Go cmd/go GOSUMDB Bypass via Malicious GOPROXY (1.27.0-rc.3)
CVE-2026-56865 Published on August 13, 2026
Fix transparency log tile verification bypass in golang.org/x/mod/sumdb/tlog
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
Products Associated with CVE-2026-56865
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Affected Versions
Go toolchain cmd/go:- Before 1.25.13 is affected.
- Version 1.26.0-0 and below 1.26.6 is affected.
- Version 1.27.0-0 and below 1.27.0-rc.3 is affected.
- Before 0.40.0 is affected.