CVE-2026-56865: Fix transparency log tile verification bypass in golang.org/x/mod/sumdb/tlog

Severity: high (CVSS 8.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% 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-347

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

Affected products

Golang.Org/X/Mod: before 0.40.0 — fixed in 0.40.0
Go Toolchain: 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: 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
golang.org/x/mod/sumdb/tlog: before 0.40.0 — fixed in 0.40.0

How to fix

Upgrade to a fixed version: Golang.Org/X/Mod 0.40.0; Go Toolchain 1.25.13, 1.26.6, 1.27.0-rc.3; Go 1.25.13, 1.26.6, 1.27.0-rc.3; golang.org/x/mod/sumdb/tlog 0.40.0. Check the vendor advisory for mitigations.

References

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