How severe is CVE-2026-56864?

No CVSS score has been assigned to CVE-2026-56864 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-56864?

CVE-2026-56864 affects the following packages: toolchain (Go), golang.org/x/mod (Go). Ecosystems affected: Go.

How do I fix CVE-2026-56864?

Update toolchain to 1.25.13 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-56864 is resolved across your whole dependency graph.

How do I detect CVE-2026-56864 in my Go dependencies?

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.

How do I mitigate CVE-2026-56864 if there is no patch (or I can't update yet)?

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.

How does O3 Security protect against CVE-2026-56864?

O3 pinpoints whether CVE-2026-56864 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-56864 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-56864 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is CVE-2026-56864?

CVE-2026-56864 is classified as CWE-347 (CWE-347). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-56864 published, and has it been updated?

CVE-2026-56864 was published on August 13, 2026 and was last updated on August 13, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-56864

CVE-2026-56864 is a CWE-347 vulnerability in toolchain. O3 Security confirms whether CVE-2026-56864 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

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

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

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
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-2026-56864.

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 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

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`toolchain`	≥ 1.26.0-0&&< 1.25.13	1.25.13
🐹Go	`golang.org/x/mod`	all versions	0.40.0

Detection & mitigation playbook

Open-source dependency

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.
Fix
Update toolchain to 1.25.13 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-56864 is resolved across your whole dependency graph.
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.
How O3 protects you
O3 pinpoints whether CVE-2026-56864 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-2026-56864. 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 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

O3 Security · Impact-Aware SCA

Is CVE-2026-56864 in your dependencies?

O3 detects CVE-2026-56864 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