CVE-2026-53965

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 running with a typical memory_limit. - Severity (suggested, maintainer to confirm): High — a remote server can reliably crash a connected client over the HTTP/SSE transport. How input reaches the sink (reachability) 1. A client connects to a server over the HTTP transport by constructing Mcp\Client\Transport\HttpTransport with the server endpoint URL, then runs the connect/request loop. 2. The transport's loop calls tick() (line 182), which calls processSSEStream() (line 194) on each iteration. 3. processSSEStream() reads up to 4096 bytes from the

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