data/reports/GO-2026-4984.yaml

id: GO-2026-4984
modules:
    - module: cmd
      versions:
        - fixed: 1.25.10
        - introduced: 1.26.0-0
        - fixed: 1.26.3
      vulnerable_at: 1.26.2
      packages:
        - package: cmd/go
summary: Malicious module proxy can bypass checksum database in cmd/go
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.
credits:
    - Mundur (https://github.com/M0nd0R)
references:
    - fix: https://go.dev/cl/775321
    - report: https://go.dev/issue/79070
    - web: https://groups.google.com/g/golang-announce/c/qcCIEXso47M
cve_metadata:
    id: CVE-2026-42501
    cwe: 'CWE-347: Improper Verification of Cryptographic Signature'
source:
    id: go-security-team
review_status: REVIEWED