CVE-2021-3856: ClassLoaderTheme and ClasspathThemeResourceProviderFactory allows reading any file available as a resource to

Q: How do I fix CVE-2021-3856?

keycloak: upgrade to 15.1.0. Exact commands for each OS are on this page.

Q: Is CVE-2021-3856 being exploited?

CVE-2021-3856 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-3856 affect?

CVE-2021-3856 affects keycloak. Severity is medium (CVSS 4.3). 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 receiv

Q: How urgent is CVE-2021-3856?

Priority 37/100. On your watchlist

Routine

MEDIUM 4.3WATCHLISTUPGRADE

CVE-2021-3856 is a medium-severity flaw (CVSS 4.3) affecting keycloak. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	1.2% (top 33%)
CISA exploited list	No
Weakness	CWE-552, CWE-22
Published	2022-08-26
Last modified	2026-06-17

Publisher (CNA): Red Hat, Inc. · Vendor: redhat

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2022-08-26 16:15 UTCPublished CVE record published
2026-06-17 04:05 UTCUpdated Record last modified

redhat keycloak UPGRADE

Affected: < 15.1.0

Fixed in: 15.1.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy keycloak || rpm -q keycloak || keycloak --version

2. UPGRADE to 15.1.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade keycloak

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade keycloak

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3856?

keycloak: upgrade to 15.1.0. Exact commands for each OS are on this page.

Is CVE-2021-3856 being exploited?

CVE-2021-3856 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3856 affect?

CVE-2021-3856 affects keycloak. Severity is medium (CVSS 4.3). 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 receiv

How urgent is CVE-2021-3856?

Priority 37/100. On your watchlist

Software pages

Keycloak vulnerabilities

Related CVEs

CVE-2023-6927 MEDIUM 4.6 MITIGATE DOWNGRADE A flaw was found in Keycloak
CVE-2023-6563 HIGH 7.7 UPGRADE DOWNGRADE An unconstrained memory consumption vulnerability was discovered in Keycloak
CVE-2023-6291 HIGH 7.1 UPGRADE DOWNGRADE A flaw was found in the redirect_uri validation logic in Keycloak
CVE-2023-6134 MEDIUM 4.6 UPGRADE UPGRADE A flaw was found in Keycloak that prevents certain schemes in redirects, but permits them if a wildcard is app
CVE-2025-8419 MEDIUM 5.3 MITIGATE A vulnerability was found in Keycloak-services

References

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