How Tanner Security Performs gRPC Penetration Tests

gRPC Penetration Testing Requires More Than a Vulnerability Scanner

gRPC has become an important technology in today’s applications, especially for microservices and distributed systems. Unlike traditional web apps, gRPC often involves several services communicating with each other using Protocol Buffers over HTTP/2. These systems depend on authentication, authorization, service meshes, API gateways, and other tools to control how services interact. According to the official documentation, gRPC is a high-performance RPC system that supports multiple languages, uses HTTP/2, offers streaming, and provides flexible authentication.

That architecture creates a different penetration-testing challenge.

A standard vulnerability scanner might spot some technical problems, but it usually can’t tell if a user can change business processes, view another customer’s data, use admin features, or move between compromised services.

This is where Tanner Security takes a different approach to gRPC penetration testing.

What makes our penetration tests different, is that we start by understanding how the application works. Then, we map the gRPC attack surface, review the services and Protocol Buffer definitions, test authentication and authorization, and examine the business logic. We use both automated and manual methods to find out what an attacker could actually do.

Tanner has performed gRPC penetration tests for multiple clients and has encountered environments where typical penetration-testing tools did not provide adequate support. In one engagement, Tanner developed a custom Burp extension specifically to facilitate testing of gRPC and Protobuf traffic.

Our goal isn’t just to list possible vulnerabilities. We want to see if weaknesses in the gRPC setup can be exploited and what impact they might have on your business.

What Makes gRPC Penetration Testing Different?

gRPC uses Protocol Buffers to define services and messages and HTTP/2 for transport. It also supports unary calls, client and server streaming, and bidirectional streaming.

Because of this, testers can’t always use the same methods they would for a typical browser-based application.

A conventional web application might expose URLs such as:

/login

/customers/123

/orders/456

A gRPC application instead exposes services and methods.

For example, an application might have methods such as:

GetCustomer

CreateOrder

ApproveOrder

TransferFunds

The security question isn’t simply whether those methods exist.

The important questions are:

“Who can call them?”

“What information can they access?”

“What parameters can be manipulated?”

“Can a user call a method they shouldn’t access?”

“Can legitimate methods be combined in an unintended way?”

“Can one compromised service reach another?”

Answering these questions takes more than automated scanning. It requires understanding the application’s architecture and how it’s supposed to work.

Tanner’s gRPC Penetration Testing Process

Each engagement is unique, so Tanner doesn’t follow a strict checklist or assume every gRPC application has the same risks.

We begin by learning about the environment, then move step by step into more detailed technical and business logic testing.

1. We Begin by Understanding the Application

Before testing individual methods, we want to understand what we’re testing.

We review the application’s architecture, how gRPC fits into the environment, how clients communicate with services, what authentication mechanisms exist, where authorization decisions occur, and what other systems the gRPC services interact with.

This can include reviewing architecture diagrams, application documentation, Protocol Buffer definitions, test accounts, API documentation, and information about important business workflows.

This early work matters because testers can only check business logic properly if they understand how it works.

For example, suppose a financial application has four gRPC methods:

CreateTransaction

SubmitTransaction

ApproveTransaction

ProcessTransaction

A scanner can discover those methods.

A penetration tester needs to understand the intended sequence.

The security question is whether a normal user can call ProcessTransaction without going through the required approval process.

That is not simply a technical configuration problem.

It is an application logic problem.

And finding those problems starts with understanding how the application is supposed to work.

2. We Set the Testing Scope and Rules of Engagement

Before active testing begins, Tanner establishes what can and cannot be tested.

We identify the systems and services in scope, testing windows, accounts, production versus non-production environments, prohibited activities, sensitive operations, and other rules designed to prevent unnecessary disruption.

Whenever possible, using a test environment that closely matches production gives a safer space for deeper testing.

However, testing production may make sense in certain circumstances.

The key is to match the testing approach to the client’s environment and risk level, rather than treating every gRPC application the same.

3. We Map the gRPC Attack Surface

Next, we determine what gRPC services and methods are actually exposed.

Depending on the environment, Tanner may use available service definitions, Protocol Buffer files, application documentation, gRPC clients, discovery techniques, and specialized tooling to understand the attack surface.

If gRPC reflection is enabled, it may provide additional information about available services and message definitions. Reflection can help with development and troubleshooting, but OWASP recommends disabling it in production unless you have a specific reason to expose it.

We don’t consider reflection a vulnerability by default.

Instead, we ask a more useful question:

“What information does it expose, who can access it, and does that information create additional risk?”

The same principle applies throughout the assessment.

4. We Review the Protocol Buffer Definitions

Protocol Buffers connect gRPC clients and services.

