What is CVE-2026-78663?

CVE-2026-78663 is a critical-severity vulnerability in Golang.org/x/net Golang.org/x/net/http2, classified under CWE-675. CVSS score: 9.1/10. Published 2026-10-08.

How severe is CVE-2026-78663?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Vulnerability in Golang.org/x/net Golang.org/x/net/http2

CVE-2026-78663

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.

Published 2026-10-08 · last modified 2026-10-09.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

Weakness classification (CWE)

CWE-675

References

Frequently asked questions

What is CVE-2026-78663?: CVE-2026-78663 is a critical-severity vulnerability in Golang.org/x/net Golang.org/x/net/http2, classified under CWE-675. CVSS score: 9.1/10. Published 2026-10-08.
How severe is CVE-2026-78663?: Critical severity. CVSS v3 base score is 9.1 out of 10.