CVE-2026-42501 Malicious module proxy can bypass checksum database in cmd/go

What is CVE-2026-42501?

CVE-2026-42501 is a high-severity vulnerability in Golang Go, published on 7 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official 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.

Weakness: CWE-347

Which versions are affected by CVE-2026-42501?

2 open source packages ship an affected version, including golang and toolchain. It affects 1 product.

Products include Golang Go.

How do I fix CVE-2026-42501?

Upgrade golang to a fixed version for your release line: 1.25.10 or 1.26.3.

Is CVE-2026-42501 being exploited?

There is no known exploitation of CVE-2026-42501 in the wild and no public exploit code.

Other vulnerabilities in Golang Go

• CVE-2020-0601 Microsoft Windows CryptoAPI Spoofing Exploited High 8.1
• CVE-2026-27143 Missing bound checks can lead to memory corruption in safe Go in cmd/compile Critical 9.8
• CVE-2024-24790 Unexpected behavior from Is methods for IPv4-mapped IPv6 addresses in net/netip Critical 9.8
• CVE-2023-39320 Arbitrary code execution via go.mod toolchain directive in cmd/go Critical 9.8
• CVE-2023-24540 Improper handling of JavaScript whitespace in html/template Critical 9.8

All Golang Go vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-98799, BELL-CVE-2026-42501, BIT-golang-2026-42501, CLEANSTART-2026-NE91671, CLEANSTART-2026-QP25328, CLEANSTART-2026-TD83938 and 799 more.