Remote Procedure Call (RPC)

A Remote Procedure Call (RPC) is a protocol that allows a computer program to execute code or a subroutine on a remote server as if it were running locally on the client machine. In a distributed network architecture, RPC abstracts the complexities of network communications, enabling seamless interaction between separate systems.

From a cybersecurity perspective, RPC is a critical focus area. Because it inherently allows one machine to request code execution on another machine, it is a high-value target for threat actors. If an RPC interface is misconfigured or contains software vulnerabilities, an attacker can exploit it to achieve unauthorized access, escalate privileges, or execute arbitrary commands across a network.

How Remote Procedure Call Works

The RPC process involves several components working together to pass a local request across a network and return the result.

• The Client Application: The local software program that triggers the procedure call, treating it exactly like a local function.

• The Client Stub: A local routine that packages (marshals) the procedure's parameters into a standardized network message format.

• The Communication Module: The underlying transport layer protocol (such as TCP or UDP) that sends the marshaled message across the network to the remote system.

• The Server Stub: The receiving routine on the remote server that unpacks (unmarshals) the network message to extract the parameters.

• The Server Application: The actual software execution environment on the remote server that processes the requested function and passes the results back through the stubs.

Security Risks and Vulnerabilities Associated with RPC

Because RPC relies on trusting incoming network requests to execute local actions, it introduces significant security exposures if not properly monitored and locked down.

• Remote Code Execution (RCE): This is the most severe risk. If the server stub fails to properly sanitize incoming variables before processing, an attacker can send a crafted payload that forces the server to execute malicious commands. Historically, vulnerabilities in Microsoft RPC (MSRPC) have been used to spread widespread network worms.

• Lateral Movement: Once inside a corporate network, threat actors frequently use legitimate RPC administrative mechanisms—such as the Windows Remote Procedure Call service—to connect to other internal servers, deploy malware, and expand their control without triggering standard perimeter alarms.

• Authentication and Authorization Flaws: By default, older or legacy RPC implementations may not strictly enforce mutual authentication. Attackers can spoof requests, impersonate authorized clients, or access sensitive administrative interfaces without valid credentials.

• Replay Attacks: If the transport layer does not use cryptographic sequence numbers or timestamps, an eavesdropper can capture a valid RPC transmission and re-transmit it later to replicate the requested action, such as creating a new administrative user.

• Reconnaissance and Enumeration: Attackers often use specialized scanning tools to query open RPC ports. By enumerating endpoints, they can discover exactly what services are running on a server, identify the operating system version, and look for known unpatched flaws.

Best Practices for Securing RPC Interfaces

Securing RPC requires a combination of network isolation, strong identity controls, and continuous patching.

• Enforce Strong Authentication: Always require robust authentication protocols, such as Kerberos, for all RPC traffic. Ensure that the platform requires encryption for the entire communication session, not just the initial handshake.

• Restrict Network Access: Limit exposure by blocking RPC ports at the perimeter firewall. Standard RPC services (such as port 135 in Windows environments) should never be accessible from the public internet and should be restricted internally using microsegmentation.

• Implement Least Privilege: Ensure that the service accounts running RPC services operate with the minimum necessary privileges on the operating system. If an interface is compromised, a low-privilege account limits the attacker's ability to take over the host.

• Maintain Continuous Patching: Because RPC is a frequent target of critical vulnerabilities, prioritizing patch management for operating systems and distributed applications is vital to closing newly discovered flaws before they are exploited.

• Monitor and Log RPC Traffic: Track network logs for unusual volumes of RPC traffic or unexpected connection requests between endpoints that do not normally communicate, which often indicates lateral movement.

Frequently Asked Questions (FAQs)

What port does Remote Procedure Call use?

The ports used by RPC vary by operating system and configuration. In Microsoft Windows environments, RPC uses port 135 as the endpoint mapper to dynamically assign communication channels, frequently utilizing ports 445, 139, and a dynamic range of high ports (49152-65535) for specific services. In UNIX and Linux environments, RPC services typically rely on port 111 (rpcbind).

Why do hackers target RPC interfaces?

Hackers target RPC interfaces because a successful exploit often grants direct execution privileges on the target server. Because RPC services are designed to manage core administrative and operational functions, compromising an RPC interface allows threat actors to bypass application layers and interact directly with the operating system.

Is RPC inherently insecure?

No, RPC is not inherently insecure, but it requires careful configuration. The risk stems from poor implementations, such as allowing unauthenticated requests, failing to encrypt network traffic, leaving administrative interfaces exposed to untrusted networks, or neglecting to patch known software vulnerabilities.

