CVE-2026-78663: Go Standard Library Net/http

Critical severity, CVSS 9.1. EPSS: 0.6% chance of exploitation in the next 30 days.

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.

Affected products

• Go Standard Library Net/http: before 1.26.9 (fixed in 1.26.9)
• Go Standard Library net/http/internal/http2: from 1.27.0-0, before 1.27.2 (fixed in 1.27.2)
• Golang.org/x/net golang.org/x/net/http2: before 0.60.0 (fixed in 0.60.0)

Published 2026-10-08. Last modified 2026-10-09.