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CRITICAL

CVE-2026-78663

CVSS v3

9.1

CRITICAL

EPSS Score

—

exploit probability

CISA KEV

No

known exploited

Exploitation

—

SSVC status

Description

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.

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Published
2026-10-08
Last Modified
2026-10-09

Frequently asked questions

What is 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.

Is CVE-2026-78663 actively exploited?

Active exploitation of CVE-2026-78663 has not been confirmed.

What is the CVSS score for CVE-2026-78663?

CVE-2026-78663 has a CVSS v3 base score of 9.1 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.

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