CVE-2026-78660: HTTP/2 transport accepts malformed framing-related headers in net/http

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-08 · updated 2026-10-09
Weakness: CWE-444

Description

Historically, we have been rather lax about malformed framing-related headers in our HTTP/2 implementation, as they cannot interfere with HTTP/2 framing. However, this makes it possible for our HTTP/2 implementation to forward responses containing such headers to an HTTP/1 client when acting as a reverse proxy. If the HTTP/1 client also does not behave strictly enough, this can result in response smuggling.

Affected products

Golang.Org/X/Net: before 0.60.0 — fixed in 0.60.0
Go Stdlib: before 1.26.9; from 1.27.0-0 before 1.27.2 — fixed in 1.26.9, 1.27.2
golang.org/x/net/http2: before 0.60.0 — fixed in 0.60.0
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

How to fix

Upgrade to a fixed version: Golang.Org/X/Net 0.60.0; Go Stdlib 1.26.9, 1.27.2; golang.org/x/net/http2 0.60.0; Go standard library net/http 1.26.9; Go standard library net/http/internal/http2 1.27.2. Check the vendor advisory for mitigations.

References

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