Bug 2508305 (CVE-2026-18209)

Summary: CVE-2026-18209 keycloak-services: keycloak-services: OIDC redirect_uri fragment bypass in HTTP parameter pollution check
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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Priority: low    
Version: unspecifiedCC: anujha, aschwart, asoldano, aszczucz, bbaranow, bmaxwell, boliveir, bstansbe, dlofthou, drichtar, istudens, ivassile, iweiss, mosmerov, mposolda, msvehla, nwallace, pberan, pesilva, pjindal, pmackay, rmartinc, rstancel, ssilvert, sthorger, thjenkin, vdosoudi, vmuzikar
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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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.
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Description OSIDB Bzimport 2026-07-29 08:20:08 UTC 
An incomplete fix for CVE-2026-9689 was identified in Keycloak's RedirectUtils.containsForbiddenOidcParameters() method. While the original fix successfully blocks forbidden OIDC parameters (such as code, state, and iss) in the URI query string, it fails to inspect the URI fragment (#).
When a client is configured with a wildcard redirect URI, an attacker can supply a redirect_uri containing these forbidden parameters within the fragment. Because matchesRedirects strips fragments during prefix matching, the crafted URI is accepted. During the authorization response, Keycloak appends its own parameters to the attacker-supplied fragment, leading to a polluted response where attacker-controlled values appear first.
Exploitation Conditions:
The target client must have a wildcard-registered redirect URI (e.g., https://app.example.com/*).

The attacker must induce a victim to follow a crafted authorization URL.

The relying party (client application) must use a first-wins parsing strategy for duplicate parameters.


Concrete Impact:
Injection of attacker-controlled iss (issuer), state, and access_token parameters.

Potential for session fixation or account confusion if the relying party does not validate parameters per RFC 9207.