URLs and HTML can be constructed securely by first creating an object and then serializing that to a string, instead of concatenating strings directly.


Wherever a string has some structure to it, it can be vulnerable to injection. When creating CSV, JSON, HTML, URL, or some other structured text, just putting a variable in it without encoding or escaping it can break the format, and result in a security issue. The naive way to solve this is to escape the variable. The better way is to create an object, and convert that to text.

URL example

You can create a URL like this:

const url = 'https://example.org/?error=' + errorMsg;

This goes wrong if errorMsg contains characters that have special meaning in a URL. The most obvious example here would be an ampersand, &, which can be used to add another parameter.

This can be solved by using escaping:

const url = 'https://example.org/?error=' + encodeURIComponent(errorMsg);

There are a few disadvantages with this:

  • you have to use the correct escaping method.
  • you have to remember to do it every time.

Another method is to use the URL API:

const url = new URL('https://example.org/')
url.searchParams.append('error', errorMsg);

By creating an object using an API, all values are automatically encoded. This way, it is impossible to create an invalid URL, or to forget escaping, or use the incorrect escaping.

HTML templates

Similarly, HTML is structured text and can be vulnerable to injection. The advice is to use a template engine, but even then there are several levels of escaping.

Most template engines are text-based with automatic escaping. The variable is automatically encoded and then interpolated in the rest of the template, as a string.

An alternative is to parse the template as a superset of HTML into a DOM, walk the DOM to replace the variables, and serialize it back to HTML. Angular does it this way. This is the most secure against injection. Consider this example:

<body class={{ variable }}>

If variable contains whitespace, this introduces a problem for a text-based template engine. However, a DOM-based template engine just assigns the variable content to the body.class property, whitespace and all.


Using the correct data types can help in securing both input and output values. By constructing objects instead of concatenating strings, injection is made impossible.

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