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
Flask:
from flask import Response
@app.route('/')
def index():
response = Response()
response.set_cookie('key', 'value') # Sensitive
return response
Compliant Solution
Flask:
from flask import Response
@app.route('/')
def index():
response = Response()
response.set_cookie('key', 'value', httponly=True) # Compliant
return response
See
