A malicious module proxy could 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.
The go command now properly checks checksum database responses to ensure
that the expected module signature is present, not just that if a signature is
present it matches the expectation.
Thanks to Mundur (https://github.com/M0nd0R) for reporting this issue.
This is CVE-2026-42501 and Go issue https://go.dev/issue/79070.
This was a PRIVATE track issue, tracked in http://b/508217711.
A malicious module proxy could 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.
The go command now properly checks checksum database responses to ensure
that the expected module signature is present, not just that if a signature is
present it matches the expectation.
Thanks to Mundur (https://github.com/M0nd0R) for reporting this issue.
This is CVE-2026-42501 and Go issue https://go.dev/issue/79070.
This was a PRIVATE track issue, tracked in http://b/508217711.