GHSA-qf3q-3h68-mmh2 · Severity: high — A malicious module proxy can exploit a flaw in the go command's validation of module checksums to...
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.
Conclusion & alert: CVE-2026-42501 is rated Low Risk (30.2/100): CVSS High severity, with low exploitation likelihood (EPSS 0.01%). Mandatory action: Monitor for updates and reassess as exploit intelligence or EPSS changes.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-05-08 | — | 0.01% | — |
Full EPSS history (1 record total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 7.5 | 3.1 | HIGH |
|
1.6 | 5.9 | 134c704f-9b21-4f2e-91b3-4a467353bcc0 |
GHSA-qf3q-3h68-mmh2 · Severity: high — A malicious module proxy can exploit a flaw in the go command's validation of module checksums to...
| vendor | priority | summary | link |
|---|---|---|---|
debian
|
not yet assigned | CVE-2026-42501 not yet assigned priority: Debian including 5 source packages (golang-1.15, golang-1.19, golang-1.24, golang-1.25, golang-1.26), 7 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 4, open 3. | https://security-tracker.debian.org/tracker/CVE-2026-42501 |
suse
|
high | CVE-2026-42501 severity important: SUSE including 7 source package names (go, go-doc, …), 33 product×package rows across 14 product lines (SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS, SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS, … (14 product lines)): Known Not Affected 29, Fixed 4. | https://www.suse.com/security/cve/CVE-2026-42501/ |
ubuntu
|
medium | CVE-2026-42501 medium priority: Ubuntu including 16 source packages (golang, golang-1.10, …), 98 status rows across 9 suites (bionic, focal, jammy, noble, questing, resolute, trusty, upstream, xenial): needs-triage 52, DNE 46. | https://ubuntu.com/security/CVE-2026-42501 |
| URL | Tags |
|---|---|
| https://go.dev/cl/775321 | Patch |
| https://go.dev/issue/79070 | Issue Tracking |
| https://groups.google.com/g/golang-announce/c/qcCIEXso47M | Release Notes |
| https://pkg.go.dev/vuln/GO-2026-4984 | Vendor Advisory |