Serverless Azure Functions are cloud-based functions that allow developers to create event-driven, single-purpose applications without having to manage infrastructure. They offer scalability, rapid deployment, and a cost-effective option, making them a popular choice for modern applications.

Unfortunately, the ephemeral nature of serverless functions introduces some security challenges, including an increased attack surface, dependency vulnerabilities, and complex permission and role management. In this dynamic environment, developers and DevOps teams need to be aware of and implement current security best practices to safeguard their serverless applications.

In this guide, you'll learn more about the importance of securing serverless Azure Functions. You'll also learn about a few practical strategies you can implement to shield your applications from potential security threats.

Why You Need to Secure Your Serverless Azure Functions

Unlike traditional development environments, where security protects a static set of resources using firewalls, intrusion prevention system (IPS) tools, and other server-based protection methods, the dynamic nature of serverless Azure Functions makes traditional security approaches less effective.

While cloud providers like Azure manage the infrastructure's security, developers must focus on securing code, configurations, and data to manage risk in a serverless ecosystem.

Following are a few reasons why securing your serverless Azure Functions is important:

Protect Sensitive Data

Because serverless architecture relies on a myriad of third-party services and APIs, sensitive data (ie user information and financial records) processed by these functions face an increased risk of exposure. Serverless functions (which could number hundreds for larger applications) often require fine-tuned permissions. This granularity opens the door to misconfiguration and overly permissive access controls—prime targets for malicious actors.

While compute workloads often run in a serverless environment, it's not uncommon for data to reside outside this environment, for example in an on-premise database or at another cloud provider, particularly during migration phases. In these scenarios, it’s important to ensure secure and private communication.

NetBird , an open source platform that combines a peer-to-peer VPN with centralized access control, ensures encrypted communication across on-premise and cloud environments. Using WireGuard , a modern, high-performance VPN protocol, along with zero-trust principles to create encrypted tunnels between devices, NetBird helps ensure only authenticated and authorized devices can establish connections. This approach protects data in transit from exposure and unauthorized access.

Prevent Unauthorized Access to Your Functions

Serverless functions frequently call third-party services and APIs, which may not be subject to Azure's stringent access controls. Poorly implemented authentication in these external services could serve as a gateway for attackers to infiltrate functions, allowing them to bypass security measures to impersonate authorized users, access sensitive information, or alter application logic.

Serverless functions processing events from varied sources, such as cloud storage or IoT devices, face risks from misconfigured permissions that could inadvertently leak sensitive data or open the door to unauthorized function calls.

Shield Your Functions from Common Attacks

Function events accessing data from untrusted sources can introduce injection vulnerabilities, such as SQL injection (SQLi) or cross-site scripting (XSS) .

SQLi can occur when serverless functions query databases using user-supplied input without adequate and proper input validation or parameterization. Attackers can exploit these vulnerabilities to manipulate databases and access, alter, or destroy sensitive information. Similarly, if a serverless function outputs the user input directly into web responses without proper escaping, attackers can embed malicious scripts into web pages to steal information or hijack user sessions.

Now that you know why you need to secure your serverless Azure Functions, let's take a look at a few best practices for securing them.

How to Secure Your Serverless Azure Functions

Securing serverless Azure Functions takes a multilayered approach that protects the functions themselves as well as the data and resources they interact with. Following are some of the ways you can safeguard your serverless Azure Functions:

Implement Authentication and Authorization

Only authorized users and services should be able to interact with your serverless Azure Functions. Authentication acts as the first line of defense against unauthorized access by confirming the identity of users or services. Authorization then determines the operations that authenticated users or services may perform, creating a security barrier around your functions.

Key strategies for authentication and authorization include the following:

• Use centralized identity management via Microsoft Entra ID (formerly Azure Active Directory [AD]). Microsoft Entra ID bolsters security with multifactor authentication (MFA) and conditional access policies, which are essential tools for verifying user identities and enforcing access controls based on predefined conditions. MFA enhances security by requiring users to provide multiple verification factors before accessing sensitive functions and data. In contrast, conditional access policies automate control, allowing or denying access based on session context, ensuring secure and context-sensitive resource access. Integrating Microsoft Entra ID user identity and application-level access controls with device-level network access management from a technology like NetBird can also create a more layered security architecture.
• Define who can do what within your Azure environment using Azure role-based access control (RBAC) . This feature allows access control to your Azure Functions at a granular level. Developers can assign precise permissions to users, groups, and applications to ensure adherence to the principle of least privilege .
• Avoid the storage of sensitive credentials in your code or configurations. Creating managed identities through Microsoft Entra ID allows you to manage credentials and access keys without hard-coding them into your configurations, reducing the risk of leaked credentials. Azure Key Vault provides a centralized, secure repository for storing sensitive information such as API keys and certificates. It also simplifies credential management with automated processes for rotation.

