When a cookie is configured with the `HttpOnly` attribute set to true, the browser guaranties that no client-side script will be able to read it. In most cases, when a cookie is created, the default value of `HttpOnly` is false and it’s up to the developer to decide whether or not the content of the cookie can be read by the client-side script. As a majority of Cross-Site Scripting (XSS) attacks target the theft of session-cookies, the `HttpOnly` attribute can help to reduce their impact as it won’t be possible to exploit the XSS vulnerability to steal session-cookies.

## Ask Yourself Whether

- the cookie is sensitive, used to authenticate the user, for instance a session-cookie
- the `HttpOnly` attribute offer an additional protection (not the case for an XSRF-TOKEN cookie / CSRF token for example)

There is a risk if you answered yes to any of those questions.

## Recommended Secure Coding Practices

- By default the `HttpOnly` flag should be set to true for most of the cookies and it’s mandatory for session / sensitive-security cookies.

## Sensitive Code Example

In php.ini you can specify the flags for the session cookie which is security-sensitive:

    session.cookie_httponly = 0;  // Sensitive: this sensitive session cookie is created with the httponly flag set to false and so it can be stolen easily in case of XSS vulnerability

Same thing in PHP code:

    session_set_cookie_params($lifetime, $path, $domain, true, false);  // Sensitive: this sensitive session cookie is created with the httponly flag (the fifth argument) set to false and so it can be stolen easily in case of XSS vulnerability

If you create a custom security-sensitive cookie in your PHP code:

    $value = "sensitive data";
    setcookie($name, $value, $expire, $path, $domain, true, false); // Sensitive: this sensitive cookie is created with the httponly flag (the seventh argument) set to false  and so it can be stolen easily in case of XSS vulnerability

By default `setcookie` and `setrawcookie` functions set `httpOnly` flag to false (the seventh argument) and so cookies can be stolen easily in case of XSS vulnerability:

    $value = "sensitive data";
    setcookie($name, $value, $expire, $path, $domain, true); // Sensitive: a sensitive cookie is created with the httponly flag  (the seventh argument) not defined (by default set to false)
    setrawcookie($name, $value, $expire, $path, $domain, true); // Sensitive: a sensitive cookie is created with the httponly flag (the seventh argument) not defined  (by default set to false)

## Compliant Solution

    session.cookie_httponly = 1; // Compliant: the sensitive cookie is protected against theft thanks (cookie_httponly=1)

    session_set_cookie_params($lifetime, $path, $domain, true, true); // Compliant: the sensitive cookie is protected against theft thanks to the fifth argument set to true (HttpOnly=true)

    $value = "sensitive data";
    setcookie($name, $value, $expire, $path, $domain, true, true); // Compliant: the sensitive cookie is protected against theft thanks to the seventh argument set to true (HttpOnly=true)
    setrawcookie($name, $value, $expire, $path, $domain, true, true); // Compliant: the sensitive cookie is protected against theft thanks to the seventh argument set to true (HttpOnly=true)

## See

- OWASP - Top 10 2021 Category A5 - Security Misconfiguration
- OWASP HttpOnly
- OWASP - Top 10 2017 Category A7 - Cross-Site Scripting (XSS)
- CWE - CWE-1004 - Sensitive Cookie Without 'HttpOnly' Flag
- Derived from FindSecBugs rule HTTPONLY_COOKIE
- STIG Viewer - Application Security and Development: V-222575 - The application must set the HTTPOnly flag on session cookies.