UBUNTU-CVE-2026-110952: Medium severity ubuntu/kafka vulnerability
A Kafka broker that authenticates clients with SASL/OAUTHBEARER using the bundled JWKS-backed validator always verifies the JWT signature, but two further checks are only performed when explicitly configured: - The issuer (iss) claim is verified only when sasl.oauthbearer.expected.issuer is set. When it is unset, a validly-signed token carrying the expected audience is accepted regardless of which issuer minted it. - An audience (aud) claim is required only when sasl.oauthbearer.expected.audience is set. When it is unset, a validly-signed token that omits the aud claim is accepted. (A token that carries an aud claim is still rejected when no expected audience is configured.) Both options default to unset and have been documented as optional (Importance LOW) since KIP-768 (3.1.0). Impact: The exposure is realizable mainly in deployments whose JWKS endpoint serves signing keys for more than one issuer (a multi-tenant identity provider, an aggregated/shared JWKS, or shared signing keys): a token minted for a different issuer, or a token lacking an audience, may be accepted to authenticate — potentially as a higher-privileged principal. In a single-issuer deployment where the provider controls the iss claim and issues audience-scoped tokens, exposure is limited. The signature trust anchor remains enforced in all cases. This is distinct from CVE-2026-33557 (acceptance of unsigned/unvalidated tokens); here the signature is validated and only the issuer/audience checks are not enforced by default. Mitigation (all affected versions, no upgrade required): Set both sasl.oauthbearer.expected.issuer (to your provider's exact iss value) and sasl.oauthbearer.expected.audience (to the audience(s) your brokers expect), per listener. Brokers that set these perform exact-match issuer/audience checks and are not affected. Both have been available since 3.1.0. Resolution: - From Apache Kafka 4.4.0: a broker configured with a JWKS endpoint fails to start unless both sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience are set (secure by default). Deployments that intentionally do not restrict the issuer and/or audience may opt out per axis with sasl.oauthbearer.allow.unverified.issuer=true and/or sasl.oauthbearer.allow.unverified.audience=true.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Apache Kafkato a version that resolves this vulnerability.Fixed in 4.4.0 - Configuration
Configure both settings per listener on all affected versions; this enables exact-match issuer and audience checks.
Apache Kafka SASL/OAUTHBEARER JWKS-backed validator sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience = Set sasl.oauthbearer.expected.issuer to the provider's exact iss value and sasl.oauthbearer.expected.audience to the audience(s) the brokers expect
Event History
Frequently Asked Questions
Are deployments using the default SASL/OAUTHBEARER validator settings affected?
Both sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience default to unset. With those defaults, issuer validation is not enforced and tokens without an aud claim can be accepted if their signature is valid.
Which environments have the most meaningful exposure?
The exposure is mainly realizable when the configured JWKS endpoint provides signing keys for multiple issuers, such as a multi-tenant identity provider, aggregated or shared JWKS deployment, or shared signing-key environment. In those cases, a token from a different issuer or one without an audience may authenticate, potentially under a higher-privileged principal.
What must an attacker have to authenticate using this condition?
The attacker needs a validly signed JWT whose signing key is available through the broker's configured JWKS source. Depending on configuration, the token may be accepted despite being minted by a different issuer or despite omitting the aud claim.
How can administrators reduce exposure through configuration?
Set sasl.oauthbearer.expected.issuer to require the expected issuer claim, and set sasl.oauthbearer.expected.audience to require an audience claim matching the expected audience. These checks are performed only when the corresponding setting is explicitly configured.
How can an administrator determine whether a broker is exposed?
Review the broker configuration for unset sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience values. Also determine whether the configured JWKS endpoint exposes keys for more than one issuer or uses shared signing keys.