# CVE-2026-78663: Go net/http double flow control refund on HTTP/2 server streams

Source: https://junglewise.ai/threats/cve-2026-78663-go-2026-6612-double-flow-control-refund-on-http-2-server-streams  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-08. Updated: 2026-10-10  
License: CC BY 4.0 with a link back

CVE-2026-78663 · Severity: critical · CVSS 9.1 · Published 2026-10-08

Go's net/http library, used to build web services and APIs, has a flow control bug in its HTTP/2 server implementation. An attacker can trigger a double refund of flow control credits by closing streams in a specific way, allowing them to send more data than the protocol permits, potentially causing denial of service or resource exhaustion.

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

## Quick answers

**Is CVE-2026-78663 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-78663 being exploited in the wild.

**How severe is CVE-2026-78663?** Critical severity, with a CVSS score of 9.1, published 8 October 2026.

**Which products and versions are affected?** Google Go <UNKNOWN>.

**Is there a fix for CVE-2026-78663?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-78663 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-78663?** Check whether you run Google 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.

## Technical details

The vulnerability is a logic error in HTTP/2 flow control handling on the server side where stream closure triggers a double refund of flow control window credits. An attacker with network access can exploit this by crafting a sequence of stream closure events, exhausting the server's flow control window and enabling unbounded data transmission. The attack requires no authentication and is remotely exploitable on any Go HTTP/2 server.

## Affected products

- Google Go <UNKNOWN>

## Identifiers

- CVE-2026-78663
- GO-2026-6612

## Timeline

- 2026-10-08: disclosed

## References

- [https://go.dev/cl/847187](https://go.dev/cl/847187)
- [https://go.dev/cl/847310](https://go.dev/cl/847310)

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