---
id: "CVE-2026-110985"
kind: "vulnerability"
aliases: ["CVE-2026-110985"]
title: "Kafka: SASL/OAUTHBEARER broker accepts unsigned JWTs without a JWKS URL"
severity: "critical"
cvss: null
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["kafka", "managed-kafka-check-config", "package", "sasl-oauthbearer", "self-hosted"]
not_affected_if: ["custom-verification-key-resolver", "jwks-endpoint-set", "non-oauthbearer-auth"]
published_at: "2026-10-28T17:17:38+00:00"
first_seen_at: "2026-10-28T17:34:08+00:00"
updated_at: "2026-10-28T17:35:16+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-110985", "first_seen_at": "2026-10-28T17:34:08+00:00"}]
---

# Kafka: SASL/OAUTHBEARER broker accepts unsigned JWTs without a JWKS URL

A Kafka broker using SASL/OAUTHBEARER with the default validator and no JWKS endpoint URL silently falls back to a validator that checks no JWT signature, algorithm or issuer. An unauthenticated remote attacker who can reach the listener can forge a token for any principal, including configured superusers, and gain full cluster access.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the broker config for `sasl.oauthbearer.jwt.validator.class` and `sasl.oauthbearer.jwks.endpoint.url`; if the URL is unset and no signature-verifying validator or verification key resolver is configured, the broker is exposed
- Check the Kafka version with `kafka-topics.sh --version` or the broker's `kafka_*.jar` version; versions before `4.2.2`, `4.3.1` and `4.4.0` are in scope per the sources

## Fix

- Set `sasl.oauthbearer.jwks.endpoint.url` for every listener and mechanism that uses OAuthBearerValidatorCallbackHandler
- Or configure a signature-verifying `sasl.oauthbearer.jwt.validator.class`
- Upgrade Apache Kafka to `4.2.2`, `4.3.1` or `4.4.0`, where a broker that would fall back fails to start instead
- Unknown whether a patch exists for older release lines beyond the mitigation above

## Timeline

- 2026-10-28T17:34:08+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-110985

## From the sources

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).

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_