Checker Framework Tutorial

This example uses the Tainting Checker to verify that user input does not contain SQL statements, thus preventing SQL injection. (If you have not already done so, download the tutorial sourcefiles.)

Outline

Run the Tainting Checker — 1 error found
Correct the error
Re-run the Tainting Checker — a new error is found
Correct the new error
Re-run the Tainting Checker — no errors

1. Run the Tainting Checker — 1 error found

Run the buildfile:
(The Ant buildfile makes use of the Checker Framework support for Ant.)

$ cd personalblog-demo
$ ant
Buildfile: .../personalblog-demo/build.xml

clean:

check-tainting:
    [mkdir] Created dir: .../personalblog-demo/bin
[cf.javac] Compiling 2 source files to .../personalblog-demo/bin
[cf.javac] javac 1.8.0-jsr308-3.12.0
[cf.javac] .../personalblog-demo/src/net/eyde/personalblog/service/PersonalBlogService.java:175: error: incompatible types in argument.
[cf.javac]                     "where post.category like '%", category,
[cf.javac]                                                    ^
[cf.javac]   found   : @Tainted String
[cf.javac]   required: @Untainted String
[cf.javac] 1 error

BUILD FAILED
.../personalblog-demo/build.xml:35: Compile failed; see the compiler error output for details.

Total time: 2 seconds

The checker issues an error for getPostsByCategory() because the possibly-tainted string category is used in the query construction. This string could contain SQL statements that could taint the database. The programmer must ensure that category does not contain malicious SQL code.

2. Correct the Error

To correct this error, add @Untainted to the type of the category parameter.

     public List<?> getPostsByCategory(@Untainted String category) throws ServiceException {

This forces clients to pass an @Untainted value, which was the intention of the designer of the getPostsByCategory method.

3. Re-run the Tainting Checker — a new error is found

Run the Tainting Checker again.

$ ant
Buildfile: .../personalblog-demo/build.xml

clean:
   [delete] Deleting directory .../personalblog-demo/bin

check-tainting:
    [mkdir] Created dir: .../personalblog-demo/bin
[cf.javac] Compiling 2 source files to .../personalblog-demo/bin
[cf.javac] javac 1.8.0-jsr308-3.12.0
[cf.javac] .../personalblog-demo/src/net/eyde/personalblog/struts/action/ReadAction.java:58: error: incompatible types in argument.
[cf.javac]                          pblog.getPostsByCategory(reqCategory));
[cf.javac]                                                   ^
[cf.javac]   found   : @Tainted String
[cf.javac]   required: @Untainted String
[cf.javac] 1 error

BUILD FAILED
.../personalblog-demo/build.xml:35: Compile failed; see the compiler error output for details.

Total time: 2 seconds

There is an error in ReadAction.executeSub(), which is a client of getPostsByCategory. The reqCategory is accepted from the user (from request object) without validation.

4. Correct the New Error

To correct, use the validate method as shown below.

    String reqCategory = validate(cleanNull(request.getParameter("cat")));

5. Re-run the Tainting Checker — no errors

There should be no errors.

$ ant
Buildfile: .../personalblog-demo/build.xml

clean:
   [delete] Deleting directory .../personalblog-demo/bin

check-tainting:
    [mkdir] Created dir: .../personalblog-demo/bin
[cf.javac] Compiling 2 source files to .../personalblog-demo/bin
[cf.javac] javac 1.8.0-jsr308-3.12.0

BUILD SUCCESSFUL
Total time: 2 seconds

You are done with the personalblog-demo project, so return to the parent directory

$ cd ..

For a complete discussion of how to use the Tainting Checker, please read the Tainting Checker chapter in the Checker Framework manual.

Previous Validating User Input

Finding a Security Error