The best free SAST tools are Semgrep as a general-purpose multi-language scanner, CodeQL for deep semantic queries on public repositories, and language-specific linters like Bandit for Python and gosec for Go. Static application security testing analyzes source code without running it, looking for patterns that indicate vulnerabilities: injection sinks, hardcoded secrets, unsafe deserialization, and weak crypto. You do not need a commercial license to get real value; a well-chosen free stack catches a large share of the common bug classes before code ships.
The trade-off with free tools is coverage and integration rather than core capability. You assemble several tools instead of buying one console, you write or curate rules yourself, and you wire the results into CI on your own. For most teams starting out, that effort is well spent, because the alternative is shipping with no static analysis at all.
Semgrep: The Best Starting Point
Semgrep is the tool to reach for first. It supports 30-plus languages, scans fast enough to run on every pull request, and its rules read almost like the code they match, so writing custom checks is approachable. It works semantically rather than by plain text search, which means a rule understands code structure and produces fewer false positives than a regex-based linter.
The open source engine plus the community rule registry cost nothing. You run it locally or in CI like this:
pip install semgrep
semgrep --config auto ./src
The --config auto option pulls a curated ruleset appropriate to your languages. As you mature, replace it with a pinned ruleset in your repository so scans are reproducible and you control what runs. Semgrep's sweet spot is broad coverage with low friction, which is exactly what a first SAST deployment needs.
CodeQL: Deep Semantic Analysis
CodeQL treats your code as a database you query. You write queries in its own language to find entire vulnerability classes, and it ships with a large library of maintained queries for common languages. Its taint-tracking is genuinely strong, tracing data from untrusted sources to dangerous sinks across function boundaries.
The catch is licensing on private code. CodeQL is free for analyzing public repositories, but scanning private repositories requires GitHub Advanced Security, which is a paid add-on billed per committer. So for open source projects it is fully free and excellent; for private code it moves into paid territory. Know that boundary before you build a workflow around it.
Language-Specific Linters: Depth Where It Counts
General scanners are broad but sometimes shallow in a given ecosystem. Single-language tools go deeper:
- Bandit scans Python for issues like hardcoded passwords, use of
assertin production paths, insecuresubprocesscalls, and weak hashing. Install withpip install banditand runbandit -r ./yourpackage. - gosec covers Go, flagging unhandled errors, SQL string concatenation, and unsafe file permissions.
- Brakeman is purpose-built for Ruby on Rails and understands the framework's conventions well enough to find issues generic tools miss.
For a single-language codebase, pairing Semgrep with the relevant specialist gives noticeably better results than either alone.
SonarQube Community Edition: Quality Plus Security Gates
SonarQube Community Edition is free and adds a persistent dashboard, quality gates, and trend tracking on top of static analysis. It leans toward code quality but includes security rules, and its gate mechanism can block a merge when new issues cross a threshold. It is heavier to operate than a CLI scanner because you run a server, but for teams that want history and a shared view, it fills that role at no license cost.
Assembling a Free Stack
A practical no-cost setup for most teams looks like this: Semgrep as the primary scanner across all languages, a language-specific linter for depth in your main stack (Bandit, gosec, or Brakeman), CodeQL if your code is public or you already pay for GitHub Advanced Security, and SonarQube CE if you want a dashboard and gates. Add a dedicated secrets scanner, since generic SAST catches only some hardcoded credentials.
Wire the CLI tools into CI so they run on every pull request and fail the build on high-severity findings. Start in a warning-only mode to tune out noise, then enforce once the signal is clean. The goal is a gate developers trust, not one they learn to ignore.
Where Free SAST Stops
Free static analysis is strong on code you write but says nothing about the dependencies you pull in, and modern applications are mostly dependencies. SAST will not tell you that a library three levels deep has a known CVE. That is the job of software composition analysis, and pairing the two closes a gap that either alone leaves open; an SCA tool covers the dependency side while SAST covers your first-party code. Runtime issues that only appear in a deployed app fall to DAST. If you later weigh a commercial platform, our Snyk comparison shows what paid tooling adds over an assembled free stack, mostly integration, triage workflow, and support rather than raw detection.
Getting Started This Week
Pick one repository, install Semgrep, and run semgrep --config auto. Read the findings, fix or triage them, and add the one language-specific linter for your main stack. Put both in a pull request check by the end of the week. You will have real static analysis running for the cost of an afternoon, which beats waiting for budget approval on a commercial tool that then sits unconfigured.
FAQ
Are free SAST tools good enough for production use?
Yes, for first-party code. A stack of Semgrep plus a language-specific linter catches a large share of common vulnerability classes. Free tools trade integration polish and support for zero license cost; the core detection is genuinely useful. They do not cover dependencies, which needs SCA.
Is CodeQL completely free?
CodeQL is free for analyzing public repositories. Scanning private repositories requires GitHub Advanced Security, a paid add-on billed per committer. For open source projects it is free and powerful; for private code, factor in that licensing.
What is the best free SAST tool to start with?
Semgrep. It covers 30-plus languages, scans fast enough for every pull request, has readable rules, and its open source engine plus community rules cost nothing. Add a language-specific linter like Bandit or gosec for deeper coverage in your main stack.
Do free SAST tools scan dependencies?
No. SAST analyzes the code you write, not third-party libraries. For known vulnerabilities in dependencies you need software composition analysis. Running SAST and SCA together covers both your own code and your supply chain.