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Vulnerability · Published Jul 31, 2026 · Assigned by redhat

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

Low 3.4

In plain English

A low-severity security flaw in Red Hat build of Keycloak 26.6 from Red Hat. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 35% of all known vulnerabilities).

Severity (CVSS 3.1)3.4Low
Exploit odds (30 days)0.4%top 64.7% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

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.

Affected products (as reported)

VendorProductVersions
Red Hat Red Hat build of Keycloak 26.6 see references
Red Hat Red Hat build of Keycloak 26.6 see references
Red Hat Red Hat build of Keycloak 26.6 see references
Red Hat Red Hat build of Keycloak 26.6.7 see references
Red Hat Red Hat build of Keycloak 26.6.7 see references
Red Hat Red Hat build of Keycloak 26.6.7 see references
Red Hat Red Hat Data Grid 8 see references
Red Hat Red Hat JBoss Enterprise Application Platform Expansion Pack see references
Red Hat Red Hat Single Sign-On 7 see references

Weakness: CWE-1288 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:L/A:N

References