CVE-2026-18209
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.
Statement
The Red Hat Product Security team has assessed the severity of this vulnerability as Low, given that it requires a specific client configuration and a non-conformant relying party to be successful. Successful exploitation allows an attacker to inject duplicate OIDC response parameters into the redirect URI fragment. The vulnerability's root cause is an incomplete validation check in RedirectUtils that fails to inspect the URI fragment for forbidden parameters.
Mitigation
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 3.4 | 4.7 | 3.4 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | High | Low | High |
| Privileges Required | None | None | None |
| User Interaction | Required | Required | Required |
| Scope | Changed | Changed | Changed |
| Confidentiality | None | None | None |
| Integrity Impact | Low | Low | Low |
| Availability Impact | None | None | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:L/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:L/A:N
Understanding the Weakness (CWE)
Other
Technical Impact: Varies by Context
Acknowledgements
Red Hat would like to thank default-cybe for reporting this issue.
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