SAST is a way of finding security vulnerabilities by analyzing an application’s source code, bytecode, or binaries without running it. It’s often called “white-box” testing because the tool sees the program’s internals. It reads the code the way a very patient security reviewer would, looking for patterns and data paths that lead to flaws like SQL injection, cross-site scripting (XSS), or hardcoded secrets.
| Approach | What it examines | When it runs |
|---|---|---|
| SAST (static) | Source code / binaries | While writing code, at commit, or in CI |
| DAST (dynamic) | The running app, from outside | Against a deployed test environment |
| IAST (interactive) | The running app, from inside via instrumentation | During functional or QA testing |
| SCA (software composition analysis) | Third-party libraries and dependencies | During the build |
SAST is the most “shift-left” of these. It can flag a bug in your IDE before the code is committed, which is when fixing it is cheapest. That’s why it sits at the center of most DevSecOps programs.
1. Parsing the code
The tool reads your source files and breaks them into tokens (keywords, variables, operators), much as a compiler does. Some tools need the code to compile or build first so they can resolve types and imports.
2. Building an Abstract Syntax Tree (AST)
The tokens become a tree that represents the program’s structure: functions, statements, expressions, and how they nest. For example, query = "SELECT * FROM users WHERE id=" + userId becomes a tree node for an assignment, with a child node for string concatenation.
3. Building graphs of program behavior
4. Running the analysis techniques
eval(), MD5 used for passwords, Runtime.exec() with string input, or hardcoded API keys. This is fast but shallow.5. Reporting findings
Each issue typically includes:
from flask import request
@app.route("/user")
def get_user():
user_id = request.args.get("id") # SOURCE: untrusted input
query = "SELECT * FROM users WHERE id = " + user_id # tainted data propagates
cursor.execute(query) # SINK: SQL execution
The SAST tool traces request.args.get → user_id → query → cursor.execute and finds no sanitizer along the way. It reports SQL Injection (CWE-89). The fix is to pass the input as a parameter instead of concatenating it:
cursor.execute("SELECT * FROM users WHERE id = %s", (user_id,))
The tool recognizes parameterized queries as safe, so this finding disappears. This is exactly the class of flaw that CISA and the FBI have urged software makers to eliminate.
For detailed comparisons, see GBHackers’ best SAST tools, the SAST features and pricing comparison, and the list of top application security testing companies.
SAST turns code into structured models (an AST, control flow, and data flow), then hunts for paths where untrusted input reaches a dangerous operation without being cleaned. It’s fast, early, and shows the exact line to fix, but it works best when tuned and paired with runtime testing.
Sources:
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