Understanding Semgrep — A Powerful Open-Source SAST Tool for Developers and Security Teams
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If you’ve ever worked on secure coding or DevSecOps pipelines, you’ve probably come across the term SAST — Static Application Security Testing. These are the tools that scan your source code for vulnerabilities, misconfigurations, or insecure patterns before your application even runs.
One of the most popular and lightweight tools in this space is Semgrep — a static analysis tool that’s fast, open-source, and surprisingly easy to customize. Let’s talk about what makes Semgrep stand out and why it’s becoming a go-to for developers and security engineers.
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What is Semgrep?
Semgrep is an open-source static analysis engine It’s designed to analyze source code across multiple programming languages, helping you find potential vulnerabilities or risky code patterns.
Unlike many bulky enterprise scanners, Semgrep is:
Lightweight – It’s easy to set up and run locally or in CI/CD pipelines.
Extensible – You can easily write your own custom rules.
Multi-language – It supports a wide variety of languages, including:
Go, Java, JavaScript, Python, Ruby, TypeScript, C#, and even markup formats like JSON and YAML.
That’s pretty rare for an open-source static analysis tool.
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How Semgrep Works
Semgrep doesn’t just perform simple text searches like grep. Instead, it parses your code into Abstract Syntax Trees (ASTs) — which basically means it understands the structure of your code.
This allows Semgrep to detect complex issues with a high level of accuracy.
Semgrep’s scanning logic is powered by rules, which are written in YAML format. These rules describe specific code patterns or behaviors you want to find.
The cool part? You can easily write, test, and publish your own rules using the Semgrep Playground.
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The Semgrep Registry
If you don’t want to start from scratch, the Semgrep Registry has you covered. It’s a large community-driven collection of over 1,000 rules, all publicly available at semgrep.dev/explore.
Some of the most popular rulesets maintained by r2c are:
r2c-ci – Focused on high-confidence, low-noise rules for CI/CD pipelines.
r2c-security-audit – A more in-depth security audit ruleset for catching subtle vulnerabilities.
You can think of these like plug-and-play templates that help you quickly integrate security scanning into your workflow.
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Running Semgrep from the Command Line
Running a Semgrep scan is incredibly simple. Once you’ve installed it (via CLI or Docker), you can run a full security audit of your codebase with just one command:
semgrep --config "p/r2c-security-audit"
This command downloads and runs over 200 security-focused rules from the Semgrep registry against your code. By default, results are printed directly to your terminal (stdout).
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Semgrep Command-Line Options
If you want to customize your scans, Semgrep’s CLI gives you several options. Here are the most useful ones:
Option | Description |
-f or -c <config> | Specify a YAML configuration file, folder of rule files, or a ruleset from the registry |
-e <pattern> | Run a custom search pattern directly from the command line |
-l <language> | Restrict scanning to a single language |
-o <output> | Save results to a file or send them to a remote endpoint |
You can explore all options by running:
semgrep --help
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Writing Custom Rules
One of Semgrep’s biggest strengths is how easy it is to write custom detection rules. You can do this using the Semgrep Playground (semgrep.dev/editor), which lets you test your rules interactively.
For example, let’s say you want to identify any AWS policy file that gives full S3 access (s3:*). Here’s a simple rule written in YAML:
rules:
- id: s3_wildcard_permissions
pattern: |
{
"Effect": "Allow",
"Action": "s3:*"
}
message: Semgrep found a match
severity: WARNING
This rule will flag any JSON configuration file where Action: "s3:*" appears — a sign of an overly permissive IAM policy.
What’s amazing here is that the same rule syntax can be used to scan source code, cloud configs, or even Kubernetes YAML files. So, whether you’re a developer or a cloud engineer, you can use one tool and one query language to identify risky patterns across your entire DevOps pipeline.
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CI/CD Integration
Semgrep was designed with automation in mind. It fits seamlessly into CI/CD pipelines using:
CLI commands
Docker containers
GitHub Actions
This makes it perfect for embedding into your build pipelines, so you can automatically catch vulnerabilities before code is merged — without slowing down developers.
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Final Thoughts
Semgrep is a great example of how open-source security tools can compete with — and often outperform — commercial solutions. It’s fast, flexible, and transparent, giving you the power to scan code in your own way.
Whether you’re a security engineer trying to enforce secure coding standards or a developer looking to clean up risky code, Semgrep is a tool worth adding to your toolkit.
If you want to try it out, here are the official resources:
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