# CVE-2026-18209: Keycloak-services: keycloak-services: oidc redirect_uri fragment bypass in http parameter pollution check

> Attackers can bypass redirect URI validation via fragment injection in wildcard configurations, potentially leading to session fixation or account takeover.

- **Severity:** Low 3.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 31 Jul 2026
- **Updated:** 16 Sep 2026
- **Also known as:** BIT-keycloak-2026-18209, CLEANSTART-2026-LI01273, GHSA-c8xx-fr3x-6m5w

## What is CVE-2026-18209?

CVE-2026-18209 is a low-severity vulnerability in Redhat Build of Keycloak, published on 31 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

A flaw was found in the keycloak-services component of Keycloak, which handles OpenID Connect (OIDC) authentication flows. The issue occurs because the security check designed to prevent HTTP parameter pollution only inspects the query portion of a redirect URL and ignores the fragment portion. When a client is configured with a wildcard redirect URI, an attacker can use this to inject duplicate security parameters into the login response. If a client application is not configured correctly, it might trust the attacker's injected data instead of the real security information from Keycloak, leading to session fixation or account confusion.

Weakness: CWE-1288 (Improper Validation of Consistency within Input)

## Which versions are affected by CVE-2026-18209?

1 open source package ship an affected version, including keycloak. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `keycloak` | Bitnami | All versions | 26.7.3 |

Products: Redhat Build of Keycloak.

## How do I fix CVE-2026-18209?

Upgrade keycloak to 26.7.3 or later.

## Is CVE-2026-18209 being exploited?

There is no known exploitation of CVE-2026-18209 in the wild and no public exploit code.

## Other vulnerabilities in Redhat Build of Keycloak

- [CVE-2026-15572](https://insights.sbomify.com/CVE-2026-15572/): Keycloak-services: keycloak-services: dcr protocol mapper type-swap policy bypass allows privilege escalation (high 8.8)
- [CVE-2026-3047](https://insights.sbomify.com/CVE-2026-3047/): Keycloak SAML Broken has Authentication Bypass by Primary Weakness (high 8.8)
- [CVE-2026-15573](https://insights.sbomify.com/CVE-2026-15573/): Keycloak-services: keycloak-services: authorization bypass via unnormalized uri matching in pathmatcher (high 8.1)
- [CVE-2026-16102](https://insights.sbomify.com/CVE-2026-16102/): Keycloak-services: keycloak-services: default dcr policy allows role forgery via user property mappers (high 8.1)
- [CVE-2026-1609](https://insights.sbomify.com/CVE-2026-1609/): Org.keycloak/keycloak-quarkus-server: keycloak: unauthorized access via jwt authorization grant with disabled users (high 8.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18209 and fix it if it is affected.

What it is: CVE-2026-18209 is a low-severity vulnerability in Redhat Build of Keycloak, published on 31 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-18209 in the wild and no public exploit code.

Affected packages and fixed versions:
- keycloak (Bitnami): affected all versions; fixed in 26.7.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-18209/ (Markdown: https://insights.sbomify.com/CVE-2026-18209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18209
- OSV: https://osv.dev/vulnerability/CVE-2026-18209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18209/
Summaries are generated and may contain mistakes; the official description above is the record.