CVE-2026-56865: Fix transparency log tile verification bypass in golang.org/x/mod/sumdb/tlog

CSIRTS triage

What

Transparency log tile verification in sumdb/tlog can be bypassed, allowing unverified log entries to be accepted.

Who is affected

All Go module users relying on sumdb transparency logs for supply-chain verification.

Urgency

High priority; CVSS 8.4 indicates critical bypass of transparency verification mechanisms.

Action

Update golang.org/x/mod to the patched version addressing CVE-2026-56865.

AI-assisted analysis generated from the source advisory — verify against the original.

Details

Source

Microsoft Security Response Center (INTL · vendor-psirt · site)

Severity

high — CVSS 8.4

Published

2026-08-11

Exploitation

Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-56865

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

• Low exploitation riskCVE-2026-568650.11% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 1% of all EPSS-scored CVEs.

Referenced CVEs

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

• high[NEW] [high] Golang Go: Multiple vulnerabilitiescert-bund
• highCVE-2026-56865: A malicious GOPROXY was previously capable of forging up to two sumdb tiles that allow for a r…nvd

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