CVE-2026-53965

Name of the Vulnerable Software and Affected Versions mcp/sdk versions 0.5.0 through 0.7.0

Description The HTTP client transport in the MCP PHP SDK reads a Server-Sent Events (SSE) response stream incrementally and appends data to an in-memory buffer sseBuffer without an upper bound. The buffer is only flushed when an SSE event delimiter (a double newline) is encountered. A malicious or compromised server can stream data without sending this delimiter, causing the buffer to grow indefinitely. This leads to memory exhaustion, triggering a fatal allocation error or an OS out-of-memory kill, resulting in a denial of service for the MCP client. This issue affects any client using HttpTransport against an untrusted server endpoint and requires no authentication or user interaction beyond initiating the connection. The vulnerability is located in the processSSEStream() function within the HttpTransport class.

Recommendations Update mcp/sdk to version 0.7.1. As a temporary mitigation, restrict the use of HttpTransport when connecting to untrusted server endpoints.