CVE-2026-53965

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

Published Aug 25, 2026 CVSS 4.0 · 6.9 Medium EPSS < 1% KEV · Not listed

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.

Analysis

Technical impact

The MCP PHP SDK’s HttpTransport reads Server‑Sent Events streams into an unbounded in‑memory buffer. Because the buffer is only flushed when a double‑newline delimiter is received, a server that streams data continuously without sending the delimiter causes the buffer to grow without limit. This leads to exhaustion of the client process’s memory, resulting in a fatal allocation error or an OS or kernel memory kill, and effectively denies service to the MCP client. The flaw is a classic resource‑exhaustion weakness, corresponding to CWE‑400 and CWE‑770.

Affected Systems

The vulnerability afflicts Model Context Protocol PHP SDK (Composer package mcp/sdk) versions 0.5.0 through 0.7.0. Any client that uses HttpTransport to connect to an untrusted MCP server is susceptible.

Risk and Exploitability

The CVSS score of 6.9 classifies the issue as moderate severity. No EPSS score is available, and the vulnerability is not listed in the CISA KEV catalog. An attacker can exploit the flaw by controlling the target MCP server or performing a man‑in‑the‑middle attack; no authentication or user interaction is required beyond initiating the HTTP connection. The attack vector is remote and relies solely on the network connection to the untrusted server.

Affected Products

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

modelcontextprotocol

php-sdk

affected

Version	Status	Constraints
`>= 0.5.0, < 0.7.1`	affected	—

No data.

No data available yet.

Remediation

No solution or workaround provided in the CVE record.

OpenCVE Recommended Actions

Apply the official patch to version 0.7.1
Ensure that HttpTransport is only used to connect to trusted, authenticated MCP endpoints
Disable or restrict the use of HttpTransport in environments that cannot guarantee server integrity

Generated by OpenCVE AI on August 26, 2026 at 02:20 UTC.

Tracking

Advisories

Source	ID	Title
Github GHSA	GHSA-7m52-jw36-44r3	MCP PHP SDK: client HttpTransport SSE buffer (sseBuffer .= chunk) grows unbounded when server withholds the event delimiter

Metrics

Attack Vector Network

Attack Complexity Low

Privileges Required None

Attack Requirements None

User Interaction Active

Vulnerable System Confidentiality Impact None

Vulnerable System Integrity Impact None

Vulnerable System Availability Impact High

Subsequent System Confidentiality Impact None

Subsequent System Integrity Impact None

Subsequent System Availability Impact None

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.615% probability within 30 days)

Exploitation poc

Automatable no

Technical Impact partial

References

Link	Providers
https://github.com/modelcontextprotocol/php-sdk/commit/055593f2592e1d7f5f48587c999ca7068a3be90d
https://github.com/modelcontextprotocol/php-sdk/releases/tag/v0.7.1
https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3

History

Aug 26, 2026 4:30 PM Scoring metrics updated View changes

Type	Values Removed	Values Added
Metrics		ssvc `{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}`

Aug 25, 2026 9:00 PM Description updated. Title updated. Weaknesses updated. Added references. Scoring metrics updated View changes

Type	Values Removed	Values Added
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.
Title		MCP PHP SDK: Unbounded SSE buffer in HttpTransport enables client-side denial of service
Weaknesses		CWE-400 CWE-770
References		https://github.com/modelcontextprotocol/php-sdk/commit/055593f2592e1d7f5f48587c999ca7068a3be90d https://github.com/modelcontextprotocol/php-sdk/releases/tag/v0.7.1 https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3
Metrics		cvssV4_0 `{'score': 6.9, 'vector': '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'}`