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
Vulnerability Analysis
CVE-2026-56865 can be exploited with local system access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to be very high.
Weakness Type
Improper Verification of Cryptographic Signature
The software does not verify, or incorrectly verifies, the cryptographic signature for data.
Products Associated with CVE-2026-56865
Want to know whenever a new CVE is published for GoLang Go? stack.watch will email you.
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.