CVE-2026-53965

Received Received - Intake

Memory Exhaustion in MCP PHP SDK via Unbounded SSE Buffer

Publication date: 2026-08-25

Last updated on: 2026-08-25

Assigner: GitHub, Inc.

Description

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.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-08-25

Last Modified

2026-08-25

Generated

2026-08-26

AI Q&A

2026-08-26

EPSS Evaluated

N/A

NVD

CVE-2026-53965

Affected Vendors & Products

Currently, no data is known.

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-400	The product does not properly control the allocation and maintenance of a limited resource.
CWE-770	The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

Attack-Flow Graph

Executive Summary

The MCP PHP SDK (mcp/sdk) versions 0.5.0 to 0.7.0 have a vulnerability where the HTTP client reads Server-Sent Events responses without a memory limit. A malicious server can send endless data without proper delimiters, causing the client's memory buffer to grow indefinitely. This leads to memory exhaustion, crashing the client process or triggering system-level out-of-memory kills.

Detection Guidance

This vulnerability can be detected by monitoring memory usage of PHP processes using MCP SDK versions 0.5.0 to 0.7.0. Check for unbounded memory growth when connecting to MCP servers. Use commands like 'ps aux | grep php' to monitor PHP processes and 'top -p <PID>' to observe memory consumption. If memory usage grows indefinitely without server responses, the vulnerability may be exploited.

Impact Analysis

If you use the vulnerable MCP PHP SDK (0.5.0-0.7.0) with an untrusted MCP server, a compromised server or man-in-the-middle attacker could crash your application by consuming all available memory. This results in denial of service for the MCP client process, potentially disrupting services relying on it.

Mitigation Strategies

Immediately upgrade the MCP PHP SDK to version 0.7.1 or later. Update your Composer dependencies with 'composer require mcp/sdk:^0.7.1'. If upgrading is not possible, restrict connections to trusted MCP servers only. Monitor network traffic for unusual server responses that may indicate exploitation attempts.

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