When SAML is configured to act as a client (SAML brokering) it does not check the NotOnOrAfter tat is defined inside the SubjectConfirmationData. This just affects in the sense that an attacker can delay the response for more of the expected time. The response cannot be tampered because SAML should configured to be signed and, therefore, the same response cannot be used twice or to respond another request (the id should be modified and therefore the signature validation fails).

<samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" Destination="http://localhost:8080/realms/master/broker/saml/endpoint" ID="ID_9e3417cd-0c39-449b-bdaa-50876264d109" InResponseTo="ID_b99bc864-e89c-4243-adbb-56898a2143b7" IssueInstant="2026-01-12T17:17:13.922Z" Version="2.0">
    <saml:Issuer>http://localhost:8080/realms/saml</saml:Issuer>
    <samlp:Status>
        <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
    </samlp:Status>
    <saml:Assertion xmlns="urn:oasis:names:tc:SAML:2.0:assertion" ID="ID_479728b1-b612-46c4-8a3a-f0dbff4409af" IssueInstant="2026-01-12T17:17:13.919Z" Version="2.0">
        <saml:Issuer>http://localhost:8080/realms/saml</saml:Issuer>
        <saml:Subject>
            <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified">samluser</saml:NameID>
            <saml:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
                <saml:SubjectConfirmationData InResponseTo="ID_b99bc864-e89c-4243-adbb-56898a2143b7" NotOnOrAfter="2026-01-12T17:22:11.919Z" Recipient="http://localhost:8080/realms/master/broker/saml/endpoint"/>
            </saml:SubjectConfirmation>
        </saml:Subject>
        <saml:Conditions NotBefore="2026-01-12T17:17:11.919Z" NotOnOrAfter="2026-01-12T17:18:11.919Z">
            <saml:AudienceRestriction>
                <saml:Audience>http://localhost:8080/realms/master</saml:Audience>
            </saml:AudienceRestriction>
        </saml:Conditions>
        <saml:AuthnStatement AuthnInstant="2026-01-12T17:17:13.922Z" SessionIndex="9RdZsk9oqv5JK9aWfX5d1saN::9ad854a5-b6fc-4402-9e2c-7b96411af415" SessionNotOnOrAfter="2026-01-13T03:17:13.922Z">
            <saml:AuthnContext>
                <saml:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:unspecified</saml:AuthnContextClassRef>
            </saml:AuthnContext>
        </saml:AuthnStatement>
        <saml:AttributeStatement>
            <saml:Attribute FriendlyName="email" Name="email" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:basic">
                <saml:AttributeValue xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xs:string">samluser</saml:AttributeValue>
            </saml:Attribute>
            <saml:Attribute FriendlyName="firstName" Name="firstName" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:basic">
                <saml:AttributeValue xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xs:string">samluser</saml:AttributeValue>
            </saml:Attribute>
            <saml:Attribute FriendlyName="lastName" Name="lastName" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:basic">
                <saml:AttributeValue xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xs:string">samluser</saml:AttributeValue>
            </saml:Attribute>
        </saml:AttributeStatement>
    </saml:Assertion>
</samlp:Response>

The NotOnOrAfter in the conditions are checked OK but the one in SubjectConfirmationData is not checked. The idea is that keycloak should also check the NotBefore and NotOnOrAfter in that element too just in case (maybe the IdP just use that data).