Apache Kafka: Broker silently skips JWT signature verification for SASL/OAUTHBEARER when the JWKS endpoint URL is not configured

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-110985
StatePUBLISHED
Assignerapache
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-28 17:17:38 UTC
Updated2026-10-28 17:17:38 UTC
DescriptionA Kafka broker using SASL/OAUTHBEARER with the default validator (sasl.oauthbearer.jwt.validator.class=DefaultJwtValidator) selects its implementation from configuration. When sasl.oauthbearer.jwks.endpoint.url is set, the validator verifies the JWT signature. When that URL is unset (the default) and no verification key resolver is supplied, DefaultJwtValidator silently falls back to ClientJwtValidator, which does not verify the JWT signature, algorithm, or issuer - and the broker starts with no warning or error. This is the residual of CVE-2026-33557, whose fix restored signature verification only on the path where the JWKS endpoint URL is set. Impact: A broker in this state accepts any structurally valid JWT, including "alg":"none" tokens. An unauthenticated remote attacker who can reach the listener can forge a token asserting any principal - including configured superusers - and gain full access to the cluster. Only brokers that enable SASL/OAUTHBEARER validation without a JWKS endpoint URL (and without a custom verification key resolver) are affected; brokers that set the URL verify signatures and are not affected. Unlike CVE-2026-110952 (issuer/audience not enforced by default), here no signature is verified at all. Mitigation (all affected versions, no upgrade required): Set sasl.oauthbearer.jwks.endpoint.url for every listener/mechanism that uses OAuthBearerValidatorCallbackHandler, or configure a signature-verifying sasl.oauthbearer.jwt.validator.class. Resolution: From Apache Kafka 4.2.2, 4.3.1, and 4.4.0: a broker that would fall back to the non-verifying ClientJwtValidator fails to start with a ConfigException instead of running without signature verification (secure by default).

Risk And Classification

Problem Types: CWE-345 | CWE-347 | CWE-636 | CWE-1287 | CWE-345 CWE-345 Insufficient Verification of Data Authenticity and its child | CWE-347 CWE-347 Improper verification of cryptographic signature | CWE-1287 CWE-1287 Improper validation of specified type of input | CWE-636 CWE-636 Failing Open

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Apache Software Foundation Apache Kafka affected 4.1.0 4.1.2 semver Not specified
CNA Apache Software Foundation Apache Kafka affected 4.2.0 4.2.1 semver Not specified
CNA Apache Software Foundation Apache Kafka affected 4.3.0 semver Not specified

References

ReferenceSourceLinkTags
kafka.apache.org/cve-list [email protected] kafka.apache.org
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: San Zhang <[email protected]> (en)

CNA: Kaixuan Li <[email protected]> (en)

CNA: Evan Zhou <[email protected]> (en)

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