Apache Kafka up to 4.1.2/4.2.1/4.3.0 JWT Validator signature verification

Summaryinfo

A vulnerability categorized as critical has been discovered in Apache Kafka up to 4.1.2/4.2.1/4.3.0. This impacts an unknown function of the component JWT Validator. Such manipulation leads to signature verification. This vulnerability is referenced as CVE-2026-110985. It is possible to launch the attack remotely. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in Apache Kafka up to 4.1.2/4.2.1/4.3.0. It has been rated as critical. Affected by this issue is an unknown functionality of the component JWT Validator. The manipulation with an unknown input leads to a signature verification vulnerability. Using CWE to declare the problem leads to CWE-347. The product does not verify, or incorrectly verifies, the cryptographic signature for data. Impacted is confidentiality, integrity, and availability. CVE summarizes:

A 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).

The advisory is available at kafka.apache.org. This vulnerability is handled as CVE-2026-110985 since 10/15/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 10/28/2026).

Upgrading to version 4.2.2, 4.3.1 or 4.4.0 eliminates this vulnerability.

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Productinfo

Vendor

• Apache

Name

• Kafka

Version

• 4.0
• 4.1
• 4.1.0
• 4.1.1
• 4.1.2
• 4.2
• 4.2.0
• 4.2.1
• 4.3.0

License

• open-source

Website

• Vendor: https://www.apache.org/

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