SAST vs DAST: Which Should Come First in Application Security?

When developing applications, security testing is as crucial for the future of a project as its code quality. Typically, two main methods are focused on: Static Application Security Testing (SAST) and Dynamic Application Security Testing (DAST). Both come into play at different points and detect different types of vulnerabilities.

In my nearly 20 years of experience, understanding these two methods correctly and using them at the right time not only speeds up development but also minimizes potential security vulnerabilities in the production environment. Sometimes, one finds what the other misses, which is why they complement each other.

What is SAST and When is it Used?

SAST is a "white-box" testing method that analyzes your code without running it. It comes into play in the early stages of the development process, meaning before the code is even compiled or turned into an application. I usually run it automatically as part of my CI/CD pipeline.

SAST tools scan your source code, bytecode, or binaries for known security vulnerability patterns. For example, when I was working on a production ERP project, SAST tools helped me catch queries that could be vulnerable to SQL injection or input sanitization deficiencies that could lead to XSS (Cross-Site Scripting) vulnerabilities early on. This prevented major security issues from arising later.

💡 The Importance of Early Detection

Detecting a vulnerability during the coding phase can be 100 times cheaper than detecting it in production. SAST is the cornerstone of the "shift-left" security approach. In my experience, a SAST scan that takes about 30 seconds in the pipeline eliminated a monthly security audit workload of 20-30 hours.

SAST tools are very effective at detecting many OWASP Top 10 vulnerabilities (like Injection, Broken Authentication, Sensitive Data Exposure) at the code level. For instance, they can easily catch situations like not using PreparedStatement instead of Statement in a Java application, or using dangerous functions like eval() in a Python application. While using FastAPI for my own side project's backend, I detected errors stemming from incorrectly handled request.json() parsing in some endpoints using SAST.

# An example that would be flagged as a potential vulnerability by a SAST scan
# In this code, if user_input is directly appended to the SQL query, there's a risk of SQL Injection.
def get_user_data(user_id):
    query = f"SELECT * FROM users WHERE id = {user_id}" # SAST will warn here
    # ... database operation

What is DAST and When is it Used?

DAST is a "black-box" testing method that tests a running application from the outside, through the eyes of an attacker. The application must be running live or in a very similar staging environment during testing. DAST tools send HTTP requests to the application, fill out forms, click on links, and analyze the responses.

DAST is particularly effective at finding vulnerabilities that arise from the application's runtime behavior. For example, situations like server configuration errors, logic flaws in authentication flows, authorization issues, or session management vulnerabilities are better detected with DAST. In our work on a bank's internal platform, DAST scans helped us identify that different user roles could access unauthorized data or that a faulty API endpoint was accessible anonymously.

DAST scans typically come into play towards the end of the development cycle, during QA or UAT (User Acceptance Testing) phases. They are even used as a final security check before production. In my Android spam app's APIs, a DAST scan revealed a deficiency in rate limiting settings. This deficiency could have left the application vulnerable to brute-force attacks. [related: Rate Limiting Experiences in API Security]

# Example output from a DAST scan (simplified)
# A DAST tool might send a request like this and analyze the response:
# Request: GET /api/v1/admin/users/123 HTTP/1.1
# Header: Authorization: Bearer <low-privilege token>
# Expected Response: 403 Forbidden
# Actual Response: 200 OK (admin data returned)
# Vulnerability: Broken Access Control

Key Differences Between SAST and DAST

SAST and DAST are two methods that approach application security from different angles but complement each other. For me, understanding these differences has been critical in deciding which tool to use and when. Here's a table summarizing the key differences:

In my practice, I've seen that SAST primarily finds errors made by developers while writing code, whereas DAST uncovers vulnerabilities in the application's overall architecture or configuration. While developing a manufacturing firm's ERP, SAST continuously improved our code quality, while DAST brought to light hidden issues in integration points or server settings.

Using Both Together: The DevSecOps Approach

After realizing that SAST and DAST complement each other, my question shifted from "Which should come first?" to "How should I integrate them?". The answer lies in the modern DevSecOps approach: integrating both into different stages of the development lifecycle.

The ideal scenario for me works like this:

1. Development and Pre-Commit: Developers run lightweight SAST tools locally or before committing. This provides instant feedback and prevents even minor errors from being committed.
2. CI/CD Pipeline: With every commit or at specific intervals (e.g., nightly), a more comprehensive SAST scan runs automatically. Found vulnerabilities are automatically logged into JIRA or a similar issue tracking system. At this stage, I prefer scans that don't exceed 15 minutes.
3. Staging Environment and QA: When the application is deployed to the staging environment, DAST scans come into play. These scans execute scenarios covering all application functionalities and look for runtime vulnerabilities. In one of my projects, a DAST scan would take an average of 45 minutes and run once a week. This was an approach I detailed in my article on [related: Automating Security in DevSecOps Pipelines].

ℹ️ Automation in DevSecOps

Integrating SAST and DAST into your CI/CD pipeline allows you to detect security vulnerabilities much faster and more consistently than manual checks. I've set up this automation using Jenkins or GitLab CI. For example, if a SAST scan fails, I can automatically stop the build and send an immediate notification to the developer.

This integration means moving security checks "left" in the development process. It transforms security from an add-on step into an integral part of the entire process. This way, security vulnerabilities are fixed at the lowest possible cost.

What to Consider When Making a Decision

There are trade-offs and points to consider when selecting and integrating SAST and DAST tools. Based on my experiences, the most important ones are:

• False Positives: SAST tools can sometimes produce false positives because they can't fully understand the code's context. This can lead to developers experiencing "alert fatigue" towards security warnings. DAST generally has a lower rate of these because it tests a running application. In one of my projects, about 20% of the warnings from SAST were false positives, and we had to manually filter them out. This led to wasted time.
• Scan Time and Performance: SAST scans can take time, especially for large codebases. DAST scans can also be lengthy depending on the application's complexity and the number of endpoints scanned. In CI/CD pipelines, I've constantly worked to optimize these times to stay within reasonable limits. For example, using incremental SAST scans that only scan changed code segments proved effective.
• Ease of Integration: How easily your chosen tools integrate with your existing CI/CD pipeline, IDE, and issue tracking systems is important. Good API support or ready-made plugins reduce integration costs.
• Scope and Language Support: SAST tools focus on specific programming languages, while DAST tools are language-agnostic. Choosing tools that match your technology stack is critical. In one project, we had a mixed system using C# and Python; finding SAST tools that supported both languages took some time.
• Cost: Both open-source and commercial SAST/DAST solutions are available. Commercial solutions often offer more features and support but come with higher costs. For smaller projects managed on my own VPS, I used open-source tools like Bandit (SAST for Python) and OWASP ZAP (DAST). For large enterprise projects, we often had to opt for commercial solutions.

By considering these factors, choosing the combination that best suits your project's needs and budget will be key to the success of your security strategy.

Conclusion: Security is a Process, Not a One-Time Action

Application security is not something that can be achieved with a single tool or a single test. Both SAST and DAST play critical roles at different layers and different lifecycle stages of an application. The biggest lesson I've learned in my 20 years of experience is that security is a continuous process and must be integrated into the development process from start to finish.

For me, the answer to the question "Which should come first?" has always been "Both, but at the right time." Catching code-level errors early with SAST and detecting vulnerabilities on the application's external face with DAST provides a comprehensive security posture. Remember, a security breach can cause severe damage to a company's reputation and financial standing. Therefore, security investments should never be overlooked. The next step should be planning how to integrate these tools into your own CI/CD pipeline.