CVE-2026-56865 Fix transparency log tile verification bypass in golang.org/x/mod/sumdb/tlog
A malicious GOPROXY can bypass sumdb checks to serve malicious module content, requiring developers to update the Go toolchain and clear caches.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-56865?
CVE-2026-56865 is a high-severity vulnerability in cmd/go, published on 13 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
A malicious GOPROXY was previously capable of forging up to two sumdb tiles that allow for a requested module to bypass the GOSUMDB check and persist attacker-controlled module content to a local Go module cache. This attack allows for a malicious GOPROXY to serve malicious module content that cannot be detected by evaluating the transparency log. All tiles are now correctly verified against their parents. In order to determine if you have been affected: rm -r go.sum go.work.sum vendor/ && go mod tidy
Weakness: CWE-347
Which versions are affected by CVE-2026-56865?
3 open source packages ship an affected version, including golang, golang.org/x/mod and toolchain.
| Package | Affected | Fixed in |
|---|---|---|
golang Bitnami | All versions | 1.25.13, 1.26.6, 1.27.0 |
golang.org/x/mod Go | All versions | 0.40.0 |
toolchain Go | All versions | 1.25.13, 1.26.6, 1.27.0-rc.3 |
How do I fix CVE-2026-56865?
Upgrade golang to a fixed version for your release line: 1.25.13, 1.26.6 or 1.27.0.
Is CVE-2026-56865 being exploited?
There is no known exploitation of CVE-2026-56865 in the wild and no public exploit code.
Sources
Also known as AZL-96647, AZL-96683, AZL-98721, BELL-CVE-2026-56865, BIT-golang-2026-56865, CLEANSTART-2026-DB44167 and 2134 more.