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Ignore unrelated, unauthenticated hashes in Lookup in golang.org/x/mod/sumdb

CVE-2026-56864

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (toolchain 1.27.0-rc.3, 1.26.6, 1.25.13; golang.org/x/mod 0.40.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • Go toolchain

    Affected
    < 1.25.13>= 1.26.0-0, < 1.26.6>= 1.27.0-0, < 1.27.0-rc.3
    Fixed in
    1.25.13, 1.26.6, 1.27.0-rc.3

  • Go golang.org/x/mod

    Affected
    < 0.40.0
    Fixed in
    0.40.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
not scored
EPSS
  • CVE-2026-56864: 0.30% estimated exploitation activity in the next 30 days; percentile 22.55 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

A malicious GOSUMDB was capable of serving arbitrary module content not contained within the transparency log. This attack allows for a coordinating GOPROXY and GOSUMDB to serve a client malicious module content that cannot be detected by evaluating the transparency log. In order to determine if you have been affected: rm -r go.sum go.work.sum vendor/ && go mod tidy

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from osv record GO-2026-6180

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-56864

Advisory: Ignore unrelated, unauthenticated hashes in Lookup in golang.org/x/mod/sumdb
IDs: CVE-2026-56864
Record: https://patchnext.com/advisories/cve-2026-56864 (revision 1, source checked 23 Oct 2026, 22:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: toolchain 1.27.0-rc.3, 1.26.6, 1.25.13; golang.org/x/mod 0.40.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go toolchain: affected < 1.25.13; >= 1.26.0-0, < 1.26.6; >= 1.27.0-0, < 1.27.0-rc.3; fixed in 1.25.13, 1.26.6, 1.27.0-rc.3
- Go golang.org/x/mod: affected < 0.40.0; fixed in 0.40.0

## Suggested inspection (read-only)
- Go toolchain: check the installed version with your package manager.
- Go golang.org/x/mod: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on toolchain, golang.org/x/mod.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GO-2026-6180 (OSV record GO-2026-6180 lists affected ranges: Go toolchain >=0 =1.26.0-0 =1.27.0-0 =0 <0.40.0 (fixed))

Commands are suggestions: review each one before running it.

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