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

Source: https://junglewise.ai/threats/cve-2026-18209-keycloak-oidc-parameter-pollution-via-redirect-uri-fragment  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-31. Updated: 2026-07-31  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/red-hat-keycloak). Vendors: [Red Hat](https://junglewise.ai/threats/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

- [https://access.redhat.com/downloads/content/package-browser/](https://access.redhat.com/downloads/content/package-browser/)
- [https://access.redhat.com/jbossnetwork/restricted/listSoftware.html](https://access.redhat.com/jbossnetwork/restricted/listSoftware.html)
- [https://access.redhat.com/security/cve/CVE-2026-18209](https://access.redhat.com/security/cve/CVE-2026-18209)
- [https://bugzilla.redhat.com/show_bug.cgi?id=2508305](https://bugzilla.redhat.com/show_bug.cgi?id=2508305)

## Related threats

- [Keycloak server arbitrary code execution in client authorization](https://junglewise.ai/threats/cve-2026-120984-a-flaw-was-found-in-the-keycloak-server-s-client-management)
- [Red Hat Build of Keycloak OIDC logout session fixation](https://junglewise.ai/threats/cve-2026-107124-a-flaw-was-found-in-the-oidc-logout-endpoint-of-the-keycloak)
- [Keycloak theme resource disk exhaustion](https://junglewise.ai/threats/cve-2026-110932-a-flaw-was-found-in-the-theme-resource-endpoint-of-keycloak-an)
- [Keycloak secure-redirect-uris-enforcer open redirect via DNS resolution](https://junglewise.ai/threats/cve-2026-118425-a-flaw-was-found-in-the-secure-redirect-uris-enforcer-client)
- [Keycloak OIDC Dynamic Client Registration offline token revocation bypass](https://junglewise.ai/threats/cve-2026-107623-a-flaw-was-found-in-the-oidc-dynamic-client-registration-dcr)