For a penetration tester, those definitions are valuable insight into how the application works.

We examine the available services, methods, request messages, response messages, data types, and relationships across the application.

This helps us understand what the application expects to receive and what it returns.

But having strongly typed Protocol Buffer messages alone doesn’t guarantee the application is secure.

OWASP notes that Protocol Buffers provide type safety but do not replace server-side validation or application-level security controls.

That distinction is important.

The question isn’t simply:

“Does this request conform to the Protobuf definition?”

It is:

“Should the application allow this request to perform this action for this user under these circumstances?”

5. We Test Authentication

Authentication establishes who is making the request.

Tanner reviews how the application authenticates clients and users and whether authentication controls apply consistently across the gRPC services and methods that require protection.

Depending on the application, this may involve evaluating tokens, certificates, API keys, custom authentication mechanisms, session behavior, and other credentials.

gRPC supports TLS, client certificates, token-based authentication, and custom credential mechanisms.

But an authentication mechanism alone isn’t enough.

We want to determine whether an attacker can bypass it, reuse credentials improperly, access protected methods without appropriate credentials, or exploit inconsistencies between services.

Authentication is just one part of the problem.

After authentication, we need to find out what that user can actually do.

6. We Test Authorization and User Roles

Authorization is often one of the most important parts of a gRPC penetration test.

A user may authenticate successfully but still have access to functionality they shouldn’t have.

Consider an application with three roles:

Customer

Employee

Administrator

The customer may legitimately be allowed to retrieve information about their own account.

That does not mean the customer should be able to retrieve another customer’s account, modify administrative settings, or invoke an internal administrative method.

Tanner tests those boundaries.

We may attempt to access another user’s data, use methods associated with another role, manipulate object identifiers, change parameters, or move between authorization levels.

OWASP’s gRPC guidance recommends granular, method-level authorization, while its API security guidance emphasizes risks such as broken object-level authorization and broken function-level authorization.

For a penetration tester, the important question becomes:

Can the authorization control actually stop someone from doing something they shouldn’t be able to do?

7. We Test Business Logic

Tanner puts significant effort into this part of the process.

Automated tools are good at finding certain classes of vulnerabilities.

They are not as good at figuring out what a business process is meant to do.

Consider an application that allows a customer to request a refund.

The application might correctly require authentication.

It might correctly verify that the customer owns the transaction.

It might correctly validate the refund amount.

But what happens if the customer submits the same refund twice?

What happens if the user changes the transaction identifier?

What happens if the user calls an approval method before the required review?

What happens if two legitimate methods can be combined in a way the developers never intended?

Those are the types of questions Tanner explores during manual testing.

We do more than check if each function works. We also look for ways functions can be combined or changed to create results the developers didn’t expect.

This is especially important for custom applications with complex workflows.

8. We Test Input Handling and Request Manipulation

Tanner also evaluates how the application handles unexpected or manipulated requests.

This can include modifying values, testing boundary conditions, sending unexpected combinations of fields, testing validation controls, and examining how the application responds to malformed or unusual requests.

Protocol Buffers provide structure, but the server still needs to validate data appropriately.

OWASP recommends validating Protocol Buffer messages and applying appropriate controls to protect against injection and other input-related attacks.

Our goal isn’t just to send random data to every method.

We want to understand how each service uses the data and whether manipulated input can meaningfully change application behavior.

9. We Evaluate TLS, mTLS, and Service-to-Service Security

Many gRPC environments rely heavily on service-to-service communication.

A modern application might look something like:

User → API Gateway → Service A → Service B → Service C → Database

Each connection represents another trust relationship.

Tanner evaluates the security controls around those relationships as appropriate to the scope.

That can include examining TLS configuration, certificate validation, client authentication, mTLS, service identities, and whether internal services trust one another too broadly.

gRPC supports TLS and mutual authentication through client certificates, while OWASP recommends TLS for production deployments and mTLS where appropriate for service-to-service communication.

But even strong transport security doesn’t eliminate the risk of authorization problems.

Two services can communicate over encrypted, mutually authenticated connections and still grant each other more access than necessary.

That is why Tanner checks identity and authorization controls in addition to encryption.

10. We Test Streaming and Resource Controls

gRPC’s streaming capabilities can create security considerations that don’t exist in exactly the same way in traditional request/response applications.

Tanner may evaluate how the application handles message sizes, connection duration, concurrent requests, streaming behavior, rate limits, and resource consumption when those risks fall within the engagement scope.

OWASP specifically recommends message-size limits, streaming controls, rate limiting, and appropriate timeouts to reduce resource-exhaustion risks.

