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HIGH

Malicious module proxy can bypass checksum database in cmd/go

Published May 7, 2026

Description

A malicious module proxy can exploit a flaw in the go command's validation of module checksums to bypass checksum database validation. This vulnerability affects any user using an untrusted module proxy (GOMODPROXY) or checksum database (GOSUMDB). A malicious module proxy can serve altered versions of the Go toolchain. When selecting a different version of the Go toolchain than the currently installed toolchain (due to the GOTOOLCHAIN environment variable, or a go.work or go.mod with a toolchain line), the go command will download and execute a toolchain provided by the module proxy. A malicious module proxy can bypass checksum database validation for this downloaded toolchain. Since this vulnerability affects the security of toolchain downloads, setting GOTOOLCHAIN to a fixed version is not sufficient. You must upgrade your base Go toolchain. The go tool always validates the hash of a toolchain before executing it, so fixed versions will refuse to execute any cached, altered versions of the toolchain. The go tool trusts go.sum files to contain accurate hashes of the current module's dependencies. A malicious proxy exploiting this vulnerability to serve an altered module will have caused an incorrect hash to be recorded in the go.sum. Users who have configured a non-trusted GOPROXY can determine if they have been affected by running "rm go.sum ; go mod tidy ; go mod verify", which will revalidate all dependencies of the current module. The specific flaw in more detail: The go command consults the checksum database to validate downloaded modules, when a module is not listed in the go.sum file. It verifies that the module hash reported by the checksum database matches the hash of the downloaded module. If, however, the checksum database returns a successful response that contains no entry for the module, the go command incorrectly permitted validation to succeed. A module proxy may mirror or proxy the checksum database, in which case the go command will not connect to the checksum database directly. Checksums reported by the checksum database are cryptographically signed, so a malicious proxy cannot alter the reported checksum for a module. However, a proxy which returns an empty checksum response, or a checksum response for an unrelated module, could cause the go command to proceed as if a downloaded module has been validated.

Affected products

Remediation

Red Hat statement

This Moderate impact flaw in the Go command (`cmd/go`) allows a malicious module proxy to bypass checksum validation, potentially leading to the execution of untrusted Go toolchains. Red Hat products utilizing the Go command are vulnerable if configured to use an untrusted module proxy or checksum database, as this could compromise the integrity of the Go development environment. Exploitation requires user interaction with a compromised proxy.

Red Hat mitigation

To verify the integrity of Go module dependencies and detect potential tampering by a malicious proxy, users can revalidate all dependencies of the current module. This process involves removing the existing `go.sum` file and then re-generating and verifying module checksums. Execute the following commands in your module's root directory: ```bash rm go.sum go mod tidy go mod verify ``` This operation will re-download and re-verify all module dependencies. Be aware that this may affect your local module cache and could require re-building projects.

Metrics

Weaknesses (1)

CWE-347

References (8)

Change history (0)

No recorded changes yet.

Sources
CVE.org / MITRE
Status PUBLISHED
Assigner Go
Published May 7, 2026
Updated May 8, 2026
Reserved Apr 28, 2026
Link CVE-2026-42501
CISA Vulnrichment
Updated May 8, 2026
NVD
Status Analyzed
Modified Jun 17, 2026
Link CVE-2026-42501
Red Hat
Severity Moderate
Public date May 7, 2026
Links CVE-2026-42501 - Bugzilla