Tracking: CVE-2026-53965
Severity: Medium (CVSS 4.0: 6.9)
Who is affected
Composer package mcp/sdk from 0.5.0 through 0.7.0, when used as an HTTP client consuming Server-Sent Events (SSE). The PHP client must connect to a malicious or compromised server; an on-path attacker may also qualify where transport protection permits.
How the attack works
The client accumulates incoming bytes while waiting for the SSE event delimiter, two consecutive newline characters. A peer that keeps sending data without completing an event can grow the buffer without a suitable limit.
The affected process can run out of memory and terminate. This is a client-side availability issue, even though the triggering input comes from an MCP server.
What to do
- Upgrade to 0.7.1 or later and restart long-running clients.
- Restrict which MCP servers clients may connect to and use appropriate TLS verification.
- Review client memory growth and repeated failures around unusually long event streams. Apply runtime resource limits as containment while patching.
Scope and limits
A server built with the PHP SDK is not automatically affected simply because it serves MCP. Confirm that the vulnerable HTTP client path is in use.