CVE-2026-56853 Apply ReadHeaderTimeout when doing unencrypted HTTP/2 check in net/http
Attackers can cause denial of service in Go servers supporting unencrypted HTTP/2 by exploiting missing timeout enforcement during preface checks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-56853?
CVE-2026-56853 is a high-severity denial of service vulnerability in Go standard library net/http, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-770
Which versions are affected by CVE-2026-56853?
2 open source packages ship an affected version, including golang and stdlib.
| Package | Affected | Fixed in |
|---|---|---|
golang Bitnami | All versions | 1.25.13, 1.26.6, 1.27.0 |
stdlib Go | All versions | 1.25.13, 1.26.6, 1.27.0-rc.3 |
How do I fix CVE-2026-56853?
Upgrade golang to a fixed version for your release line: 1.25.13, 1.26.6 or 1.27.0.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-56853 being exploited?
There is no known exploitation of CVE-2026-56853 in the wild and no public exploit code.
Sources
Also known as AZL-96644, AZL-96680, AZL-98613, BELL-CVE-2026-56853, BIT-golang-2026-56853, CLEANSTART-2026-AF18240 and 2155 more.