CVE-2026-78663: net/http: net/http/internal/http2: golang: golang.org/x/net/http2: golang: flow control limit bypass via double refund in HTTP/2 server

Severity: Critical

CVSS Score: 9.1

The HTTP/2 server can refund connection-level flow control twice for the same data: Once when a client resets a stream (refunding data for any sent-but-unread portion of the stream), and again when a request handler reads the buffered data. A malicious client can exploit this to bypass the configured connection-level flow control limit (MaxReceiveBufferPerConnection). Total buffered data is still limited by the concurrent stream limit and stream-level flow control.