Again, our goal isn’t just to check if a limit exists.

We want to find out whether an attacker could misuse normal features to consume resources or disrupt the application’s availability.

11. We Look for Attack Paths, Not Individual Findings

A key part of penetration testing is seeing how different weaknesses can combine to create bigger risks.

Imagine Tanner identifies three findings:

A standard user can access a method they shouldn’t.

That method exposes an internal identifier.

A second service trusts requests from the first service too broadly.

Each finding may be severe on its own.

Together, they could create a much more serious attack path.

That’s why Tanner looks at the bigger picture, not just individual vulnerabilities.

We ask what an attacker could do after finding the first weakness.

“Could the attacker access another service?”

“Could they obtain additional credentials?”

“Could they access another user’s information?”

“Could they escalate privileges?”

“Could they reach sensitive business functionality?”

“Could they move from the application into another part of the environment?”

This is where manual penetration testing adds value that automated scans cannot provide.

12. We Use Automation Where It Helps and Build Custom Tools When Needed

Automation is an important part of Tanner’s penetration-testing process.

It helps testers perform repetitive tasks, increase coverage, process large amounts of information, and efficiently validate certain classes of vulnerabilities.

However, Tanner does not consider automated scanning to be a full penetration test.

gRPC presents an additional challenge because specialized support within conventional application-security tools can vary.

Tanner encountered that issue during an earlier gRPC/Protobuf engagement and developed a custom Burp extension to support application testing. Tanner has also described developing custom solutions when existing penetration-testing tools don’t adequately support a client’s technology.

This experience matters because a testing provider shouldn’t stop just because their usual tools don’t support the technology.

If the technology needs a different approach, the testing process should adapt.

13. We Validate Findings Manually

Automated tools can generate false positives.

They might also find technical issues without showing if those issues actually create real risks.

Tanner validates findings manually whenever appropriate.

For example, a scanner might identify that an endpoint appears accessible without authorization.

The next question is:

“What can an attacker actually access?”

If the endpoint returns only public information, the business impact may be limited.

If the endpoint provides another customer’s financial information, the risk is substantially different.

Manual validation allows the report to distinguish between those situations.

14. We Report What the Findings Mean to the Business

The final deliverable should not simply be a list of technical problems.

A useful penetration-testing report needs to help the technical team understand how to fix the issue while helping management understand why the issue matters.

Tanner’s reporting approach focuses on the vulnerability, evidence, affected functionality, potential attack path, business impact, and recommended remediation.

For example, rather than simply reporting:

“Authorization bypass found.”

The report should explain what the tester accessed, how the control was bypassed, which users or systems were affected, and what should change.

The goal is to give your team clear next steps.

What Tanner Looks For During a gRPC Penetration Test

Although every engagement is different, Tanner’s testing can have many interconnected areas.

Authentication determines whether the application properly establishes identity.

Authorization determines whether that identity has the right to perform a particular action.

Input validation determines whether the service safely handles the information it receives.

Transport security protects communications between clients and services.

Business-logic testing determines whether legitimate functionality can be manipulated to produce an unintended result.

Service-to-service testing examines whether trust relationships create additional attack paths.

And attack-path analysis brings those pieces together to determine what an attacker could actually accomplish.

This last point is particularly important.

A penetration test should do more than list technical problems. It should help you understand what those weaknesses mean for your business.

When Should You Consider a gRPC Penetration Test?

gRPC penetration testing can be valuable when a business launches a new gRPC application, significantly changes an existing application, exposes services to customers or partners, introduces new authentication or authorization mechanisms, adds important integrations, or moves toward a more distributed microservices architecture.

It also makes sense when a business needs independent validation before releasing an application or when customers, contracts, regulators, or compliance requirements call for security testing.

When to test should depend on the level of risk and how often the application changes.

A system that changes frequently may need testing more often than a relatively stable application.

What Tanner Needs From Your Team Before Testing

A good gRPC penetration test starts with good information.

Tanner may request architecture information, application documentation, Protocol Buffer definitions, test credentials, user roles, important business workflows, API documentation, and information about the testing environment.

That doesn’t mean Tanner expects your development team to explain every line of code.

The goal is to understand the application well enough to test it effectively.

In particular, test accounts and business workflows can be extremely valuable.

If the application has customer, employee, manager, and administrator roles, Tanner needs to understand what each role should be able to do.

If the application handles financial transactions, approvals, healthcare data, or other sensitive tasks, knowing these workflows helps us focus business-logic testing where it matters most.

