Is CVE-2026-18209 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-18209 being exploited in the wild.

How severe is CVE-2026-18209?

Low severity, with a CVSS score of 3.4, published 31 July 2026.

Which products and versions are affected?

Red Hat build of Keycloak unspecified.

Is there a fix for CVE-2026-18209?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-18209 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-18209?

Check whether you run Red Hat build of Keycloak at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-18209: Keycloak OIDC parameter pollution via redirect URI fragment

CVE-2026-18209 · Severity: low · CVSS 3.4 · Published 2026-07-31

Keycloak is an open-source identity and access management solution used to secure modern applications and services. A security flaw in its authentication component allows attackers to bypass certain safety checks when a client application is configured with overly broad 'wildcard' redirect settings. This could allow an attacker to manipulate login sessions, potentially leading to account confusion or session fixation where a user's session is controlled by the attacker.

Technologies: Red Hat build of Keycloak. Vendors: Red Hat.

Quick answers

Is CVE-2026-18209 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-18209 being exploited in the wild.
How severe is CVE-2026-18209?: Low severity, with a CVSS score of 3.4, published 31 July 2026.
Which products and versions are affected?: Red Hat build of Keycloak unspecified.
Is there a fix for CVE-2026-18209?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-18209 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-18209?: Check whether you run Red Hat build of Keycloak at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A vulnerability exists in the keycloak-services component, specifically within the RedirectUtils.containsForbiddenOidcParameters() method. While the system checks the query portion of a redirect URL for forbidden OIDC parameters (like 'code', 'state', or 'iss'), it fails to inspect the URI fragment (#). If a client is configured with a wildcard redirect URI, an attacker can craft a redirect_uri containing these parameters in the fragment. Because Keycloak appends its own parameters to the attacker-supplied fragment during the authorization response, it results in a polluted response. If the relying party application uses a 'first-wins' parsing strategy, it may trust the attacker's injected data over the legitimate security information from Keycloak.

Affected products

Red Hat Red Hat Build of Keycloak unspecified

Timeline

2026-07-29: other: Reported to Red Hat Bugzilla
2026-07-31: advisory: NVD and Red Hat published advisory

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.