Client HttpTransport SSE buffer grows unbounded when server withholds the event delimiter

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 or inject into that server’s SSE response body (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 note

The source advisory suggests High (a remote server can reliably crash a connected client). Shopmon currently publishes this as Low via enrichment override.

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

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 active SSE stream and appends them to $this->sseBuffer (line 203).
4. The buffer is only drained inside the while (false !== ($pos = strpos($this->sseBuffer, "\n\n"))) loop (line 207). If the server never emits "\n\n", strpos never matches, the buffer is never flushed, and it grows on every tick() until OOM.

203php private string $sseBuffer = '';
   php private function processSSEStream(): void {
     if (null === $this->activeStream) { return; }
     if (!$this->activeStream->eof()) {
       $chunk = $this->activeStream->read(4096);
       if ('' !== $chunk) {
         $this->sseBuffer .= $chunk; // unbounded append
       }
     }
207  while (false !== ($pos = strpos($this->sseBuffer, "\n\n"))) {
       $event = substr($this->sseBuffer, 0, $pos);
       $this->sseBuffer = substr($this->sseBuffer, $pos + 2);
       if (!empty(trim($event))) { $this->processSSEEvent($event); }
     }
     if ($this->activeStream->eof() && empty($this->sseBuffer)) {
       $this->activeStream = null;
     }
   }

Proof of concept

End-to-end reproduction against the released Composer package.

Environment

macOS arm64, PHP 8.5.6 (cli), Composer 2.9.8. The package under test is the published release mcp/sdk v0.5.0 (the version that introduced this HTTP client transport), installed from Packagist — not a re-packaged tree.

Full reproduction steps are in the upstream GHSA. Shopmon has not added a shop-specific PoC.