How severe is CVE-2026-8204?

No CVSS score has been assigned to CVE-2026-8204 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-8204?

CVE-2026-8204 affects the following packages: concrete5/concrete5 (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-8204?

Update concrete5/concrete5 to 9.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-8204 is resolved across your whole dependency graph.

How do I detect CVE-2026-8204 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for concrete5/concrete5, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-8204 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.

Is CVE-2026-8204 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-8204 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 is the EPSS score for CVE-2026-8204?

CVE-2026-8204 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 27th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-8204?

CVE-2026-8204 is classified as CWE-639 (CWE-639). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-8204 published, and has it been updated?

CVE-2026-8204 was published on May 21, 2026 and was last updated on August 12, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2026-8204: Fixed in 9.5.1

Exploitation Status

No confirmed exploitation observed yet

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-2026-8204.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk+0.14%

Lower risk than most CVEs27th percentile — riskier than 27% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐘concrete5/concrete5

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

Description

Concrete CMS 9.5.0 and below is vulnerable to authorization Bypass in the Calendar Event Frontend Dialog which can allow cross-calendar data disclosure. A public calendar block can be used as a pivot point to access private calendar data. The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 6.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Winston Crooker for reporting.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`concrete5/concrete5`	all versions	9.5.1`composer require concrete5/concrete5:^9.5.1`

Affected Products

1 product · 1 configurations

Application

concrete cmsconcretecms

< 9.5.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for concrete5/concrete5, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update concrete5/concrete5 to 9.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-8204 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.

Frequently Asked Questions

Concrete CMS 9.5.0 and below is vulnerable to authorization Bypass in the Calendar Event Frontend Dialog which can allow cross-calendar data disclosure. A public calendar block can be used as a pivot point to access private calendar data. The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 6.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Winston Crooker for reporting.

O3 Security · Impact-Aware SCA

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CVE-2026-8204 — concrete5/concrete5