Auth bypass in Corebunch Instatic
CVE-2026-121458
Instatic before 0.0.21 contains a missing authorization vulnerability in its AI/MCP content read tools that allows low-privileged authenticated users with ai.chat and content.edit.own capabilities to access data rows belonging to other users by calling content_get_document and content_list_documents without the object-level authorization checks enforced by the normal HTTP API. Attackers can use the MCP interface to retrieve private or unpublished draft content and enumerate rows from the posts system table, resulting in cross-user data disclosure.
Vulnerability class: Broken Access Control
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
Weakness classification (CWE)
References
- github.com/CoreBunch/Instatic/releases/tag/v0.0.21 (issue-tracking, patch)
- github.com/CoreBunch/Instatic/security/advisories/GHSA-x5cc-8486-8qqx (vendor-advisory, technical-description)
- www.vulncheck.com/advisories/instatic-before-authorization-bypass-via-mcp-conte… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-121458?
- CVE-2026-121458 is a medium-severity vulnerability in Corebunch Instatic, classified under Missing Authorization. CVSS score: 6.5/10. Published 2026-11-03.
- How severe is CVE-2026-121458?
- Medium severity. CVSS v3 base score is 6.5 out of 10.