Securing Remote Procedure Call (RPC) Interfaces with ThreatNG

A Remote Procedure Call (RPC) interface is a powerful network mechanism that enables seamless code execution between systems. However, when an RPC port is inadvertently exposed to the public internet, it creates a critical pathway for unauthorized access, lateral movement, and remote code execution (RCE). Protecting an organization from RPC-based attacks requires absolute visibility into the external network perimeter and the ability to detect configuration errors before threat actors discover them.

ThreatNG serves as a connectorless, agentless Integrated External Risk Management Platform designed to eliminate these blind spots. By providing an unauthenticated, outside-in attacker's perspective without intrusive penetration testing, ThreatNG continuously transforms unstructured internet data into prioritized intelligence, enabling security teams to effectively secure exposed RPC interfaces.

Agentless External Discovery to Map Exposed RPC Interfaces

Before an organization can secure its RPC services, it must know exactly which systems are connected to the internet. Threat actors aggressively scan the global IPv4 and IPv6 space looking for open RPC ports, particularly port 135 (Microsoft RPC) and port 111 (UNIX rpcbind), often targeting forgotten shadow IT infrastructure.

ThreatNG eliminates this visibility gap through continuous, agentless external discovery. Operating entirely from the outside-in without requiring internal software installations or access credentials, the platform actively crawls public registries, global routing tables, and cryptographic logs. This discovery engine recursively maps all registered domains, active subdomains, and public IP blocks connected to the enterprise brand. By establishing a complete, real-time inventory of the external attack surface, ThreatNG ensures that unmanaged cloud instances or rogue staging servers that might be broadcasting RPC services are immediately brought under security oversight.

Deep External Assessment for RPC Vulnerability Validation

Once the digital footprint is mapped, ThreatNG performs non-intrusive external technical assessments to evaluate active configuration errors and software exposures, translating these risks into clear Security Ratings.

• Detailed Assessment Example: Detecting Exposed Microsoft Endpoint Mappers

  During an external assessment of a newly provisioned cloud environment, ThreatNG interrogates the external network boundary from the outside-in. The assessment engine detects that TCP port 135 is publicly accessible and actively returning MSRPC endpoint mapper responses. ThreatNG flags this critical configuration error, providing the exact host IP address, the open port number, and the specific service banner being broadcast. This technical intelligence allows network engineers to update cloud security groups and block external access to the port before an attacker can enumerate the server's internal services.

• Detailed Assessment Example: Identifying Vulnerable UNIX rpcbind Services

  ThreatNG directly assesses exposed UNIX and Linux infrastructure. If the assessment engine discovers that port 111 is exposed on a public-facing server, it analyzes the service response. By correlating the banner data with known vulnerability databases, ThreatNG identifies that the server is running an outdated, vulnerable version of rpcbind susceptible to a known denial-of-service or remote code execution flaw. The platform delivers the precise version string and IP data, enabling administrators to patch the system and restrict the port to internal traffic only.

Deep-Dive Investigation Modules for RPC Threat Hunting

Threat actors often bypass perimeter scans by finding architectural blueprints or credentials that grant them direct access to internal RPC services. ThreatNG deploys highly specialized investigation modules to hunt for these off-perimeter threat indicators across the open, deep, and dark web.

• Detailed Investigation Example: Sensitive Code Exposure Module

  Software engineers use infrastructure-as-code (IaC) to deploy network environments, but misconfigurations can be accidentally published. ThreatNG's Sensitive Code Exposure module continuously scans public development repositories such as GitHub and GitLab for corporate markers. In a live scenario, the module discovers a public repository containing a Terraform script that explicitly opens RPC ports to the global internet (0.0.0.0/0) for a corporate cloud environment. ThreatNG captures the exact repository URL, author details, and the flawed code block in real time, allowing the security operations center to correct the deployment script before the vulnerable infrastructure goes live.

• Detailed Investigation Example: Dark Web and Infostealer Intelligence Module

  If an attacker compromises a virtual private network (VPN) gateway, they can access internal RPC services just like a legitimate employee. Driven by the DarCache Infostealer Intelligence Repository, ThreatNG’s Dark Web Presence module continuously scans underground marketplaces and ransomware leak forums. If an attacker uploads an info-stealer log containing valid corporate credentials for a remote access portal, ThreatNG intercepts the breach. The module uses its Context Engine™ to deliver precise attribution, allowing the organization to secure the account, force a password reset, and prevent the attacker from establishing the internal presence needed to exploit internal RPC flaws.

Continuous Monitoring for RPC Configuration Drift

Because enterprise networks are highly dynamic, a firewall rule that properly blocks RPC traffic today might be accidentally modified tomorrow during routine maintenance, instantly exposing a critical server.

