psalm/docs/security_analysis/avoiding_false_positives.md

# Avoiding false-positives

When you run Psalm’s taint analysis for the first time you may see a bunch of false-positives.

Nobody likes false-positives!

There are a number of ways you can prevent them:

## Escaping tainted input

Some operations remove taints from data – for example, wrapping `$_GET['name']` in an `htmlentities` call prevents cross-site-scripting attacks in that `$_GET` call.

Psalm allows you to remove taints via a `@psalm-taint-escape <taint-type>` annotation:

```php
<?php // trackTaints

function echoVar(string $str) : void {
    /**
     * @psalm-taint-escape html
     */
    $str = str_replace(['<', '>'], '', $str);
    echo $str;
}

echoVar($_GET["text"]);
```

## Specializing taints in functions

For functions, methods and classes you can use the `@psalm-taint-specialize` annotation.

```php
<?php // trackTaints

function takesInput(string $s) : string {
    return $s;
}

echo htmlentities(takesInput($_GET["name"]));
echo takesInput("hello"); // Psalm detects tainted HTML here
```

Adding a `@psalm-taint-specialize` annotation solves the problem, by telling Psalm that each invocation of the function should be treated separately.

```php
<?php // trackTaints

/**
 * @psalm-taint-specialize
 */
function takesInput(string $s) : string {
    return $s;
}

echo htmlentities(takesInput($_GET["name"]));
echo takesInput("hello"); // No error
```

A specialized function or method will still track tainted input:

```php
<?php // trackTaints

/**
 * @psalm-taint-specialize
 */
function takesInput(string $s) : string {
    return $s;
}

echo takesInput($_GET["name"]); // Psalm detects tainted input
echo takesInput("hello"); // No error
```

Here we’re telling Psalm that a function’s taintedness is wholly depenedent on the input to the function.

If you're familiar with [immutability in Psalm](https://psalm.dev/articles/immutability-and-beyond) then this general idea should be familiar, since a pure function is one where the output is wholly dependent on its input. Unsurprisingly, all functions marked `@psalm-pure` _also_ specialize the taintedness of their output based on input:

```php
<?php // trackTaints

/**
 * @psalm-pure
 */
function takesInput(string $s) : string {
    return $s;
}

echo htmlentities(takesInput($_GET["name"]));
echo takesInput("hello"); // No error
```

## Specializing taints in classes

Just as taints can be specialized in function calls, tainted properties can also be specialized to a given class.

```php
<?php // trackTaints

class User {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }
}

/**
 * @psalm-taint-specialize
 */
function echoUserName(User $user) {
    echo $user->name; // Error, detected tainted input
}

$user1 = new User("Keith");
$user2 = new User($_GET["name"]);

echoUserName($user1);
```

Adding `@psalm-taint-specialize` to the class fixes the issue.

```php
<?php // trackTaints

/**
 * @psalm-taint-specialize
 */
class User {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }
}

/**
 * @psalm-taint-specialize
 */
function echoUserName(User $user) {
    echo $user->name; // No error
}

$user1 = new User("Keith");
$user2 = new User($_GET["name"]);

echoUserName($user1);
```

And, because it’s form of purity enforcement, `@psalm-immutable` can also be used:

```php
<?php // trackTaints

/**
 * @psalm-immutable
 */
class User {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }
}

/**
 * @psalm-taint-specialize
 */
function echoUserName(User $user) {
    echo $user->name; // No error
}

$user1 = new User("Keith");
$user2 = new User($_GET["name"]);

echoUserName($user1);
```

## Avoiding files in taint paths

You can also tell Psalm that you’re not interested in any taint paths that flow through certain files or directories by specifying them in your Psalm config:

```xml
    <taintAnalysis>
        <ignoreFiles>
            <directory name="tests"/>
        </ignoreFiles>
    </taintAnalysis>
```