Kafka OAuth JWT issuer and audience checks off by default (CVE-2026-110952)
The bundled JWKS validator always checks the signature, but iss and audience checks need explicit settings.
48
CVE-2026-110952
Optional JWT issuer/audience checks in Kafka SASL/OAUTHBEARER
- CVSS
- —
- EPSS
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- Published
- ·
- Modified
AI analysisSummaries by Vani AI
Apache Kafka brokers using the bundled JWKS-backed SASL/OAUTHBEARER validator always check the JWT signature, but verify the issuer and require an audience only when sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience are set. Both default to unset since 3.1.0. On a shared or multi-issuer JWKS, a validly signed token from another issuer, or one that omits aud, can authenticate, possibly as a higher-privileged principal. Single-issuer, audience-scoped deployments are much less exposed.
What to do: Set sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience per listener (available since 3.1.0; no upgrade required). From 4.4.0, a JWKS-configured broker will not start unless both are set or explicitly opted out.
| Product | Affected | Fixed in |
|---|---|---|
| Apache Kafka | SASL/OAUTHBEARER JWKS validator since 3.1.0 (KIP-768), with both checks optional by default | 4.4.0 requires both settings at startup unless explicitly opted out; setting both configs mitigates all affected versions |
Estimated exposure
No basis for an estimate.
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description · NVD
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.
- Weakness
- CWE-345, CWE-1188