# CVE-2026-56853: Go HTTP/2 timeout bypass in unencrypted connections

Source: https://junglewise.ai/threats/cve-2026-56853-go-http-2-timeout-bypass-in-unencrypted-connections  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-13. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-56853 · Severity: high · CVSS 7.5 · Published 2026-08-13

Go's HTTP/2 implementation fails to apply the configured read timeout when establishing unencrypted HTTP/2 connections. An attacker can exploit this by sending partial HTTP/2 handshake data slowly, bypassing the timeout protection and causing the server to hang indefinitely or consume resources. This affects Go-based web services and APIs that support HTTP/2 without TLS.

Technologies: [stdlib (Go)](https://junglewise.ai/threats/technologies/go-stdlib). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-56853 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-56853 being exploited in the wild.

**How severe is CVE-2026-56853?** High severity, with a CVSS score of 7.5, published 13 August 2026.

**Which products and versions are affected?** stdlib (Go) SEMVER:introduced=0,fixed=1.25.13,introduced=1.26.0-0,fixed=1.26.6,introduced=1.27.0-0,fixed=1.27.0-rc.3.

**Is there a fix for CVE-2026-56853?** Yes. stdlib (Go): upgrade to 1.27.0-rc.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-56853 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-56853?** Check whether you run stdlib (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- stdlib (Go): upgrade to 1.27.0-rc.3 or later

  ```sh
  go get stdlib@v1.27.0-rc.3
  ```


## Technical details

When a Go server is configured to support plaintext HTTP/2, it reads the HTTP/2 client preface (24 bytes: "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n") from new connections to detect the protocol. The vulnerability is that the ReadHeaderTimeout setting is not enforced during this initial read operation. An attacker can send bytes slowly or incompletely to the preface detector, causing the server to wait indefinitely without triggering the timeout. The attack requires a network-reachable HTTP/2 server with plaintext (h2c) enabled and no other timeouts configured at a lower level. This leads to connection exhaustion and denial of service.

## Affected products

- Go Go <UNKNOWN>

## Identifiers

- CVE-2026-56853
- GO-2026-6089

## Timeline

- 2026-08-13: disclosed

## References

- [https://pkg.go.dev/](https://pkg.go.dev/)
- [https://go.dev/cl/795540](https://go.dev/cl/795540)

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