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Not in CISA KEV

CVE-2026-56865

CVE-2026-56865 is a CWE-347 vulnerability in toolchain. O3 Security confirms whether CVE-2026-56865 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Fix transparency log tile verification bypass in golang.org/x/mod/sumdb/tlog

Published
Aug 13, 2026
Updated
Aug 13, 2026
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Aug 13, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • A successful exploit gives an attacker total control of the affected component, not partial access.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-56865.

Real-World Exposure

2 pkgs affected
🐹toolchain🐹golang.org/x/mod

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

A malicious GOPROXY was previously capable of forging up to two sumdb tiles that allow for a requested module to bypass the GOSUMDB check and persist attacker-controlled module content to a local Go module cache.

This attack allows for a malicious GOPROXY to serve malicious module content that cannot be detected by evaluating the transparency log.

All tiles are now correctly verified against their parents.

In order to determine if you have been affected:

rm -r go.sum go.work.sum vendor/ && go mod tidy

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🐹Gotoolchain≥ 1.26.0-0&&< 1.25.131.25.13
🐹Gogolang.org/x/modall versions0.40.0

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for toolchain. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

  2. Fix

    Update toolchain to 1.25.13 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-56865 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 pinpoints whether CVE-2026-56865 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-56865. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

A malicious GOPROXY was previously capable of forging up to two sumdb tiles that allow for a requested module to bypass the GOSUMDB check and persist attacker-controlled module content to a local Go module cache. This attack allows for a malicious GOPROXY to serve malicious module content that cannot be detected by evaluating the transparency log. All tiles are now correctly verified against their parents. In order to determine if you have been affected: rm -r go.sum go.work.sum vendor/ && go mod tidy
O3 Security · Impact-Aware SCA

Is CVE-2026-56865 in your dependencies?

O3 detects CVE-2026-56865 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works