Why Tanner’s Approach Matters

A gRPC penetration test is more than just:

• Connect a scanner.
• Run a tool.
• Export a report.

gRPC applications can contain relationships between users, services, APIs, authentication systems, cloud infrastructure, databases, and business processes.

A thorough penetration test must account for these relationships.

Tanner combines automated testing, manual analysis, application understanding, controlled exploitation, and attack-path analysis to evaluate how the application behaves from an attacker’s perspective.

And when conventional tools don’t adequately support the technology, Tanner has demonstrated a willingness to develop the capabilities needed to test it. In a prior gRPC/Protobuf engagement, that meant developing a custom Burp extension rather than accepting limited tool coverage.

This is what sets thorough application testing apart from simply running a scan.

Is Your gRPC Application Ready for a Penetration Test?

If your business relies on gRPC for customer applications, internal services, APIs, financial transactions, sensitive data, or critical business processes, it is worth understanding what an attacker could actually do if one of those services contains a security weakness.

Tanner Security can review your application architecture, discuss your gRPC environment, and help determine the appropriate penetration-testing scope.

You don’t need to wait for a security incident to learn whether your gRPC services are at risk.

Contact Tanner Security to discuss a gRPC penetration test and discover what an attacker could really do.

How Tanner Security Performs gRPC Penetration Testing FAQ’s 

Does Tanner actually perform gRPC penetration tests?

Yes. Tanner Security has performed gRPC penetration tests for multiple clients and has developed testing approaches for gRPC and Protocol Buffer environments where tools did not provide sufficient coverage.

How is Tanner’s gRPC penetration testing different from a vulnerability scan?

A vulnerability scan primarily looks for technical conditions that may indicate security weaknesses. Tanner’s penetration testing combines automated testing with manual analysis and controlled exploitation to determine whether those weaknesses can actually be used and what an attacker could accomplish.

Does Tanner test gRPC authentication?

Yes. Depending on scope, Tanner can evaluate authentication mechanisms, credentials, tokens, certificates, protected methods, and attempts to bypass authentication controls.

Does Tanner test gRPC authorization?

Yes. Authorization testing is an important part of the assessment. Tanner can test whether users can access methods, data, or functionality outside their intended permissions.

Does Tanner test business logic?

Yes. Business-logic testing is one of the most important parts of a meaningful gRPC penetration test because automated tools may not understand how your application’s workflows are supposed to operate.

Does Tanner need our .proto files?

Providing Protocol Buffer definitions can make testing more efficient because they describe services, methods, and message structures. Tanner can discuss what information is appropriate for the specific engagement.

Can Tanner test gRPC applications hosted in AWS or Azure?

Yes. gRPC applications can be tested in cloud-hosted environments. The appropriate scope depends on how the application, cloud infrastructure, gateways, authentication systems, and other services interact.

Can Tanner test production gRPC services?

Potentially, depending on the application and rules of engagement. Tanner works with clients to establish appropriate testing boundaries and safeguards designed to reduce unnecessary disruption.

Does Tanner use automated tools for gRPC penetration testing?

Yes. Automation can improve coverage, but Tanner does not rely exclusively on automated scanning. Tanner combines automated tools with manual testing and controlled exploitation.

What happens if standard penetration-testing tools don’t support our gRPC implementation?

Tanner adapts the testing approach to the technology. Tanner has previously developed custom tooling for a gRPC/Protobuf engagement when existing tools did not provide adequate support.

What will Tanner provide after the test?

Tanner provides a report documenting identified vulnerabilities, supporting evidence, affected functionality, risk, potential attack paths, business impact, and remediation recommendations.

Can Tanner retest the application after remediation?

Yes. Retesting can verify whether identified vulnerabilities have been addressed and whether the implemented fixes resolve the security issue.

How often should a company conduct a gRPC penetration test?

The appropriate frequency depends on risk and application changes. Significant architecture changes, new functionality, authentication changes, major integrations, or security incidents can all justify additional testing.

Does a gRPC penetration test satisfy compliance requirements?

A penetration test may support certain compliance or contractual requirements, but it does not establish compliance. Tanner can help determine what testing is appropriate for your specific compliance framework and business requirements.

Test Your gRPC Application Before an Attacker Does

Your gRPC services may have strong authentication, TLS, Protobuf definitions, and carefully designed APIs.

But that does not guarantee an attacker cannot change workflows, bypass authorization, access other users’ data, or move between services.

That’s exactly what penetration testing is meant to uncover.

Tanner Security combines technical testing, manual analysis, business-logic testing, controlled exploitation, and attack-path analysis to determine what an attacker could actually accomplish.

If your company relies on gRPC, contact Tanner Security to discuss a gRPC penetration test.