads a Server-Sent Events response stream incrementally and appends each chunk to an in-memory buffer with no upper bound. The buffer is only flushed when an SSE event delimiter, a double newline, is found, so a remote MCP server that streams response bytes without ever sending the delimiter causes the buffer to grow without limit. A malicious, compromised, or man-in-the-middle-controlled server that the client connects to over the HTTP transport can exploit this to exhaust the client process's memory, triggering a fatal allocation error or OS out-of-memory kill and denying service to the MCP client. The issue affects any client using HttpTransport against an untrusted server endpoint and does not require authentication or user interaction beyond initiating the connection. This issue is fixed in version 0.7.1.

Affected: Packagist: mcp/sdk

## Summary

The HTTP client transport in `mcp/sdk` reads a Server-Sent-Events (SSE) response stream incrementally and appends each 4 KiB chunk to an in-memory buffer (`$this->sseBuffer .= $chunk;`) with **no upper bound**. The buffer is only ever flushed when an SSE event delimiter (`"\n\n"`) appears. A remote MCP server (the peer the client connects to) that streams response bytes without ever sending the `"\n\n"` delimiter makes `$sseBuffer` grow without limit until the client process exhausts its PHP `memory_limit` (fatal "Allowed memory size … exhausted") or is killed by the OS OOM-killer.

This is a denial-of-service against the MCP **client**: any server it talks to — or a network position that controls the server's response body — can crash the client by withholding the event delimiter while streaming data.

## Impact

- **Type:** Denial of service (memory exhaustion / process crash) of the MCP client. - **Who can trigger it:** The remote MCP server endpoint the client connects to via `HttpTransport`, or any party that can control/inject into that server's SSE response body (e.g. a man-in-the-middle on a plaintext endpoint, or a malicious or compromised server). The buffer growth happens while the transport is reading the response stream, before a complete event is ever parsed. - **Effect:** A response stream of N bytes containing no `"\n\n"` drives the client's resident buffer to track N. A few hundred MB of delimiter-free data is enough to kill a client…