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MEDIUM severity

CVE-2021-3856 — keycloak-core

MEDIUMFix: keycloak/keycloak@73f0474

CVE-2021-3856 is a medium-severity (CVSS 4.3) CWE-552 vulnerability in org.keycloak:keycloak-core. A fix is available for org.keycloak:keycloak-core — see the affected versions and patch details below.

Keycloak has Files or Directories Accessible to External Parties

Also known asGHSA-3w4v-rvc4-2xpw
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.2%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs67th percentile — riskier than 67% of all scored CVEsHighest risk
0.00%0.55%1.11%1.66%0.4%1.2%May 26Sep 26Nov 26

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
☕org.keycloak:keycloak-core

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

Description

ClassLoaderTheme and ClasspathThemeResourceProviderFactory allows reading any file available as a resource to the classloader. By sending requests for theme resources with a relative path from an external HTTP client, the client will receive the content of random files if available.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenorg.keycloak:keycloak-coreall versions15.1.0org.keycloak:keycloak-core:15.1.0

Affected Products

1 product · 1 configurations
Application
keycloakredhat
< 15.1.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

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

  2. Fix

    Update org.keycloak:keycloak-core to 15.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3856 is resolved across your whole dependency graph.

  3. Workarounds

    Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatLow
ProductFixed inAdvisory
Red Hat Single Sign-On 7.6 for RHEL 7rh-sso7-0:1-4.el7ssoRHBA-2022:5452
Red Hat Single Sign-On 7.6 for RHEL 8rh-sso7-0:1-4.el8ssoRHBA-2022:5454

Frequently Asked Questions

ClassLoaderTheme and ClasspathThemeResourceProviderFactory allows reading any file available as a resource to the classloader. By sending requests for theme resources with a relative path from an external HTTP client, the client will receive the content of random files if available.
O3 Security · Impact-Aware SCA

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CVE-2021-3856: keycloak-core — Fixed in 15.1.0