ThreatNG addresses this by providing continuous monitoring across the entire external digital footprint. The moment an automated pipeline exposes a new cloud instance with port 135 open, or a network engineer inadvertently drops an access control list protecting port 111, ThreatNG flags the change immediately. This real-time visibility ensures that organizations maintain a secure perimeter and immediately detect configuration drift that could lead to an RPC-based intrusion.

Intelligence Repositories for RPC Attack Path Context

ThreatNG aggregates all discovered external assets, active network exposures, and dark web threat indicators within DarCache, its centralized operational intelligence data store. DarCache organizes data into distinct sub-repositories to give defenders an integrated view of their attack surface.

To transform isolated port exposures into actionable defense strategies, ThreatNG uses the DarChain engine to perform contextual hyper-analysis of digital attack risk. DarChain models the exact path an external threat actor would take. For example, DarChain can illustrate how an adversary could discover an exposed MSRPC port (found via external assessment), utilize leaked credentials (found via the infostealer module) to authenticate, and execute commands to pivot deeper into the network. This predictive analysis helps organizations evaluate their overall risk through an External Open FAIR Assessment and prioritize firewall adjustments based on structural impact.

Standardized Reporting for Strategic Remediation

To ensure that technical findings lead to definitive action, ThreatNG structures its continuous data into the eXposure paradigm, automatically generating specialized Executive, Technical, and Prioritized reports. Executive Reports convert complex port exposures into clear Security Ratings, allowing leadership to track perimeter risk trends over time. Concurrently, Technical and Prioritized Reports deliver actionable data directly to network engineering queues. These documents feature an embedded Knowledgebase complete with precise technical definitions, risk reasoning, and clear remediation instructions—such as specific iptables rules or Windows Firewall configurations—ensuring that infrastructure teams can rapidly block exposed RPC interfaces.

Accelerating Defenses Through Cooperation with Complementary Solutions

ThreatNG serves as an automated external discovery and intelligence engine, focusing on seamless integration with complementary internal security solutions to accelerate remediation of exposed RPC interfaces and automate response actions.

• Cooperation with Security Orchestration, Automation, and Response (SOAR) Complementary Solutions: Upon identifying a high-risk external exposure—such as an unauthenticated RPC port facing the public internet—ThreatNG streams a zero-latency alert to enterprise SOAR complementary solutions. The SOAR platform cooperates by automatically executing a predefined response playbook, interfacing with the perimeter firewall to instantly drop all external traffic attempting to reach the exposed port while the engineering team conducts a formal review.

• Cooperation with Vulnerability Management Complementary Solutions: Internal vulnerability scanners focus on auditing servers within the network, but may not identify those accidentally exposed to the internet. ThreatNG cooperates with these systems by continuously feeding its outside-in discovery baseline directly into the central vulnerability management platform. This cooperation ensures that internal scanning tools prioritize patching for servers that ThreatNG has identified as publicly broadcasting RPC services.

• Cooperation with Identity and Access Management (IAM) Complementary Solutions: If ThreatNG’s dark web investigation modules detect compromised administrative credentials that could be used to authenticate against RPC services, it routes this technical intelligence directly to internal IAM complementary solutions. The IAM system cooperates by instantly enforcing conditional access rules, invalidating active administrator sessions, and requiring immediate credential rotation, neutralizing the threat actor's ability to utilize the stolen access.

Frequently Asked Questions (FAQs)

How does ThreatNG find exposed RPC ports without using local software agents?

ThreatNG utilizes an agentless architecture, meaning it maps the organization's footprint entirely from the outside-in. By interrogating public DNS records and IP blocks, and performing non-intrusive banner grabbing, ThreatNG can identify which IP addresses belonging to the organization are actively listening on common RPC ports (such as 135 or 111) without requiring any internal access or software deployments.

Why is continuous monitoring critical for securing Remote Procedure Call services?

Network perimeters are not static. While a security audit might verify that all RPC ports are closed today, a single misconfigured cloud deployment or temporary firewall bypass implemented by an engineer can expose those ports tomorrow. Continuous monitoring ensures that the moment an RPC port becomes publicly accessible, the security team is alerted, closing the window of opportunity for attackers.

Can ThreatNG safely assess RPC vulnerabilities without crashing legacy systems?

Yes. ThreatNG relies strictly on non-intrusive external assessment techniques. It analyzes standard network responses, protocol handshakes, and service banners to identify configuration errors and outdated software versions. It does not deploy exploit payloads, execute denial-of-service checks, or perform active penetration testing, ensuring that delicate or legacy systems remain stable during the evaluation.