CVE-2026-53965
MediumCVSS 6.9Exploitation Probability (EPSS)
Low risk29th percentile - higher than 29% of all known CVEs
Summary
MCP PHP SDK versions 0.5.0 through 0.7.0 have a vulnerability to unbounded buffer growth in the HTTP transport when reading a Server-Sent Events stream. A malicious MCP server can send data without delimiters, causing client process memory exhaustion.
Risk Assessment
A remote, compromised, or man-in-the-middle-controlled server can crash the MCP client, causing denial of service.
Recommendation
Update the mcp/sdk package to version 0.7.1 or later.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

