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Vulnerability · Published Aug 25, 2026 · Assigned by GitHub_M

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

Medium 6.9

In plain English

A medium-severity uncontrolled resource consumption in php-sdk from modelcontextprotocol. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 50% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)6.9Medium
Exploit odds (30 days)0.6%top 50.4% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Packagistmcp/sdk0.7.1GHSA-7m52-jw36-44r3

Affected products (as reported)

VendorProductVersions
modelcontextprotocol php-sdk >= 0.5.0, < 0.7.1

Weakness: CWE-400, CWE-770 · 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

References