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CVE-2026-53965— MCP PHP SDK: Unbounded SSE buffer in HttpTransport enables client-side denial of service

Quick assessment

Affected
modelcontextprotocol php-sdk
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

MCP PHP SDK（Composer 包 mcp/sdk）是 Model Context Protocol（模型上下文协议）的官方 PHP 软件开发工具包。在 0.5.0 至 0.7.0 版本中，HTTP 客户端传输机制会逐块读取服务器发送事件（Server-Sent Events, SSE）响应流，并将每个数据块追加到一个没有上限的内存缓冲区中。该缓冲区仅在检测到 SSE 事件分隔符（即双换行符）时才会被清空。因此，如果远程 MCP 服务器持续推送响应字节但永不发送分隔符，会导致内存缓冲区无限增长。 当客户端

CVSS 6.9 · Medium EPSS 0.36% · P29

Affected Version Matrix 1

VendorProduct Version RangeStatus
modelcontextprotocol php-sdk >= 0.5.0, < 0.7.1 affected
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I. Basic Information for CVE-2026-53965

Vulnerability Information
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Vulnerability Title
MCP PHP SDK: Unbounded SSE buffer in HttpTransport enables client-side denial of service
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
未加控制的资源消耗(资源穷尽)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
modelcontextprotocol php-sdk >= 0.5.0, < 0.7.1 -

II. Public POCs for CVE-2026-53965

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III. Intelligence Information for CVE-2026-53965

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Patches & Fixes for CVE-2026-53965 (1)

Vendor Advisories for CVE-2026-53965 (1)

Vendor Pages for CVE-2026-53965 (1)

IV. Related Vulnerabilities

Same vendor: modelcontextprotocol
Same weakness: CWE-400

V. Comments for CVE-2026-53965

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