Perform Input Validation and Sanitization

To protect against malicious input, you should always validate and sanitize all incoming data from API calls, databases, file uploads, and other cloud services.

The following are a few key strategies for input validation and sanitation:

• Define strict input schemas that ensure your serverless functions accept only well-formed data matching specific criteria (eg type, format, length). Use Azure Functions bindings to apply these schemas directly within your function configuration and reject noncompliant requests automatically. Tools such as JSON Schema or XML Schema Definitions (XSD) are also invaluable for validating structured data, offering an additional layer of defense by ensuring data integrity from the start.
• Use regular expressions to enforce strict input formatting rules against complex patterns, such as email addresses, phone numbers, or custom identifiers. Integrating regex checks into function code provides an effective mechanism for detecting and rejecting malformed inputs early in the data processing pipeline.
• Configure Azure API Management to prevalidate requests before they reach your serverless functions. These preemptive checks add a security layer that ensures that functions process only properly authenticated and sanitized requests.

Enable Monitoring and Logging

During the reconnaissance phase of a cyberattack , where attackers search for vulnerabilities to exploit, the transient nature of serverless functions can obscure threats, making a monitoring and logging system for real-time anomaly detection and response crucial.

Key strategies for effective monitoring and logging include the following:

• Connect your serverless Azure Functions with Azure Monitor to track standard function metrics, including execution count, duration, and memory usage. Azure Monitor also provides features to define custom metrics. Developers can create alert rules based on these metrics to trigger alerts and notifications for performance issues, critical conditions, or anomalies.
• Integrate application insights to extend the capabilities of Azure Monitor, including collecting log, performance, and error data for deeper insights into application performance and usage. The Azure Live Metrics stream feature gives developers real-time visibility into the health of the serverless function. Application Insights also collects telemetry across various components of your application for end-to-end transaction tracing. This helps identify bottlenecks or faults within larger, interconnected applications.
• Use Azure Log Analytics to aggregate logs from various sources, including serverless Azure Functions, into a single repository for advanced queries and analysis. This centralized log repository offers a unified view of application behavior and performance, making it easier to spot patterns, anomalies, or potential security concerns across your serverless environment. Azure Log Analytics also ensures that logs are securely stored and managed, supporting compliance with security policies and regulations.

Validate Third-Party Library Dependencies

Third-party libraries and frameworks can help with faster development for serverless Azure Functions; however, these dependencies can introduce risks if attackers exploit their known vulnerabilities. Addressing this concern requires a proactive approach to validating and managing these dependencies.

The following are some key measures for safeguarding your dependencies:

• Integrate tools such as the Open Worldwide Application Security Project (OWASP) Dependency-Check or Snyk into your Azure DevOps pipelines to check third-party library dependencies for known vulnerabilities before deployment. NetBird's secure, encrypted connections ensure that automated tools in continuous integration, continuous delivery (CI/CD) pipelines can safely communicate with external services, such as OWASP Dependency-Check and Snyk, without introducing additional security risks.
• Keep your libraries up-to-date. Dependabot in GitHub Actions automates this process by scanning your dependencies for vulnerabilities and suggesting updates through pull requests. This minimizes the risk of using outdated libraries with known vulnerabilities.
• Utilize Microsoft Defender for Cloud , which offers continuous security assessments and recommendations for addressing vulnerabilities in your dependencies. It offers a centralized view of the security posture of Azure resources that aids in the early detection and remediation of potential security vulnerabilities. This proactive security management approach helps protect your serverless functions against the latest security threats, ensuring the integrity and reliability of your cloud applications.

Wrapping Up: Streamlining the Security of Serverless Azure Functions

A serverless environment demands a rigorous approach to security to mitigate risks such as unauthorized access, data breaches, and operational disruptions. You need to implement multilayered security measures, including authentication, authorization, schema validation, and monitoring, to keep your functions safe.

Integrating NetBird into your security strategy is a straightforward way to manage access and protect your serverless functions against unauthorized use. Its hassle-free security framework simplifies secure, encrypted connectivity between serverless applications like Azure Functions and other cloud and on-premise resources across multiple cloud and hybrid environments.

To learn more about how NetBird can enhance your serverless Azure Functions security, check out the following resources: