Apache Kafka: Unauthorized principal can read arbitrary broker files via IncrementalAlterConfigs

An authenticated Kafka principal with no authorization grants (not even ALTER_CONFIGS) can make a broker read a file or an environment variable from the broker host and return its contents. An IncrementalAlterConfigs (or AlterConfigs) request targeting a BROKER resource reaches the broker's configuration validation before any authorization check is applied to that path. Validation resolves configuration variables such as ${file:/path:key} or ${env:VAR} using a ConfigProvider named in the request, and this resolution bypasses the org.apache.kafka.automatic.config.providers allowlist that was added as the mitigation for CVE-2024-31141. An arbitrary ConfigProvider (for example FileConfigProvider or EnvVarConfigProvider) is therefore instantiated and the referenced file or environment variable is read, and a type-validation error returns the resolved content to the caller. Impact: On a cluster with an authorizer enabled, an authenticated principal with no authorization grants can read the contents of files accessible to the broker process, and the broker's environment variables, exposing credentials, keystores, or other secrets, defeating the CVE-2024-31141 allowlist. The read is bounded by the broker process's file permissions. The brokers and the cluster remain available; the issue is disclosure of file and environment- variable contents. Clusters with no authorizer configured are not affected in practice, because every principal already holds ALTER_CONFIGS there. Mitigation: Upgrade to Apache Kafka 4.2.2, 4.3.2, or 4.4.0. As defense in depth, run brokers under a least-privileged operating-system account so that sensitive files are not readable by the broker process. Resolution: From Apache Kafka 4.2.2, 4.3.2, and 4.4.0: broker configuration changes are authorized (ALTER_CONFIGS on the cluster) before the configuration is validated, and configuration- variable resolution enforces the config-providers allowlist.