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Injection attacks are the #1 security vulnerability on the web.

This happens when an attacker inputs a string that is later used inside of a command or query. The malicious string contains illegal characters that change how the command is executed.

The most common targets of attack are:

To mitigate this risk, THT introduces TypeStrings, which are literal strings prefixed with a type identifier.

How TypeStrings Work 

Unlike regular strings, TypeStrings can ONLY be combined with other strings through the use of placeholders.

Placeholders (aka parameterized queries) are an industry best practice for securing SQL queries. TypeStrings expand this tactic to cover all types of sensitive strings.

This approach is effective because:

How to Use TypeStrings 

Just prepend the type to any literal string to mark it as a TypeString.


let query = sql'select * from users';

Supported types:

TypeStrings can’t be modified like regular strings, so they can’t be accidentally mixed with insecure data.


// The safe kind of value that we normally
// expect from a form field.
let userId = '123';

// Instead, here is a malicious value that attempts
// to delete an entire database table by using a
// semicolon to split it into 2 commands.
let userId = '123; drop table users;';

// ✖ ERROR
// The TypeString can't be joined with a normal string
let q = sql'select * from users where userId = ' ~ userId;

Filling Placeholder Values 

You can attach dynamic values to a TypeString via the fill method.

These will be safely inserted into placeholders (e.g. {}), which will be safely escaped by the TypeString class.


let query = sql'select * from users where userId = {}';

// The Db modules only accepts TypeStrings
let row = Db.selectRow(query);

Appending TypeStrings 

You can join two TypeString together. Placeholder values will be merged into one list.

let name = html'Name: <b>{}</b>';
let job = html'Job: <b>{}</b>';

let profile = name ~ job;

print(profile.fill('Theresa', 'Therapist & Teacher'));
//= Name: <b>Theresa</b>
//= Job: <b>Therapist &amp; Teacher</b>

Aside: TypeStrings vs Immutable Strings 

TypeStrings are not the same as “immutable” strings in other languages.

Immutable strings mostly exist for thread safety and performance. They provide no real security against injection: They can still be combined with unsafe user strings to create new (unsafe) Immutable strings.


See the TypeString class for a list of methods.