What’s Vulnerable › Vulnerabilities

CVE-2026-56853: Apply ReadHeaderTimeout when doing unencrypted HTTP/2 check in net/http

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-13 · updated 2026-09-03
• Weakness: CWE-770

Description

When a server is configured to support unencrypted HTTP/2, it reads a few bytes from each new connection to see if they contain the HTTP/2 client preface. ReadHeaderTimeout is unexpectedly not being applied when doing this.

Affected products

• Go Stdlib: before 1.25.13; from 1.26.0-0 before 1.26.6; from 1.27.0-0 before 1.27.0-rc.3 — fixed in 1.25.13, 1.26.6, 1.27.0-rc.3
• Go standard library net/http: before 1.25.13; from 1.26.0-0 before 1.26.6; from 1.27.0-0 before 1.27.0-rc.3 — fixed in 1.25.13, 1.26.6, 1.27.0-rc.3

How to fix

Upgrade to a fixed version: Go Stdlib 1.25.13, 1.26.6, 1.27.0-rc.3; Go standard library net/http 1.25.13, 1.26.6, 1.27.0-rc.3. Check the vendor advisory for mitigations.

References

• https://go.dev/cl/795540
• https://go.dev/issue/80205
• https://groups.google.com/g/golang-announce/c/94pEornpRlI
• https://pkg.go.dev/vuln/GO-2026-6089

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